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Photometry of Star Clusters - im AIfA

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1. Figure 18 The coaddition window of THELI magnitudes can improve when you choose 0 8 here Exit the configuration menu and start the coaddition After this step you are done with the given filter Proceed with the reduction of the next filter before you go to the next section After finishing all available filters you should make a new folder e g XunShi_AndreasK pper_20091224 M92_FINAL and copy all the coadded pictures and their weights into this folder 5 3 8 Prepare color picture After the reduction of the images you have to make sure that all pictures cover the same field This is usually not the case Therefore we crop the borders of the images such that all images have the same size and show the same part of the sky This can be easily done with the Prepare color picture routine of THELI which you find in the top bar under Miscellaneous 31 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Coaddition reference RA 17 17 07 27 reference DEC 43 08 11 5 Identification string blue Coadd these chips Pixel scale Sky position angle Outlier threshold Proper motion gt f inpajoec LL mini Resampling kernel LANCZOS3 ze Projection TAN u Celestial type NATIVE Combine type WEIGHTED lt _4 Interpolate bad pixels Figure 19 The coaddition setup window of THELI In the Create color picture window Fig 20 write the location of y
2. Create source cat box and push the Configure button In the create source 29 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Astro photometry Scamp Astrometrix POSANGLE_MAXERR deg POSITION_MAXERR arcmin 2 0 PIXSCALE_MAXERR factor 1 DISTORT_DEGREES SN_THRESHOLDS low high 5 20 FGROUP RADIUS deg 1 0 dekk CROSSID_RADIUS arcsec Create source cat ASTRINSTRU_KEY FILTER DETECT_THRESH 5 PHOTINSTRU_KEY STABILITY_TYPE INSTRUMENT DETECT_MINAREA te MOSAIC_TYPE UNCHANGED DEBLEND_MINCONT 0 0005 ith which FOCAL_PLANE Use default FP Minimum FWHM 1 _4 Use previously created headers LiL Maximum FLAG _ Update FITS header with astrometric solution Figure 17 The create source catalogue window left and the astrometry configuration window right of THELI catalog window Fig 17 set the DETECT_THRESH and the DETECT_MINAREA pa rameters to one of the following values 5 5 10 10 40 15 or 100 20 Begin with the lowest numbers and switch to a higher set in case THELI finds no astrometric solution in the following reduction step For very noisy data with a low S N ratio you can try 1 5 first Also you can increase the Minimum FWHM to 4 pixels as it is very unlikely that you will get a better seeing than that Close the configuration window by pushing Ok and hit Start Finally make the astrometric and photometric solution Therefore check Astro
3. Initialise Preparation Calibration Superflatting Weighting Astrom Photom coaddition mAstrometry and photometry SCIENCE OFFTARGET Astrometric reference RA 17 17 07 27 Mag limit 20 A DEC 43 08 11 5 Radius 30 Clear scripts Start Web CDS lt Defaults Abort USNO B1 Get catalog p Messages FLIP X f Configure _ Astrosphotometry scamp connigure m Commands Figure 16 The astrom photometry window of THELI 5 3 6 Astrom Photom The Astrom Photom window of THELI contains essential steps of the data reduction pro cess of your data which have to be done with care First of all get the coordiantes of your object e g from ALADIN and enter them in to the RA and DEC fields Make sure that you use the same coordinates for all filters Then you have to download an astrometric reference catalog Therefore select Web CDS and USNO B1 catalog from photographic plates or 2MASS catalog from a digital sur vey Make sure the radius of your requested catalog covers a few arcsec more than your field of view choose an appropriate magnitude limit e g 15 and hit Get catalog The larger your radius and the fainter the limiting magnitude the more sources you will get in your catalogue Don t have too many sources in this catalogue as the astrometric solution will get less accurate with more such degrees of freedom Check the
4. OK Figure 33 Configuration dialogue for Scamp If the image quality is good you can have both DETECT THRESH and DETECT_MINAREA as low as 5 5 If you have many hundreds or thousands of sources in an image then choose a higher pair of thresholds such as 10 10 or 40 15 or even 100 20 THELI has been used successfully with the latter parameters even for extremely crowded fields such as the Magellanic Clouds or exposures taken near the galactic centre The object catalogues created are saved in SCIENCE cat How to redo the task Just rerun Old catalogues in SCIENCE cat will be overwritten A 6 3 Astro photometry Here you can choose between three methods Scamp Astrometriz and Shift only The lat ter determines only relative astrometric offsets and relative photometric zeropoints It does 51 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS not care for sky position angles or absolute sky coordinates It can not handle mosaicing and only works for single chip data It requires a reasonable overlap of sources between exposures This method is most useful if you work with images that show just one or a few objects It serves as a fall back solution if the other two solutions fail for whatever reason Scamp usually fast and Astrometrix usually slower on the other hand are the entire opposite They calculate the linear offsets between the reference pixel CRPIX and the reference coordinates CRVAL as well as two dime
5. TECT_MINAREA and DETECT_THRESH just like we used them in THELI You can set those parameters by editing the script with a text editor or by simply adding the two num bers to the command line argument gt get_catalogue PATH B V R DETECT_MINAREA DETECT_THRESH where you should choose one of the pairs 5 5 10 10 15 40 or 20 100 You can play with the values and compare the resulting numbers of detections The larger values will give 34 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS you less false detections but also much less sources You have to decide which detections you will base your analysis on 35 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 6 Analysis After extracting the catalogue from the observational data you can begin the analysis Aim of the analysis will be to determine the most basic parameters of the clusters the distance and age We recommend to do the analysis with GNUPLOT since it is a very convenient tool for such tasks as plotting data from many different files For help on the functionality of GNUPLOT we recommend some on line help pages 6 1 Calibration Begin your analysis by drawing a first color magnitude diagram Therefore start GNUPLOT in a terminal window by typing gt gnuplot Plot the color index B V versus the V magnitude with the following command gt plot result_ascii cat using 3 4 4 where the 3 and 4 are the columns in the file Note that the magn
6. That makes it crucial for our understanding of galaxy formation and galactic stellar populations to investigate these objects in detail Moreover star clusters are unique test beds for stellar evolution theories Their most im portant property that all member stars were born in a single star burst out of one giant molecular cloud enables us with a few simple observations to get a snap shot of stellar evolution of a whole population of stars at a certain age distance and metallicity Without star clusters our knowledge on stellar evolution wouldn t be nearly as detailed as it is today The other way round observations of star clusters offer the possibility to easily derive their ages distances and metallicites by comparing the observations to theoretical stellar population models of certain compositions and evolutionary stages This latter application is the main objective of the underlying lab course project For this purpose the Hertzsprung Russel diagram HRD Fig 1 can be used which during the last century proofed to be the optimal tool for studying stellar populations and stellar evolution Main objective of this lab course project is therefore the understanding prepa ration and analysis of a color magnitude diagram CMD which is a direct derivative of the HRD ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS This script is organized as follows Section 2 gives an introduction to the subject of this project i e star clus
7. 13 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 3 Basic Knowledge of Astronomical Observations Almost all of our astronomical knowledge has been devised from the measurement of electro magnetic waves emitted from various forms of matter e g stars molecular clouds etc in the universe Due to the large distances from these emitters to us the intensity of radiation we can measure is very small thus sensitive equipments are needed Nowadays for optical observations like the one you will do radiation photons is collected and imaged by an optical telescope and detected converted into electronic signal by a CCD detector This section tends to give you a basic introduction to the equipments you will use and what you need to do to produce scientific data 3 1 Telescope optics Figure 9 Light path in a Cassegrain reflector telescope The red star indicates the Cassegrain focus where the CCD camera should be placed The telescope used for this lab course is a 50 cm Cassegrain reflector telescope placed in the dome on top of Argelander Institut f r Astronomie Bonn It has an f number f ratio of f 9 at its Cassegrain focus and f 3 at its primary focus We will use the telescope here only in the Cassegrain focus T3 1 Calculate the focal length of the telescope in both foci 3 2 CCD detector CCDs Charged coupled devices are now widely used as photon detectors both in the largest astronomical optical telescopes
8. 6 10 Table 2 List of Open Clusters The first column gives the Messier and or New General Catalogue NGC catalog name of the star cluster RA is the Right Ascension of the star cluster for epoch 2000 hour minute second while DEC is the Declination of the star cluster for epoch 2000 degree minute second my gives the cluster s total integrated visual magnitude and the last column gives its angular diameter in arcminutes Catalog Name RA DEC my dim NGC 288 00 52 47 5 26 35 24 81 13 NGC 2419 07 38 08 5 38 52 55 10 4 5 M 3 NGC 5272 13 42 11 2 28 22 32 6 2 18 M 92 NGC 6341 17 17 07 3 43 08 11 6 4 14 M 15 NGC 7078 21 29 58 3 12 10 01 6 2 18 Table 3 List of Globular Clusters Columns are the same as for Tab 4 1 4 2 Observing Schedule Since time is short during the night every step has to be planned in advance Therefore an observing schedule has to be prepared beforehand For details on the tasks which have to be performed during the night see the observer s manual at the telescope 1 Telescope and CCD camera setup 2 Cool down the CCD to about 25 degrees below the current temperature 3 FLAT frames 10 FLATs filter Note that there s very limited time in which a SKY FLAT FLAT frame towards evening morning twilight sky can be taken So prepare everything beforehand and hurry up 4 Focusing 5 Science frames of your objects 6 science frames object apply dithering in between reference sta
9. ASTROM PHOTOM processing group A 6 1 Astrometric reference catalogue Retrieving a catalogue from the web This is the default state You can select the GSC2 the USNO B1 or the 2MASS as a database The catalogue will be retrieved from CDS and is reformatted for use with THELI You can control the limiting magnitude of the objects retrieved in the catalogue s mag nitude system and therefore their number density The catalogue will be retrieved around the reference coordinates within the radius specified The coordinates themselves can be entered in decimal or hexagesimal values If left empty they will automatically be deter mined from the current data set You retrieve the catalogue by clicking on the Get catalog PushButton The GUI will not accept any other input during the time it takes to get the catalogue seconds to minutes It appears frozen but it is not will change in the future Once downloaded and processed the message window will tell you how many sources it got from the catserver at CDS You may then rerun the task with a modified limiting magnitude if you are not happy too few or too many objects 49 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Processing group AstromiPhotom 1 Create source cat DETECT _THRESH 10 0 DETECT _MINAREA DEBLEND MINCONT 0 0005 Minimum FWHM li Maximum FLAG Defaults all pages y Figure 32 Configuration for the catalogue creation How to
10. M92 B XunShi_AndreasK pper_20091224 M92_V Also name your files properly otherwise you will get confused on the next day or whenever you work with the data again 22 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 5 Data Reduction For reducing the data and extracting a catalogue of sources out of the pictures a number of steps are necessary and a variety of scripts and programmes will be used In this section we will introduce the tools you need for creating a color magnitude diagram this will be done in the order in which you will have to use them 5 1 ALADIN SERIEN File Edit Image Catalog Overlay Tool Help S Wj ocation cs v Pixel 452 S 1 0 ru Tv ER ESO POSSZUKSTU_Blue 1 Server selector EN Sri E aio Erov trol The Digitized Sky Survey from ESO Garching Fill in all these fields and press the SUBMIT button 17 17 07 27 43 08 11 5 Grab co DSS2 blue POSS2UKSTU_Blue E 17 01 x 16 8 Width arcmin 30 INFO on this serv zZ c 1999 2008 ULPICNRS Centre de Donnees astronomiques de Strasbourg Oselj0are 20mb Reset Clear Help SUBMIT Close Figure 10 Main window left and server selector window right of ALADIN Before starting any work on the data make sure you know how your object looks like Therefore start ALADIN a virtual telescope in which you can view parts o
11. Omega Centauri Hubble leritage 1 light year Figure 6 The globular cluster w Centauri is the largest known cluster in the Milky Way It is about 50 pc in diameter and consists of about 10 million stars On the right is a close up of the innermost region of w Centauri observed with the Hubble Space Telescope pictures taken from the Digitized Sky Survey NASA and the STScI clusters of the Milky Way and other galaxies However as mentioned above there are some young luminous clusters that resemble in size and mass a globular cluster Despite some exceptions the two groups therefore seemed to be of completely different origin Open clusters form continuously while globular clusters came into existence when the Universe was much younger and thus denser Actually globular clusters are even some of the oldest objects in the Universe and therefore put a strong limit on the age of the Uni verse but also on the understanding of stellar evolution and on structure formation after the Big Bang Another hint for a fundamental difference of their origin was seen in the distribution of the two types of clusters in the Milky Way and other spiral galaxies as for example An dromeda Globular clusters are randomly distributed in the halo of the host galaxy and follow random orbits while open clusters are solely found in the galactic disk All this was explained by taking their different formation processes into account e As mentioned above op
12. You can check your own images and compare them with the virtual telescope data by using SKYCAT or DS9 For the first one type gt skycat in a terminal window Open any image with the menu File Open and select the the file you want to open To adjust the view of the image push the button Auto Set Cut Levels first and then go to the View Colors menu In the new window you can choose various color scalings color maps and intensity functions Pick for example Linear heat and gamma for a good representation of your star cluster Fig 11 With SKYCAT you can also access on line catalogues like DSS Simply choose an appropriate catalogue from the Data Servers menu DS9 is similar to SKYCAT but offers slightly different functionality To start DS9 type gt ds9 in a terminal window and the DS9 window will show up Fig 11 As in SKYCAT open any file with the File Open menu The scaling of the view can be adjusted via the Scale and the Color menu Choose for example Square Root 99 5 and Grey You can now easily adjust the scaling by holding the right mouse button and moving the cursor up and down or left and right 5 3 THELI The data reduction itself will be done with THELI a freely available software package for the reduction of astronomical imaging data which was in part developed at AIfA An in troduction to THELI is given in the appendix of this document therefore we only give the necessary steps here but strongly recommend going throu
13. all filters If you go through this task for a second set of pictures for a different filter make sure that you remove the specific lines from the Commands window before hitting the Start button 5 3 3 Calibration In the calibration window first check the Process biases darks box and then hit Start Note again that this task also has to be done just once for the other filters simply skip this step Then check Process flats and start the procedure 27 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Ld THELVA 61 0 File Edit Settings View Reduction Miscellaneous Delete Help PUTT SS ON NS RR AS ES OS ES ib RP PER EE Pal AGRE TAS TZE PAE aL EE al z Le ery a aah aca pata Ei La aE EER E a sack ae anil sae Ea ehe eft Figure 15 The weighting window of THELI Finally you have to calibrate your data Therefore check also the Use DARK box for those cases where you need a dark current subtraction i e long exposures but not the Create SUPERFLAT box before you push the start button 5 3 4 Superflatting Superflatting will not be applied in this lab course project 5 3 5 Weighting In the weighting window you have to start the Create global weights process and then the Create WEIGHT process 28 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS THELI v 1 6 1 0 Jol File Edit Settings View Reduction Miscellaneous Delete Help
14. and in your pocket digital camera A CCD detector consists of a two dimensional array of picture elements pixels which are produced as a light sensitive metal oxide semiconductor MOS capacitor on a silicon substrate CCDs make use of the inner photoelectric effect to convert the distribution of photons to the dis tribution of electrons which are then collected in capacitors and sequently read out after an exposure is terminated the collected charge is shifted column by column to a readout column by an alternating voltage impressed on the picture elements The readout column is finally read out pixel wise and the resulting signal amplified and converted to a digital signal by an analogue digital converter 14 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS As a detector a CCD has the advantages of e high sensitivity high quantum efficiency of up to 90 high dynamical range the limits of luminance range that a detector can capture linearity over almost the entire dynamical range e large spectral range mid infrared to ultraviolet for optical optimised detectors e direct availability for further computer aided data analysis A less advantageous property of CCDs is the so called dark current At room tempera ture dark current brings CCD pixels to their saturation level within a minute or even less Therefore astronomical CCDs are always cooled Cooling is done thermo electrically tem perature difference to ambient tempe
15. data set you are reduc ing into the Current LOG file line a new log file will be created Start with one filter and don t start reducing a second filter set before you finish the first Just to make sure that no old preferences are set hit the Reset button next to the LineEdit afterwards For the given filter enter the directory names into the appropriate lines Make sure the Main path is an absolute path and not a relative one i e that it gives the full path starting from root Finally before proceeding to the next panel choose the camera you used from the in strument list 26 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Superfiatting Weighting Astrom Photom Coaddition m Debiasing flatfielding creation of superflat SCIENCE OFFTARGET Do not apply BIAS DARK _Configure_ Do not apply FLAT Clear scripts Start min max Defaults Abort s blases darks J 7 7 E aa poi groups Length _ Spread sequence IR 3 12 me ee Create SUPERFLAT M Use DARK Figure 14 The calibration window of THELI 5 3 2 Preparation In the preparation window you have to check the Split FITS correct header box and hit the Start button Note that this task has to be done for the BIAS and DARK frames just once as they are used for
16. e each pixel would get a value between 0 and 65535 For a better coverage of the available range of counts the logarithm of the analog signal is taken first This is because you want to be very accurate for pixels with only a few counts but do not need to be that accurate for pixels with many counts But since the analog digital converter can only handle positive values and fails for a value of zero an offset has to be added electronically to every pixel value during the read out process before the logarithm is taken Otherwise small voltages would lead to negative numbers thus would make the converter give out high positive values as the next value below 0 is 65535 in the example of a 16 bit converter This so called BIAS has to be recorded by a zero time exposure called the BIAS frame which then has to be subtracted from every image as the first step of data reduction 3 4 2 Dark current subtraction Even if the CCD chip is NOT exposed to optical light there will still be a current flowing in it due to thermal fluctuations which is called the dark current Dark current is one of the main sources for noise in image sensors such as a CCD The pattern of different dark currents in the pixels across the CCD can result in a fixed pattern noise Taking DARK frames and subtract them from the science and FLAT frames can remove an estimate of the mean fixed pattern but there still remains a temporal noise due to the fact that the dark current itself
17. mean has lower noise when more images are stacked How to redo the task Delete the OFC images in SCIENCE and play back the previous exposures from the SCIENCE SPLIT_IMAGES directory Images in SCIENCE SUB_IMAGES are overwritten A 4 The SUPERFLATTING processing group A 4 1 Smooth SUPERFLAT create fringe model This step smoothes the SUPERFLAT that was created previously yielding an illumination correction that can be used to superflat the data The difference between the smoothed and the unsmoothed SUPERFLAT gives the fringing model The illumination correc 46 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Superflatting Weighting Astrom Photom Coaddition Superflatting defringing SCIENCE OFFTARGET Smooth SUPERFLAT y j m create fringe model Smoothing SIZE 256 Clear scripts Start Superflat data pr Use unsmoothed SUPERFLAT Defaults Abort FT Defringe data Subtract SUPERFLAT THELI messages l Chop Nod sky subtraction Pattern Invert l Merge sequence IR Number of groups 3 Collapse correction Commands that will be executed you have the last word Figure 29 The Superflatting PG tion images are called SCIENCE_i_illum fits whereas the fringing models are named SCIENCE_i_fringe fits How to redo the task Just rerun Previous results will b
18. more or less tightly bound strongly depending on the initial conditions and the birth parameters If the initial gas loss is too violent the cluster will dissolve within a short time otherwise it will virialise within a few million years and from then on dissolve slowly Boily amp Kroupa 2003a b Moreover based on observations of pre main sequence stars a primordial i e initial bi nary fraction of about 100 has been found Kroupa 1995 which means that almost every clump in the collapsing gas cloud splits into two subclumps and in the end yields two dis tinct stars which form a binary system The actual value of the binary fraction of observed clusters is still a debated topic though Due to insufficient spatial resolution this question cannot be answered directly by observations yet hence uncertainties are still quite large The binary fraction of all stars in the Milky Way is believed to be about 50 i e every ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Figure 4 Two young star clusters On the left NGC602 a star forming region where the massive stars in the centre already started to blow out the gas On the right the centre of the Pleiades cluster which is about one hundred million years old and exhibits almost no more gas pictures taken from NASA and the STScI second star on the sky has a companion which in most cases cannot be seen with the naked eye while relatively young clusters like the Pleiades Fig 4
19. stars Fig 3 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS logio amp log m 0 log o m Mo Figure 3 Logarithmic representation of the canonical IMF solid histogram The histogram gives the probability of forming a star of mass m in a star cluster Between 0 08 My and 0 5Mo the IMF has the slope 1 35 For higher stellar masses it has a slope of 2 35 From this figure it is obvious that it is much more likely to form low mass stars than high mass stars Figure taken from Kroupa 2001 T2 1 The canonical IMF m has the form E m 0 237m t for m lt 05 Me 1 m 0 114m for m gt 0 5Mo 2 which is normalized such that the integral over from m 0 08 Mo to m 150 Mo is equal to 1 If you draw 1000000 stars from this IMF say for the simulation of a globular cluster how many will be below 0 5 M How much mass will be in stars below 0 5 M When the first newly formed stars ignite the cluster will still be embedded in its birth gas cloud If the initial cloud was rich there will be a couple of large so called O and B stars with masses of up to 150 M where this upper mass limit is still wildly discussed Kroupa 2005 As can be seen in Fig 4 these luminous stars will soon blow out the left over gas and free the cluster from its birth cradle with their enormous radiation pressure and as a cause of ongoing supernovae What is left is an ensemble of stars which is
20. start working on a new R band data set of NGC 1234 then NGC1234_R would 41 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Superfiatting Weighting astrom Photom Coaddition Pipeline settings Select Instrument Create new instrument Current LOG file of CPUs a E Refresh instrument list ngc1234_r LIRIS WHT el cuisye fans E MOSAIC1_0 9M KPNO Astrometric reference catalog optional MOSAICI_4m KPNO ei pial NFRAMES E E MOSAIC I KPNO MOSCA 22 NOT NACOSDI VLT NICS TNG NOTcam_highres NOT NOTcam_lowres NOT Data directories OASIS4x4 WHT ceaun OASIS WHT Main path Restore ALL PFC WHT PAELLA RIBEIRA SOFI NTT Bias Restore ORIG ST10_AOT NEWTON E rn STIO FFC Dark Restore ORIG STIO NEWTON Flat Restore ORIG STB FFC 3 STLIIK TAK Science O Restore ORIG SuSl2_2x2 NTT SSS SuSl2old_2x2 NTT Offtarget Restore ORIG a A VIMOS VLT VISIR VLT WFC INT WFI MPGESO WFI SSO Mem 467 MB Disk 80 15855 MBIEN Figure 25 The Initialise PG qualify as a nice LOG name Enter it in the corresponding LineEdit then click on the Clear processing status PushButton This will flush the GUI from all settings that might be left over from a previous reduction run Parameters in the GUI and the LOG are set to m
21. sticky What s going on A Your machine is swapping If you work under Linux you can see this in the memory ProgressBar at the bottom of the window It becomes red and tells you how much swap space is currently used Solaris users don t see this ProgressBar Lower the number of images loaded simultaneously into memory the NFRAMES parameter in the Initialise PG such that they consume not more than about 40 of your physical RAM Astrom Photom Q I want to retrieve an astrometric reference catalogue from the web through the orange window in the Astrom Photom PG but no results are returned What is wrong A Sometimes the CDS server is down Check the status of your local network as well A 10 Image statistics Calling Image statistics from the Miscellaneous pull down menu presents you with the dialog shown in Fig 36 Image directory Specify here the path to the images for which you want to obtain some statistics 55 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Name filter This is a string for filtering a subset of images out of all files in the specified directory For example it can simply be something like A OFCS fits If left empty all images fits in the directory will be considered How it works Clicking on the Get statistics PushButton will retrieve the statistics This can take a while and the GUI will not allow any other action during this time may change in a future release The table obtained will auto
22. tips that are displayed when you hover with your mouse button over a specific PushButton or other element unless the meaning of those is obvious see Fig 24 e More extensive help for the various PGs and reduction steps is available under Help What s this The cursor will change to a question mark Move it anywhere into a GroupBox with the reduction steps to obtain general information of what is happening in this particular PG Or click on the CheckBox of a particular reduction step to obtain more detailed information for this task such as if this step is mandatory or optional and if you have to provide any parameters e The Help menu provides you with further support such as dialogue containing an overview of the functionality of the various GUI elements Furthermore you can access this document as well the general pipeline documentation or a rather technical paper analysing the performance of THELI e If the GUI expects some parameters as input it will highlight the according fields for you with a red background colour etc The colour coding is explained in detail in the following subsection e Each reduction step you run dumps all programme and script output into the SCRIPT LOG You can access these for each PG separately through View Processing logs in the menu bar A 1 The INITIALISE processing group A 1 1 Pipeline Settings The LOG file The four PushButtons and the first LineEdit are associated with the LOG Say you
23. 2 MNRAS 336 1188 Lada C J Lada E A 2003 ARA amp A 41 57 Lejeune T Schaerer D 2001 A amp A 366 538 Nasi E Bertelli G Girardi L Marigo P 2008 MmSAl 79 738 Russell H N 1913 Obs 36 324 Salpeter E E 1955 ApJ 121 161 57
24. A 3 The CALIBRATION processing group A 3 1 Process biases darks Combines all BIASes in the BIAS subdirectory set in the Initialise PG You can override this by entering a different subdirectory in the LineEdit next to the CheckBox The more BIASes you have the better your master BIAS will be and the smaller the calibration noise that is introduced into your SCIENCE images We recommend to use at least 10 BIASes Overscans are corrected and trimmed How to redo the task Delete the BIAS_i fits images from the BIAS dir and rerun They have to be deleted because otherwise they are taken as an input image 43 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Superflatting Weighting Astrom Photam Coaddition Naming sorting splitting linking In this directory is the raw data Rename images to this FITS key ARCFILE Sort data using this FITS key OBJECT j al m Start Biases Darks Dometlats Skyflats Standards _Clear seripts_ SE Bias Dark FLATDOME FLATSKY STD Detauts Abort THELI messages Work on images in this directory only X Split FITS correct header Create links Chips going to scratch Scratch directory Commands that will be executed you have the last word Jprocess_split WFI MPGESO_eclipse sh home mischa DATA BIAS Jprocess_split WFI MPGESO_eclipse sh ho
25. Advanced Lab Course Photometry of Star Clusters Argelander Institut fiir Astronomie Universitat Bonn 28th November 2010 written by Xun Shi amp Andreas Kiipper ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Contents 1 2 Motivation amp Overview Basic Knowledge of Star Clusters and CMDs 21 Star Clusters 2 2dr EA a le A a eet ai Ret ei 2 2 Open Clusters vs Globular Clusters 2 2 2 Corn nn 28 ERDICMO sata os e A A Bee Se A 2 4 Colors amp Magnitudes sorgor porod nn nen Basic Knowledge of Astronomical Observations 3 1 Telescope optics eh gou ee 3 2 CCD detector o es a eee ak el 3 3 Observational conditions and requirements 2 22 2 m nn nennen 3 4 Image Reduction Steps s sooo eso m nn nen 3 5 Photometric calibration reference stars 2 22 2 2 mo nn Observations 4 1 Choosing Your Objects 2 22 22 on Emm nn 4 2 Observing Schedule nn nn AS Data storage lea ra nn AE BOR EE Ra aS Data Reduction 5 ALADIN usara Den Death 5 2 FOR IDO we Lene a Bite git cae a aie Bal Soe A Beet wee SO o AE a ce ha Bld en a IS A eo Bid OSHXtRACLOR a A A A BON 8 A asi Oe Analysis 6 12 Calibration 2 0 2 3 4 4 A eS ee ee a Be ee F 6 2 EXtinctionin g Ye 2 eS ne eden D ee ee Bo Eh eo A A ee me da 6 3 Distance Determination 2 2 CC m mn nn 6 4 Tsochrone Fitting y ats 22 es au a ek en lan THELI user manual lab course short version A 1 The INIT
26. EIGHTING processing group A 5 1 Create globalweights The normalised FLAT is taken and has bad pixels replaced by zero values Whether a pixel is bad is determined by one or more threshold pairs which refer to the normalised FLAT itself How to redo the task Just rerun A 5 2 Create WEIGHTS All cosmics hot pixels and other chip defects are detected on an image by image basis in this step How to redo the task Just rerun 48 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Superflatting Weighting Astrom Photom Coaddition Astrometry and photometry SCIENCE OFFTARGET Astrometric reference RA Mag limit 20 catalog from MEE Radi ao PEREN za Clear scripts Start Berus J won csc22 22 v Get catalog THELI messages l Transform images FLIP X E x Create source cat FE Astro photometry Commands that will be executed you have the last word create_stdcat_fromWEB sh home mischa DATA NGC1234 GSC2 null null 40 20 Jparallel_manager sh correct_crval_para sh homelmischa DATA NGC1234 nora nodec parallel_manager sh create_astromcats_weights_para sh homelmischa DATA NGC1234 WEIGHTS weight Jcreate_scamp sh home mischa DATA NGC1234 ceso EEE Disk 81 14866 ME IEM Figure 31 The Astrom Photom PG A 6 The
27. IALISE processing group e A 2 The PREPARATION processing group 2 2 2 2m rn nn nn A 3 The CALIBRATION processing group o A A The SUPERFLATTING processing group 2 2 22 nn nn nenn A 5 The WEIGHTING processing group 2 2 2 nn A 6 The ASTROM PHOTOM processing group 2 2 2 A 7 The COADDITION processing group nn A 8 Guidelines for observers using THELI 2 2 2222 2 nn nn A 9 FAQ and Troubleshooting 2 2 2 2 nn 2 000000 eee eee A 10 Image statistics oa se u an au Din aa ann en a 14 14 14 15 17 19 20 20 21 22 23 23 24 24 32 36 36 36 36 38 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 1 Motivation amp Overview Temperature K 25 000 10 000 6 000 3 000 Superglants Luminosity Maln Sequence Absolute Magnitude White Dwarfs A bh A se Ge m Spectral Class Figure 1 A typical Hertzsprung Russell temperature luminosity diagram each dot represents a single star Valuable information on a star s evolutionary state can be derived from its position within the diagram picture taken from NASAexplore com Star clusters belong to the most important objects in the Universe First of all they are the fundamental building blocks of galaxies because it is nowadays believed that most if not all stars are born in such groups of a few dozen up to several million stars Hence nearly all stars in galaxies have once been member of a cluster
28. I_RILD NTT EMMI_RIMG NTT Astrometric reference catalog optional Figure 23 The two main GUI elements the menu bar top and the tabbed menu A THELI user manual lab course short version General description of the GUI and its elements Main elements and nomenclature The main window Fig 23 consists of two main parts e a menu bar at the top of the window and e a dialogue with a tabbed menu We will refer to the seven tab menues of the latter as processing groups PG for short These are e Initialise Preparation Calibration Superflatting Weighting e Astrom Photom e Coaddition and contain the various reduction steps or other settings The data is reduced by making some initial settings in the first processing group tell the software where the data is which instrument was used etc and then one works his way through the remaining six PGs 40 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS _ Use unsmoothed SUPERFLAT seo Correct your data with an unsmoothed SUPERFLAT instead of a smoothed one Number of groups 3 IN along 3 12 Mx y Figure 24 A tooltip appears whenever you hover with the cursor over certain GUI elements Integrated Help system Besides the help described here see also the troubleshooting section in App A 9 A very extensive help layer has been integrated that comes in various active and passive forms e On the lowest level there are simple tool
29. ailable T2 6 How does the distance of your object from T2 4 change in the case you have 0 2 mag of extinction 11 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 2 4 4 Metallicities In astronomy all chemical elements heavier than He are called metals In the first few minutes after the Big Bang just a very low percentage of all baryons was synthesized to metals while most baryons synthesized to hydrogen and helium In fact accurate primordial abundances have already been predicted by Alpher Bethe amp Gamow 1948 out of theoretical Big Bang nucleosynthesis considerations before first accurate measurements were made The standard values which are assumed for primordial abundances in most stellar evolution calculations are X 0 765 for hydrogen Y 0 235 for helium and Z 0 for metals Through stellar evolution the metallicity in the Universe increases as hydrogen and he lium are processed into heavier elements Nuclear fusion in stars in combination with stellar winds and supernova explosions permanently enriches the interstellar material The solar metallicity for instance is about Z 0 02 hence it has formed out of already enriched gas Stars which form out of enriched material evolve differently from metal poor stars as the metal content has a large influence on stellar evolution and the stellar structure For instance higher metalicities cause stars to become dimmer and cooler below a stellar mass of about 4 M Above thi
30. and Hyades show fractions of about 60 70 Kroupa 1995 T2 2 The binary fraction of a star cluster fin is defined as Noin foin N Niin 3 where Nin is the number of binary systems whereas N is the number of single stars Imag ine you observe a star cluster and you detect 1000 point sources but you know the cluster has a binary fraction of 0 7 how many stars are in this cluster 2 2 Open Clusters vs Globular Clusters Historically star clusters are split up into two distinct populations open clusters and glob ular clusters Although these two families of stellar groups exhibit significant differences see Table 2 2 the definitions as many other things in astronomy are not strict and there is even a number of objects like the cluster Westerlund I that cannot be clearly assigned to one of the two Open clusters present the lower mass range of star clusters They assemble from a dozen up to several thousand stars in a region with a diameter of 1 10 pc Hence densities vary significantly from cluster to cluster and reach from about 0 1 Mopc which may rather ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Figure 5 The colliding galaxies NGC4038 and NGC4039 also known as the Antennae galaxies During this merger of two gas rich spiral galaxies thousands of star clusters have formed and are still being formed Actually the bright blue points in this Hubble view are not single stars or star clusters but clus
31. and filter characteristics see for example Binney amp Merrifield 1998 p 53 For the lab course project the Johnson filters U B V I and R are available Fig 7 Note Nowadays the Johnson filter system is being replaced in many applications by the Sloan filter set Fukugita et al 1996 which has a similar wavelength coverage u g r 12 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 0 8 0 6 Transmission 0 2 0 h pe A E E ii 3 3000 4000 5000 6000 7000 8000 9000 10000 A angstroms Figure 7 The transmission functions of the five available Johnson filters picture taken from the University of G ttingen 0 4 4 2500 5000 7500 10 1 25x10 Figure 8 The transmission functions of the five Sloan filters picture taken from Fukugita et al 1996 i z but allows a higher transmission in each filter Fig 8 and therefore requires shorter exposure times Due to this varity of available filters conversions of observed fluxes into apparent magnitudes have to be done carefully It is crucial that the measured fluxes of the reference stars in a given filter are compared to the listed magnitudes of the reference stars in the specific filter i e if you observe with Johnson filters make sure the magnitudes of the reference stars you use are listed in the Johnson system and not in Sloan or anything else The same holds for the theoretical isochrones which you fit to your data
32. cluster By measuring f for the ref erence star as well as for the cluster stars and by knowing the magnitude of the reference star we can obtain the magnitudes of the cluster stars using equation 4 Note the flux of a single star is mostly determined by fitting a two dimensional Gaus sian distribution to the CCD image This Gaussian results from the point like appearance of the star which gets convolved with the telescope point spread function PSF By inte grating over this distribution function the total flux f of the star can be determined and converted to magnitudes as described above In contrast to stars galaxies and star clusters often appear as extended sources on the CCD Their magnitude is determined by fitting appropriate distribution functions to the CCD image and integrating over these functions out to a pre defined cut off radius In this way it is possible to define magnitudes for ex tended sources but these values have to be handled with care as there are always underlying models which have been assumed and which may differ significantly from case to case 10 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS T2 4 You observe a reference star and measure 16000 counts from the literature you know that this star has an apparent magnitude of 15 0 For a second object you measure 4000 counts What is the apparent magnitude of this object 2 4 2 Distance Modulus The observed flux f of an object depends not only on its in
33. ctual resolution is much worse for ground based telescopes And the PSF can be better described by a two dimensional Gaussian The size of such a stellar image can be described by the full width at half maximum FWHM of the Gaussian which is called the seeing of the image Naturally seeing measures the actual 15 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS resolution in a particular observation and it reflects the stability of Earth s atmosphere on a given night at a given location as seen through the telescope The best seeing on the surface of earth is about 0 4 It s found at high altitude observatories on small islands such as Mauna Kea or La Palma For the condition above AIfA Bonn a 2 seeing is already good To measure the seeing on your image you can choose a non saturated star and measure its FWHM in number of pixels using the telescope software and then convert it to arcsec onds using the relation pixel size um 206 pixel 10 focal length um pe en The pixel size of the CCD and focal length of the telescope can be found in the above subsections T3 3 Derive the factor 206 in equation 10 3 3 2 Focusing If the CCD camera is out of focus you will get a big blob for each star on your image So before taking science frames make sure you have the right focus by adjusting the focus until you get the sharpest image 3 3 3 Linearity saturation dynamical range and exposure time When speaking about l
34. e artifacts will influence the quality of your data dead pixels on the detector and cosmic rays The former results to that several pixels on each of your obtained images only represent noise but not any signal The latter on the other hand leads to the saturation of several pixels around the cosmic ray impact position While the number and positions of the dead pixels are fixed the number of cosmic rays depends on the exposure time and their positions are random These artifacts will hinder you from getting information on those pixels which correspond to certain positions on the sky To avoid totally losing information at those positions several frames 6 is suggested here are taken of each scientific object and the telescope is moved slightly between each exposure This method is called dithering The multiple frames are then aligned and the median of each pixel of the combined image is determined with the consequence that all extreme values are discarded This method also has the advantage that the signal to noise ratio is improved without saturating brighter sources 3 4 Image Reduction Steps 3 4 1 BIAS subtraction The signal of the CCD is first converted from an analog count signal electrons in the pixel i e a voltage value into a digital number by an analog digital converter For example with a 16 bit analog digital converter you would transform your analog voltage values from the pixels into 21 65536 discrete levels i
35. e ones Mealib Minstr Z 11 So you can calibrate your instrumental magnitudes by observing some reference stars or standard stars whose calibrated magnitudes are well known and stable Observe such stars using the same configurations as your scientific objects For this lab course project it is sufficient to look up the magnitudes in the different filters for some reference stars in your chosen clusters from a catalogue or compare it to a well calibrated CMD of this cluster from the literature and add appropriate offsets 19 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 4 Observations After reading this section you are supposed to know what you need to do during your night time observations step by step Furthermore by reading this section you should choose your objects to be qualified for the observations Feel free to discuss with your tutors if you have questions The data files you should obtain during your observations are e 10 FLATS in each of the 2 filters B and V e 10 BIAS frames e 6 science exposures in each of the 2 filters for each object 4 1 Choosing Your Objects As a first exercise choose yourselves the two star clusters one open cluster and one glob ular cluster to observe from the short list provided below Your choice should base on the visibility of the objects during the night of observations i e their tracks across the sky It would be desirable to have the object as high in al
36. e overwritten A 4 2 Superflatting This step superflats your data by dividing each frame by the illumination correction How to redo the task Delete the OFCS images in SCIENCE and move back the images in SCIENCE OFC_IMAGES A 4 3 Defringing If you want to defringe your data you must have superflatted your data previously by a smoothed SUPERFLAT the illumination correction This task scales the fringing models according to the sky background of your exposures in relation to the SUPERFLAT and then subtracts the rescaled fringe model 47 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Superflatting Weighting Astrom Photom Coaddition Weighting cosmetics and target separation SCIENCE OFFTARGET Debloom images Saturation threshold 55000 BESAS Start Defaults Abort THELI messages Create binned mosaics Binning Ja E DS9 x Create WEIGHTS FT Distribute target sets Minimum overlap 1000 Commands that will be executed you have the last word Atransform_ds9_reg sh home mischa DATA NGC1234 Jparallel_manager sh create_weights_para sh home mischa DATA NGC1234 Mem 334 MB Disk 81 14866 MEIEN Figure 30 The Weighting PG How to redo the task Delete the OFCSF images in SCIENCE and move back the images in SCIENCE OFCS_IMAGES A 5 The W
37. eaningful default values If the LOG entered does not yet exist it is created at this moment It is automatically updated or created if not yet existing if you switch to any other PG or shut down the GUI The LOG will contain all reduction steps parameter and GUI settings you have done or chosen for a particular data set Choose a new LOG name if you reduce a different data set If you leave this field empty and start processing anyway the LOG will be named noname Whenever you launch THELI it will read the LOG that was used last updates all GUI elements and internal variables correspondingly and switches to the PG that was active when you closed your last THELI session You can continue with your reduction at the point where you left it the last time LOGs are usually stored in qt and linked to theli reduction_logs The previ ous path may vary depending on your Qt installation You will never have to touch a LOG file apart from loading an old one into the GUI 42 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Clear Clears the processing status in the LOG and resets all GUI elements to default values This is useful if you want to start again with your data reduction from the very beginning or start with an entirely new data set A 1 2 Data directories The LineEdit fields collected in this GroupBox tell THELI where the data is that you want to process You specify the main directory path hereafter maindir that co
38. en clusters form in the disk as this is the only place where enough material can assemble to induce star formation After birth the newly formed cluster still moves around the galactic centre in the plane of the disk just as the birth cloud has done before e In contrast the majority of globular clusters may have formed while larger structures as the Milky Way have not existed yet hence do not have to share the rotation of the final galactic disk of their host galaxy Some of the globular clusters may have even ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Abundance No of stars Diameter pc Age yr Open clusters 104 10 104 10 10 10 10 Globular clusters 10 10 10 10 10 1010 Table 1 Rough overview of the basic paramters of open and globular clusters Abundance gives the estimated number of clusters in the Milky Way been captured by their host galaxy during merger events for further details see for example Binney amp Merrifield 1998 The modern picture interprets both families of clusters as the low mass and high mass part of the same initial cluster mass function ICMF which similar to the IMF is a power law Observations of star burst galaxies like the Antennae galaxies Fig 5 show that in one star formation event clusters of all masses are formed of course depending on the available material and the local star formation rate SFR That is globular clusters like objects can only
39. entres of globular http www astromatic net software sextractor 32 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Coadditions ELum Convolution Color calibration FITS gt gt TIFF Movie SCIENCE dir Select coadditions coadd_blue Get coadded images fain 34_1 akuepper data M92 M92_all Remove image s from list Restore all images Crop maximum overlap Statistics region xmin xmax zz ymin ymax E Figure 20 The create color picture window of THELI clusters but will suffice for our purposes Here we will extract the sources with a small home made shell script that makes use of the SExtractor routines Therefore create a folder e g XunShi_AndreasK pper_20091224 M92_CATALOGUE and copy the following files into this directory e get_catalogue sh e get_catalogue assoc e get_catalogue makessc the cropped images the cropped weights Run the script by typing the following command in a terminal window gt get_catalogue PATH B V R where PATH is the full path of the current folder B is the name of the blue cropped im age V is the green cropped image and R is the third in case you have made observations in a third filter Otherwise simply repeat argument V in this way you will get an R column in the catalogue which is redundant since it repeats the magnitude of the V band The script will create source catalogues just as
40. f the sky in all sky catalogues like the Sloan Digital Sky Survey or 2MASS if ALADIN is not installed on your computer SKYCAT can do the same see next section Open a terminal window and type gt aladin and the ALADIN window will show up Fig 10 ALADIN enables you to access a variety of survey data For our purposes an all sky catalogue with a good coverage of the northern hemisphere is recommended To select for example one of the servers of the Digitized Sky Survey DSS go to File Load astronomical image DSS and choose one of the available locations The server selector window will show up Fig 10 Enter the name of your target into the appropriate box e g M92 and select a survey from the Sky Survey drop down menu e g DSS2 blue Also make sure that the requested field of view is at least as large as the FOV of your telescope After pushing SUBMIT the data will be received from the server and displayed in the main window Additionally the coordinates of the object will be given at the top of the window 23 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS SAO Image dso JBG Fle Edit View Frame Bin Zoom Scale Color Region WCS Analysis Hep File blue_cropped fits Object Me edt view frame zoom scale color region wes help exit about oper saveimage header page setup print Figure 11 Main windows of SKYCAT left and DS9 right 5 2 SKYCAT DS9
41. form in rich molecular clouds with a high SFR conditions which especially existed in the very early Universe or during the merging of two spiral galaxies Kroupa amp Boily 2002 Today in the Milky Way as in most spiral galaxies star formation is restricted to the Galactic disk since this is the only place where enough gas can accumulate to induce gravitational collapse The Milky Way exhibits 150 to 200 globular clusters with typically 10 stars Open clusters are more frequent as they are produced continuously so there are about 1000 known in the Milky Way But since they all lie in the Galactic plane most of them are heavily obscured by dust The total number of open clusters in the Milky Way therefore is supposed to be about 20000 Probably the most famous open cluster of the Milky Way is the Pleiades Cluster which is a typical galactic cluster with about 1000 stars a diameter of approximately 10 pc and an age of about 125 million years The Pleiades can be well seen with the naked eye due its small distance of roughly 135 pc and the bright heavy stars in its centre see Fig 4 By far the most massive cluster of the Milky Way is w Centauri Fig 6 with more than 10 million stars and a diameter of about 50 pc With its enormous mass w Centauri represents the upper mass limit of a star cluster therefore may also be classified as an ultra compact dwarf galaxy or the nucleus of a stripped dwarf spheroidal galaxy e g Fellhauer amp Kro
42. gh the appendix first Before you start working on your data make sure you have made a backup of the whole data set Also ensure that all files are stored in appropriate folders as described in Sec 4 Inttp www astro uni bonn de mischa theli html 24 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Superfiatting Weighting Astrom Photom Coaddition AAA A __E _ e Select Instrument a t LOG file laad 4 Create new instrument EL i oad of CPUs fh Refresh instrument list m92_new_b Reset gt sl x GUI style 2 6 El INGRID WHT ISAACOVLT LAICA_2x2 CAHA LAICA CAHA LBC_BLUE LBT LBC_RED LBT LDSS3 LCO LIRIS_POL WHT LIRIS WHT MEGAPRIME CFHT MOIRCS SUBARU MOSAIC1_0 9mM KPNO MOSAIC1_4m KPNO MOSAIC I KPNO MOSCA_2x2 NOT NACOSDI VLT NICS TNG NOTcam_highres NOT NOTcam_lowres NOT OASIS4x4 WHT OASIS WHT PFC WHT QHY8 SOFI NTT ST10_AO7ONEWTON ST1O FFC Astrometric reference catalog optional i fo 4 Ran NFRAMES Clear dirs A ath i Y 2 PR ALL Bias m Restore ORIG_ 15 _ Restore ORIG Flat ar Restore ORIG_ Science DD M92B _Restoreonic Offtarget a Restore ORIG 4 Fully process OFFTARGET data as well ST10_RED NEWTON ST2000XM ST8 FFC STLI1K TAK SuprimeCam_2001010 lt h4 E ES Figure 12 The i
43. has a shot noise Note that the level of dark current is strongly dependent on the temperature of the CCD chip and the length of the exposure For a liquid nitrogen cooled CCD camera the dark current can be neglected for many observational purposes But in this lab course it has to 17 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS be taken into account Thus you should also make sure that the CCD temperature and exposure time for the DARK frames match those of the light science frames 3 4 3 Flat fielding You also need to take some FLAT frames by making exposures towards a uniformly illu minated background e g twilight sky or a carefully constructed and illuminated dome flat field screen currently we use the former However your obtained image will be far from uniform On your FLAT frame you will probably see donuts which represent dust grains somewhere within the light path and other variations of light due to the telescope optics Also the response of the CCD is not exactly the same from pixel to pixel such pixel wise variation is also recorded in a FLAT frame The exposure time for the FLAT frames should be determined such that the peak brightness level of a FLAT frame is 1 2 or 2 3 of the saturation level saturation full well ine ADU gain And for each filter used for sci ence exposures a new FLAT frame should be made This is because the pixel response to incoming light is wave length dependent During da
44. ilar diagram the temperature luminosity or color magnitude diagram Fig 1 The advantage of the latter lies in the way the corresponding quantities can be obtained While it is rather hard to determine the spectral class of a star it is much easier to obtain its temperature in means of a color index So just by taking pictures of a star cluster with two different color filters the temper ature color and the apparent luminosity magnitude can be derived and a color magnitude diagram CMD can be drawn How this is done will be subject of this lab course project The evolution of stars within a CMD has been widely studied and is a major subject of every basic astronomy training The most common classifications of stellar objects like main sequence star red giant white dwarf etc are derived from this type of diagram and are taken to be well known by the students carrying out this lab course project An ad vanced overview on stellar evolution can be found in Binney amp Merrifield 1998 p 258 or more detailed in de Boer amp Seggewiss 2008 2 4 Colors amp Magnitudes To understand the various flux measurements we make with our telescope setting we have to clearify first which quantities exactly we measure and how we can derive physical quantities out of them Here we introduce you to the basic principles you need for carrying out this project a more complete picture is given in Binney amp Merrifield 1998 p 26 2 4 1 Appare
45. inearity linear means the increase in measured signal is propo tional to the increase in the incoming photon flux It is a desired property of the detector since it enables a direct measurement of the incoming photon flux A CCD detector has good linearity over almost the entire dynamical range but only over the dynamical range When the detector reaches the saturation level it is no longer possible to derive the exact number of photons that reached the detector originally Therefore it is very important not to saturate the objects of interest by a too long exposure time For bright stars a few sec onds exposure suffices to reach saturation On the other hand one needs a long exposure to detect weak sources And the longer the exposure time T is the larger the signal to noise ratio remember that S N is proportional to 1 T The selection of exposure time is thus dependent on the specific scientific goal of the observation Only if a large number of objects of interest is not saturated photometric analysis and calibration are possible Thus better make many shorter exposures of your object and co add the images see next section T3 4 Fora standard star of magnitude m 3 0 you get 30000 counts after an exposure time of t 10s How long do you have to make an exposure for a star of magnitude ma 5 0 to get the same number of counts 16 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 3 3 4 Cosmics and dithering Two mor
46. ion due to the Earth s at mosphere T The amount of extinction furthermore depends on the column density of air along the line of sight and increases with lower observing angles Therefore the best possible observing conditions are achieved near the zenith where the length of the light path through the atmosphere is minimal Extinction and disturbing seeing effects get worse for observations at lower elevations These effects are often expressed in terms of the ob jects airmass a which tells you through how much atmosphere column density the light travels compared to vertical in fall For an angular distance z from the zenith it can in good approximation be computed as a 1 cos z such that a 1 for an object at the zenith and formally a inf at the horizon The filter transmission F is readily determined for well defined filters like the Johnson filters which will be used in this lab course Sec 2 4 5 The instrumental efficiency R is a composite of the efficiency of the telescope s optical system and the sensitivity of the CCD In order to get well defined magnitudes of star cluster members for a color magnitude di agram we can correct the observed fluxes of the stars by observing a reference star which is nearby the cluster and whose magnitudes in different frequency ranges filters are well known since the observing conditions air mass filter telescope optics CCD for such a reference star are approximately the same as for the
47. ith metallicity Based on this part of the main sequence we can estimate the distance without regard to the age and metallicity of the cluster Therefore take the youngest isochrone of one of the isochrone directories e g isochrones geneva 008 iso_c008_0600 UBVRI JHKLM and plot column 9 B V versus column 7 absolute V magnitude into your open cluster CMD i e gt plot result_ascii cat using 3 0 3 4 0 1 4 0 1 SCPHOT geneva 008 iso_c008_0600 UBVRIJHKLM using 9 7 with line Apply a constant shift to the V magnitudes such that the high mass part of the main sequence fits the observations From this shift you can calculate the distance d by using d 1002m M 5 pc 12 37 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 6 3 2 Position of the Horizontal Branch Old globular clusters do not show an extended main sequence Fig 22 But as stellar populations evolve massive stars move onto the horizontal branch where they burn helium in the core The absolute magnitude at which they shine in this stage is well known and can be used to determine the distance to old star clusters My of horizontal branch stars is approximately 0 5 mag Thus identify the horizontal branch in your CMD shift your V magnitudes accordingly and apply equation 12 6 4 Isochrone Fitting After we have produced a well calibrated CMD we can fit a theoretical isochrone to the data and see which parameters reproduce the cluster best Fo
48. itudes in your catalogue are not calibrated yet This has to be done by hand by comparing the measured magnitudes of a few stars with reference magnitudes of on line archive data and applying a correction constant to each magnitude You can do that for example with gt plot result_ascii cat using 3 0 3 4 0 1 4 0 1 For the calibration of the magnitudes you will either use the magnitudes of the reference stars which you have observed seperately or your tutor will provide you with the reference magnitudes of some non variable stars in your field of view For the latter identify the reference stars in your catalogue by their right ascension and declination and calculate the correction constant for each filter If the quality of your data does not allow for this method your tutor will provide you with a literature CMD of this specific cluster such that you can shift your magnitudes accordingly Now you can draw a calibrated CMD What is the limiting magnitude of your observations 6 2 Extinction Extinction by interstellar dust makes stars appear dimmer than they are This can be corrected for by adding a constant factor to each magnitude In principle these factors can be determined through a color color diagram in which two color indices are plotted versus each other But therefore you need reliable images in three colors and sophisticated analyses This is hardly possible with the data taken from Bonn due to light pollution thus we
49. maller than the field of view of the detector How to redo the task Just rerun Old results will be overwritten A 7 2 Coaddition The coaddition goes in three steps First global information about the data set is obtained and the reduction settings are made accordingly Then the SCIENCE images and their associated WEIGHTs are resampled Lastly the resampled images are combined 52 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Superflatting Weighting Astrom Photom Coaddition Sky subtraction and coaddition SCIENCE OFFTARGET X Sky subtraction Clear scripts Start Defaults Abort SExtractor catalog THELI messages in this directory full path l Resolve links Commands that will be executed you have the last word parallel_manager sh create_skysub_para sh home mischa DATA NGC1234 sub Jprepare_coadd_swarp sh m home mischa DATA s NGC1234 e sub w sub eh parallel_manager sh resample_coadd_swarp_para sh home mischa DATA NGC1234 sub home mischa THELIAhelilguiiscripts Jperform_coadd_swarp sh home mischa DATA NGC 1234 ceso MEET Disk 81 14866 MBER Figure 34 The Coaddition PG A 8 Guidelines for observers using THELI Do this e When acquiring your data orient the camera such that its axes are parallel t
50. matically be stored in the directory you specified The name of this file will include the name filter if such a filter was put If the table is obtained a second time with an identical filter the old file will be overwritten with out warning You can manually save the table to a different file name or load a previously created table If the Create source cat and Astrometry processing steps were done as well seeing and relative photometric zeropoints will be shown as well respectively That requires the pres ence of a cat and or a headers directory in the specified path 56 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS References Adams F C Myers P C 2001 ApJ 553 744 Allen L et al 2007 prpl conf 361 Alpher R A Bethe H Gamow G 1948 PhysRev 73 803 Binney J Merrifield M 1998 Galactic Astronomy Princeton University Press NJ Boily C M Kroupa P 2003a MNRAS 338 665 Boily C M Kroupa P 2003b MNRAS 338 673 de Boer K S Seggewiss W 2008 Stars and Stellar Evolution EDP Sciences France Fellhauer M Kroupa P 2003 Ap amp SS 284 643 Fukugita M Ichikawa T Gunn J E Doi M 1996 AJ 111 1748 Hilker M Kayser A Richtler T Willemsen P 2004 A amp A 422 L9 Johnson H L Morgan W W 1953 ApJ 117 313 King I R 1962 AJ 67 471 Kroupa P 1995 MNRAS 277 1491 Kroupa P 2001 MNRAS 322 231 Kroupa P 2005 Nature 434 148 Kroupa P Boily C M 200
51. me mischa DATA FLAT Jprocess_split WFI MPGESO_eclipse sh home mischa DATA NGC1234 MPGESO MCU EEN Figure 26 The Preparation PG A 3 2 Process flats MANDATORY Combines all FLATs in the FLAT subdirectory Again you can override this by entering a different subdirectory A FLAT correction is very useful even if the camera appears to be illuminated very homogeneously This is because the FLAT does not only correct for vignetting effects but also for different sensitivities on a pixel to pixel basis The more FLATs you have the better the master FLAT will be and the smaller the calibration noise you introduce into your SCIENCE images We recommend at least 10 FLAT exposures The FLAT exposures are debiased overscan corrected and scaled to the highest mode in the stack before combination How to redo the task Delete the FLAT_i fits images from the FLAT dir and rerun They have to be deleted because otherwise they are taken as an input image A 3 3 Calibrate data Images are overscan corrected debiased and flat fielded If suitable a SUPERFLAT is cal culated from the data as well 44 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Supertiatting Weighting Astrom Photom Coaddition Debiasing flatfielding creation of superflat SCIENCE OFFTARGET Configure FT Do not appl
52. ndard star if not in the same frame with the object 6 BIAS and DARK frames 10 each They can be taken at any time during the night The exposure time of DARK frames should be the same with the science frames Note depending on the telescope software it may be possible to automatically take DARK and BIAS frames with each exposure and automatically subtract them from the science frames This may be convenient but is not always recommendable 21 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 4 3 Data storage Since you will take a number of frames during the night it is recommended to create an appropriate tree folder on the local hard drive of the telescope beforehand The data reduc tion software THELI requires a certain folder structure which you should stick to from the very beginning to avoid confusion This structure looks as follows The top folder name should have your names and the date of observations in it e g XunShi_AndreasK pper_20091224 There should be a separate folder for any BIAS frames e g XunShi_AndreasK pper_20091224 BIAS If you take DARK frames then you also need a separate folder e g XunShi_AndreasK pper_20091224 DARK Each set of FLAT frames for a specific filter should have a separate folder e g XunShi_AndreasK pper_20091224 FLAT_B XunShi_AndreasK pper_20091224 FLAT_V Each set of science frames for a specific object and filter gets its own folder e g XunShi_AndreasKiipper_20091224
53. nitialise window of THELI Throughout the reduction we recommend to follow the processing of your files by checking the specific folder content after each reduction step and following the renaming of the files This might be very helpful in case a reduction step fails or has to be redone or undone Note that THELI is not fail safe and needs the caution of the user Note also that not all reduction steps have to be repeated for each filter e g BIAS processing and might even lead to errors when repeated Start THELI by typing gt theli in a terminal window and the main window will show up which has seven panels which have to be gone through from left to right Thus in the following sections we will go through each of the seven panels in the correct order 25 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS File Edit Settings View Reduction Miscellaneous Delete Help Initialise Preparation Calibration Supertiatting Weighting Astrom Photom Coaddition m Sorting splitting linking data Is in the main directory _ Sort data using FITS key Configure Clear scripts Start data is in the individual directories Split FITS correct header _ Create links Chips going to scratch Scratch directory Abort Configure Figure 13 The preparation window of THELI 5 3 1 Initialise In the initialise window of THELI enter an appropriate name for the
54. nsional distortion polynomials of higher order There is no difference in running them on a single chip or on a multi chip camera In the latter case the solution is for the entire mosaic The results of the astrometry step regardless of the method chosen will be written to a headers subdirectory inside SCIENCE Configuration We recommend to use Scamp for this experiment You can run it in its default configuration with one small modification the degree of the distortion polynomial should be set to 2 instead of 3 as there are not many sources in our data that can be used for distortion correction All other parameters can be left unchanged How to redo the task Just rerun Old results in SCIENCE headers will be overwritten A 7 The COADDITION processing group A 7 1 Sky subtraction Configuration This step subtracts objects above certain user provided thresholds from the image From the remaining sky an estimate is determined The Configure dialog Fig 35 presents you with the following options lab course here we present only the option we are going to use Automatic sky modelling In a first pass SExtractor is run to remove all objects from the image The result is then smoothed and subtracted To this end you must provide the usual detection threshold minimum number of connected pixels and the extent pixels of the smoothing kernel This is the default method and useful for all exposures where the largest object is significantly s
55. nt Magnitude The apparent magnitude m is a measure of a star s brightness as seen by an observer on Earth In other words it is the integrated radiation flux f measured in W m contained in a particular frequency range Av Measuring fluxes of celestial bodies in different frequency ranges is called astronomical photometry which is the main objective of this lab course project Based on a magnitude system first introduced by ancient Greek astronomers the apparent magnitude is a comparative scale in which brighter stars are given smaller magnitudes than fainter stars such that m1 ma 2 5logig 2 4 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS where the indices denote the magnitudes and fluxes of two distinct stars But there is extinction of radiation through intergalactic gas interstellar gas and of course through the earth s atmosphere In addition there s also flux getting lost in our telescope Thus the flux which reaches the solar system is not the flux which was emitted by the source and the flux received by our detector f is not the flux f that reaches the solar system While former varies from source to source the latter is fixed and can be quantified for any instrument We measure FE J AT 5 where T is the transmission of the atmosphere F is the transmission of any applied filter and R is the efficiency of the telescope system Depending on the frequency range there is significant extinct
56. ntains everything followed by the names of the subdirectories that contain the BIASes FLATs etc Behind each subdirectory you will find the number of images that was found in the corresponding place Note that these numbers may not correspond to the actual number of images in the corresponding directory due to an un solvable bug in THELI All fields do not accept blank characters as input in addition the subdirectories do not accept a slash You only need to specify those subdirectories which you actually need Restore ORIG This deletes all data in the corresponding directory apart from the very raw data that has been moved into the ORIGINALS subdirectory The ORIGINALS data is played back thus restoring the original state before any processing has been launched If no ORIGINALS subdirectory is present nothing will be deleted Clear dirs Clears all LineEdits A 2 The PREPARATION processing group A 2 1 Split FITS correct header The main job of this task is to split multi extension FITS files into single chips thus allowing for parallel processing It also writes a new FITS header conformed with the THELI pipeline If single chip images are given only the FITS header will be updated This reduction step will be applied automatically to all subdirectories that are specified in the Initialise PG How to redo the task Delete all split images in the corresponding directories and move back the images from the ORIGINALS directory
57. nto the sky coordinates is called astrometric calibration This can also be done by softwares with the help of a reference catalogue 18 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 3 4 6 Sky subtraction During exposures the CCD not only collects light from your target of interest but also receives radiation from the background sky In addition there will be ADU counts from unresolved objects and glow from objects not even in your field of view e g a bright star nearby or the moon By determining this background level and removing it from your image only the source flux will remain The usual way of modelling the background is to first remove all objects in your frame and then smooth the image with a specific kernel width Then this background image can be subtracted from the original frame 3 4 7 Co adding By stacking all science images into one and making sure that each object falls onto the same pixel the final image will have a higher S N value than each image alone This process is called co adding 3 5 Photometric calibration reference stars For the same star on the sky observations through different telescopes under different weather conditions will observe different fluxes Converted to magnitudes you get a partic ular instrumental magnitude which does not directly reflect the true magnitude of the star The most simplistic ansatz is to assume you only have an offset Z between your observed magnitudes and the tru
58. o the right ascension and declination axes THELI prefers North up and East left but will handle any orientation of 0 90 180 and 270 degrees including flips The coadded image will always exactly have North up and East left e When planning your observations make sure that you dither your exposures apply small telescope offsets after each image Do not do this e Do not reduce different data sets in parallel especially from different filters This leads only to confusion and you might use up your disk space sooner Finish one reduction and then go to the next data set e You currently can not run two instances of THELI at the same time since both of them will overwrite each others parameter and configuration files You can do this 53 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS No sky substraction Manual override individual exposures E f Automatic sky modelling Constant sky estimate manual fine tuning Constant sky estimate resist EN sa Show mosaics only Box from which the estimate is taken xminjxmax ff ymin ymax Mode From each chip Figure 35 The configuration window for manual sky subtraction safely if you have two separate installations in two different paths owned by two different users and having THELI launched by these two users A 9 FAQ and Troubleshooting General Q I want to activate the CheckBox of some reduction task but the acco
59. ound likely to collapse and form a new star system picture from the FORS Team with the 8 2 meter VLT ESO Star clusters play a key role in the development of our understanding of the Universe But what exactly is a star cluster In principle any agglomeration of more than a few stars may be called like this where a few is not well specified and may be taken to be around ten In terms of star cluster dynamics since the constituent stars mutually attract each other by the force of gravity a star cluster may also be defined as a dynamically bound system of a number of stars The fundamental property of a star cluster is its origin which is assumed to be a single giant molecular cloud for all members of one cluster Fig 2 The according formation scenario of star clusters is quite well understood nowadays a collapsing cloud fragments into small clumps which form the progenitors of the cluster stars Depending on the size of the molecular cloud and on the conditions of its collapse a fraction of about 10 30 of the gas is consumed by star formation Adams amp Myers 2001 Lada amp Lada 2003 Allen et al 2007 The masses of the stars produced thereby follow a more or less universal distribution function which is a quite simple power law the so called initial mass function e g Salpeter 1955 Kroupa 2001 An important feature of the initial mass function IMF is that it predicts a large number of low mass and just a few very massive
60. our folder with the coadded images created in the previous reduction step and hit Get coadded images Remove all images from the list which are not necessary and crop the images by pushing the Crop maximum overlap button If this step was successful your are done with the data reduction with THELI The next step will be to extract the final source catalog from the coadded and cropped images This will be done with SExtractor in the next section But before proceeding with the next step you can create a color picture of your object if you have managed to take images in three filters Therefore go to the Color calibration tab of the Create color picture window and select the three images in the drop down menus at the top Fig 21 First try to let THELI create the color picture automatically by pushing Calibrate and then Preview If the colors of the final picture are not according to your taste hit the Reset button and try to adjust the weighting factors for the three colors by hand The result will be stored in preview tif in your current working directory 5 4 SExtractor The final extraction of the sources will be done using SExtractor a program that builds a catalogue of objects from an astronomical image Although it is particularly oriented towards reduction of large scale galaxy survey data it can perform reasonably well on mod erately crowded star fields Thus it may lead to bad results in the c
61. pho tometry select Scamp from the drop down menu and enter the Configure menu In the astrometry configuration window Fig 17 and try the following values POSAN GLE_MAXERR 5 POSITION_MAXERR 5 DISTORT_DEGREES 1 Press Ok to exit this window and start the process If THELI finds no astrometric solution try creating a new source catalog with different parameters as stated above or change the two MAXERR parameters to about 10 5 3 7 Coaddition In the coaddition window Fig 18 first start the sky subtraction by checking the corre sponding box and pushing the Start button Then check the Coaddition box and enter the Configure menu In the configuration window Fig 19 enter the coordinates which you also entered in the previous reduction steps Furthermore enter an identification string into the specifc box The name should be clear e g blue green red or B V R Further more enter the pixel scale of the CCD i e 0 4 in some cases the quality of the extracted 30 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS THELI v 1 6 1 0 File Edit Settings View Reduction Miscellaneous Delete Help B o x Initialise Preparation Calibration Superfiatting Weighting Astrom Photom Coaddition SCIENCE OFFTARGET m Sky subtraction and coaddition Clear scripts Start Abort r Messages _ Resolve links in this directory full path m Commands
62. r this task we use the Geneva database of isochrones Lejeune amp Schaerer 2001 named after the university of origin Note that these isochrones are based on simplified assumptions and that they obey an in trinsic uncertainty There are other databases of isochrones coming e g from Padova Nasi et al 2008 which in some cases differ substantially from the Geneva models For simplicity we stick to the Geneva database here The available set of isochrones covers metallicities of Z 0 001 0 004 0 008 0 020 0 040 0 1 and ages of 10 yr to 15 Gyr in a reasonable step width The data is stored in the folder geneva This folder contains subfolders for each metallicity and in every subfolder are sep arate files for each age For example the file geneva 001 iso_c001_0950 UBVRIJHKLM contains the isochrone for Z 0 001 and for an age of 10 yr The columns in the files are always the same Use column 9 for B V and column 7 for the absolute V magnitude For each cluster go through the data set and compare the theoretical isochrones with your data For young clusters you should stick to high metallicities whereas for old clusters you should apply low metallicities When you have a good fit you can change the output to a file instead of the screen and give an output name Afterwards you must change to the screen again though gt set terminal eps postscript enhanced gt set output figurel eps gt plot result_ascii cat u
63. rature about 30 C with closed cycle systems or by liquid nitrogen CCD temperature can reach 100 C The CCD camera in use for the lab course is of type SBIG STL 6303E which has 3072 x 2048 pixels where the size of each pixel is 9 um x 9 um It has a Full Well Capacity of 100000 e ie in each pixel it can store 100000 electrons and an ADU Analog To Digital Converter Unit gain of 1 4 e ADU It can be thermo electrically cooled to about 30 below room temperature T3 2 Calculate the field of view FOV of the telescope and the theoretical angular res olution What limits the angular resolution during your observations 3 3 Observational conditions and requirements There are some undesirable yet existing effects which will hinder you from getting good data if you don t take care of them correctly Here we list those of them which will be encountered in this lab course The treatment of these effects during data reduction will be done rather automatically using sophisticated software 3 3 1 Seeing The light distribution of a point source e g a star on the image plane is called point spread function PSF In the idealised case a PSF is determined by the telescope aperture With a normal circular aperture with diameter D it is an Airy disc with an angular resolution of A A9 1 227 9 where A is the wave length of incoming light However when the blurring effect of earth s atomosphere is taken into account the a
64. rding command does not appear in the command window What am I doing wrong A You forgot to provide either a necessary parameter look for a red LineEdit field or you did not specify all necessary directories in the Initialise PG For example you might have forgotten or misspelled the BIAS directory Whenever a command does not appear then information is missing Q Bother The GUI crashed hung up while a reduction task was running What do I have to do now A Probably NOTHING apart from relaunch The GUI will come up in the same configu ration it had when it died Besides the GUI is just a graphical engine it does nothing else than launching reduction processes Those are running stand alone without supervision by the GUI If the GUI crashes then this has no effect on the job running Just see top to learn when the job has finished then bring up a new instance of the GUI Besides a crash of the GUI should happen very rarely only 54 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Image directory Get statistics Name filter Load table Image section D o Mean rms Mode Seeing Clear Show Median rel phot zeropoint Lower quartile Close Statistics fmegename Mean CEI CEI CEEI CCA CT CS Figure 36 The dialogue for obtaining image statistics Calibration Q During the pre processing steps in the Calibration and Superflatting PGs my computer gets horribly slow and
65. redo the task Old results will be overwritten when a new reference catalogue is retrieved A 6 2 Create source cat Here we extract source catalogues from all images The detection characteristics can be fine tuned by means of the configuration dialogue Fig 32 Parameter configuration DETECT_THRESH is the detection threshold in sigma of the sky background noise and DETECT_MINAREA is the minimum number of connected pixels above that threshold The latter depends on the seeing and the pixel scale of your instrument Three more parameters that usually do not need to be modified are available too Fig 32 If you leave a field empty then the default value will be used No warning message will be printed General tips for the catalogue creation The WEIGHT maps created previously are taken into account in the catalogue creation process guaranteeing a clean catalogue that is largely free from spurious detections 50 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Processing group Astrom Photom 2 Astro photometry Astrometrix POSANGLE MAXERR deg 2 0 POSITION _MAXERR arcmin 2 0 DISTORT DEGREES 2 SN_THRESHOLDS low high 5 20 FGROUP_RADIUS deg 1 0 CROSSID_RADIUS arcsec 2 0 ASTRINSTRU_KEY Po FILTER PHOTINSTRU_KEY Po FILTER STABILITY_TYPE INSTRUMENT 4 MOSAIC_TYPE UNCHANGED which FOCAL_PLANE Use default FP lt lt gt gt Defaults this page Cancel Defaults all pages
66. s mass an increase in Z only causes a decrease in a star s tem perature For a single star this metallicity effect can only be taken into account by taking deep spectra and fitting theoretical models of stellar atmospheres to the observed spectra Since all stars in a star cluster have the same Z the metallicity of a cluster can be determined from a CMD by fitting various stellar evolution models with a range of metallicities to the data and using the fact of the mass dependent reddening and dimming of stars with increasing metallicity Unfortunately the precission of the underlying data has to be very high for this method Due to the rather bad seeing conditions in Bonn we will refer in this experiment to literature values T2 7 Draw two schematic CMDs in one plot one of a globular cluster with an age of 11 Gyr and a metallicity of Z 0 001 and the other of an open cluster with an age of 100 Myr and Z 0 02 Include the main sequence the giant branch and the horizontal branch if applicable in your sketch 2 4 5 Johnson Filter System The most commonly used photometric system is the UBV system based on the work by Johnson amp Morgan 1953 The acronym stands for ultraviolet blue and visual and denotes the wavelength coverage of the three most often used filters Meanwhile this system has been extended to the infrared with the filters R red I infrared and then J H K L and M for an overview of the corresponding wavelengths
67. sing 3 4 4 SCPHOT geneva 008 using 9 7 with line gt set terminal xil Our aim is it to understand anything in our CMD since there should be an explaination for any position of any data point in the graph Therefore answer the following questions e What age has the specific cluster e How large are the uncertainties in this determination How can you estimate reason able uncertainties e What influences the width of the main sequence i e why is the main sequence not as thin as the isochrone predicts Think about how the isochrone is produced and what is assumed in this process 38 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS e Can you identify stars that lie beyond the turn off point i e are bluer than the turn off point but still appear to lie on the main sequence How can this happen e How many stars can you identify in this way to appear abnormal in age color and what does it tell you about the stellar population of this cluster Do this for all clusters for which you have taken data and for each additional catalogue which your tutor hands out to you 39 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS THELI v 1 0 5 g Pipeline settings Select Instrument test Apply Delete Create new instrument Current LOG file Open amp apply existing LOG Save current status Refresh instrument list Clear processing status Create splitting script EFOSC2 3 6m EMM
68. stage and you do not have to worry too much about it A good starting point for optical data is 1 0 5 250 depending on the flatness of the image and the detector size If your images exhibit strong fringing then you can no longer use very low detection thresh olds since then the fringes themselves are detected as objects and thus removed from the SUPERFLAT In this case one can no longer calculate a fringing model from the SUPER FLAT We recommend to use a high S N threshold if strong fringing is present Try starting with 5 0 5 250 in this case With near IR detectors DT and DMIN often must be increased to 10 in order to not mask features in the very inhomogeneous sky background If one or 45 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Processing group Calibration Rejecting nlow and nhigh pixels from a stack nlow nhigh Overscan lo Poo 1 Bias Dark i a Flat i 1 i 1 Superflat Calibrate data Superflat parameters DT DMIN SIZE 2 Subtract r Window size Defaults this page Cance Defaults all pages KO A Figure 28 Configuration for the outlier rejection during image stacking and the creation of aSUPERFLAT more of those three LineEdits is left empty then the default values will be used without warning You can choose between a median and a mean combination for the SUPERFLAT The median delivers a more stable result for a small number of stacked images whereas the
69. ta reduction a FLAT frame will be normalized to an average value of 1 and used to divide the science image T3 5 What can you infer from the sizes of the donuts T3 6 How many counts do you expect your FLAT frames to have 3 4 4 Masking weighting When one pixel on your CCD is more sensitive to another one which always happens you would like to trust the sensitive one more since it gives you data with higher S N This can be done by assign individual weights for every pixel a process called weighting The weighting factors can directly be taken from the normalised FLAT Weighting is also neccessary when you try to co add frames see co adding your object on each frame may be at different positions usually the case see dithering when co adding them a proper weighting ensures maximised S N in the final image For those bad pixels columns on the CCD chip and cosmic rays a simple way of treat ment is to mask them using softwares and assign them less weight There are two types of masks global and individual One global mask is made for a particular CCD detector and can be applied to all images produced by it One can also create an individual mask for each image counting also for cosmic rays 3 4 5 Astrometric calibration Your obtained image is a telescope configuration dependent projection of a curved sky Thus the pixel detector coordinates and the sky coordinates do not have a simple relation To project your image back o
70. ters and the color magnitude diagram Section 3 introduces the basic concepts of astronomical observations as well as the techniques and instruments you will use in this project Section 4 covers the observations you will take out how you prepare them properly and how to carry them out such that you can get valueable data Section 5 is about the data reduction of your images and how you extract the infor mation you need from the images Section 6 is the main scientific part where you will be guided through the analysis of your data The appendix will help you with the THELI software package which you will use during this project Please read all sections except the appendices carefully before you start Make sure you answered all questions in the text since you are not allowed to start the project before answering all questions ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 2 Basic Knowledge of Star Clusters and CMDs 2 1 Star Clusters Figure 2 Picture of the giant molecular cloud Barnard 68 which is relatively nearby with a distance of about 200 pc and a diameter of about 0 2 pc The cloud can only be seen indirectly in optical wavelengths as it hardly emits light whereas it absorbs all light coming from background sources It is not known exactly how molecular clouds like Barnard 68 form but it is known that these clouds are themselves likely places for new stars to form In fact Barnard 68 itself has recently been f
71. ters of star clusters Credit Brad Whitmore STScI and NASA be called associations than clusters up to 10 Mopc Globular clusters on the other hand are rich clusters with 10 to 10 stars and diame ters of 20 to 150 pc Unlike open clusters which are often asymmetric and less centrally concentrated these systems are quite smooth and spherical They furthermore show a very high concentration in the core which extends from 0 3 up to 10 pc Typical densities of these core regions are 10 M pc thus lie clearly above open clusters and even represent one of the densest stellar environments in the Universe In addition to their different dimensions and shapes the two species of clusters show other fundamental differences Taking a closer look at pictures of open and globular clusters in the Milky Way immediately shows that the former in many cases have diffuse emission from gas while the later do not Also open clusters show bright blue stars which are young and massive while they are completely missing in globular clusters From the location of the turn off point in the Hertzsprung Russell diagram the age of a cluster can be derived quite accurately The ages of open clusters found in this way range from a few million years up to about 10 billion years while globular clusters may have solely formed about 11 13 billion years ago at least this is true for most of the known globular ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS
72. titude as possible during the time of observations One other factor to take into account is the position of the moon You wouldn t like to have your object to be close to a full moon since a bright moon would significantly increase the level of your sky background and thus decrease the signal to noise ratio S N of your object One convenient way to visualize the tracks of the objects during one night is to produce a visibility plot Make one yourself according to the instructions in the task below figure out its meaning and choose your objects basing on it T4 1 Generate a visibility plot for the date of your observation Procede as follows e go to http catserver ing iac es staralt e select mode Staralt Date your expected date for observation e specify the coordinate of AIFA Bonn 07 04 01 50 43 46 75 e load the file s containing the object coordinates provided by your tutor e include Moon distance in Options e generate one plot for open clusters and one for globular clusters Which direction does the peak of one object track correspond to T4 2 Pick two objects one OC and one GC according to your visibility plots 20 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Catalog Name RA DEC my dim M103 NGC 581 01 33 23 60 39 00 7 4 5 M36 NGC 1960 05 36 18 34 08 24 6 0 10 M50 NGC 2323 07 02 42 08 23 00 5 9 14 M11 NGC 6705 18 51 05 06 16 12 5 8 13 M29 NGC 6913 20 23 57 38 30 30 6
73. trinsic brightness but also on its distance d In fact the flux decreases with d7 If we want to calculate the flux F of a certain object assuming that it was at distance D we can therefore use f Pr 6 In this context the absolute magnitude M is defined as the apparent magnitude an object would have if it was located at some standard distance D where this distance is always taken to be 10 pc Using equation 4 we then get f d m M 2 5 logio 5 5logig 5 5logig d 5 7 where the quantity m M is called the distance modulus of the specific object Hence by knowing m and d we can correct the apparent magnitude for the non standard distance On the other hand if we know m and M we can infer the distance d T2 5 From the literature you know that the abolute magnitude of your object from T2 4 is M 10 mag What is the distance of the object and what is the distance modulus 2 4 3 Interstellar Extinction Absorption and scattering of photons in the interstellar medium can cause stars to appear dimmer than they actually are This effect is called interstellar extinction and has to be handled with care If we have A magnitudes of extinction then equation 7 has to be rewritten as Fortunately interstellar extinction is unlike the distance effect strongly wavelength depen dent such that it can be measured by taking images in multiple color filters For many objects reliable extinction measurements are therfore av
74. upa 2003 In addition recent observations show that it is indeed much more complex than a regular star cluster since its temperature luminosity diagram shows pronounced substruc ture i e w Centauri may consist of more than one stellar population Hilker et al 2004 T2 3 Calculate the angular size of e a typical globular cluster in the halo 50 pc diameter at a distance of 10 kpc e a typical open cluster in the disk 5 pc diameter at a distance of 1 kpc e the full Moon 107 pc diameter at a distance of 1071 kpc ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS 2 3 HRD CMD In 1913 Henry Norris Russell presented his recent work at a meeting of the Royal Astronom ical Society There he showed a diagram that represented a relation between the spectral classes and the absolute magnitudes of all stars for which fairly reliable distances had been obtained so far Russell 1913 Soon the importance of this discovery became clear to the astrophysical community and since then the so called Hertzsprung Russell diagram Ejnar Hertzsprung was the first to anticipate the existence of a relation between the two quanti ties has given a great contribution to the understanding of stellar evolution Hitherto much effort has been put into this specific type of diagram and it was found that it is possible to replace spectral class on the abscissa by temperature and absolute magnitude on the ordinate by the star s luminosity to obtain a sim
75. was done in THELI for each filter and cross reference the three catalogues Only sources which get detected in all three filters will be written to the resulting ascii catalogue which will be located in 33 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS Coadditions ELum Convolution Color calibration FITS gt gt TIFF Movie Red image red_cropped fits Green image green_cropped fits Blue image blue cropped fits Method PHOTOMETRIC CATALOG f Photometric reference catalog NOMAD ue BV 0 62 0 68 G2 star VR fos hoz defaults Factor Error Red 1 0 0 0 Blue 1 0 0 0 Calibrate Green 1 0 0 0 Reset Based upon 3 stars CE ES L Max level 5 Preview Figure 21 The create color preview window of THELI XunShi_AndreasKiipper_20091224 M92_CATALOGUE result result_ascii cat where the columns are 1 right ascension J2000 in degrees 2 declination J2000 in degrees 3 B magnitude 4 V magnitude 5 R magnitude You additionally get catalogues for each filter and the combined catalogue in an enhanced file format LDAC comparable to the fits format for images which you don t need here any more but which you should keep in mind in case you ever need to deal with more sophisticated catalogues You can quickly count the number of detected sources by typing gt wc 1 result_ascii cat within the result directory The number of detections depends on the parameters DE
76. will stick to values listed in the literature Often the extinction factor is smaller than the achievable accuracy in magnitudes which can be achieved from Bonn Thus assume an extinction factor E B V of zero if not stated otherwise by your tutor 6 3 Distance Determination The distance to star clusters can be determined in different ways e g with variable stars or supernovae With our CMD we can also derive the distance since there are stars in each cluster with well defined absolute magnitudes By comparing the observed magnitudes of those stars with their theoretical absolute magnitudes we can evaluate the distance modulus 3http www gnuplot info faq faq html http ti6web lanl gov Kawano gnuplot index e html 36 ADVANCED LAB COURSE PHOTOMETRY OF STAR CLUSTERS My Z 0 Z 0 Z 0 Z 0 N o y NNNN B V Figure 22 Isochrones for high and low metallicity stellar populations The main sequence of young clusters is almost independent of metallicity for B V lt 0 For old clusters older than 10 yr the horizontal branch is independently of metallicity located at 0 5 of the cluster But the stellar populations of open clusters and globular clusters can differ significantly such that not every method can be applied to every cluster 6 3 1 Main Sequence Fitting From Fig 22 we can see that open clusters of young age have a well defined main sequence for high mass stars which does not vary significantly w
77. y BIAS DARK E Stan Do not apply FLAT Defaults Abot min max THELI messages X Process flats M Spread sequence IR of groups length 3 12 X Calibrate data Use DARK Calculate SUPERFLAT Commands that will be executed you have the last word parallel_manager sh process_bias_eclipse_para sh home mischa DATA BIAS Jparallel_manager sh process_flat_eclipse_para sh home mischa DATA BIAS FLAT parallel_manager sh create_norm_para sh home mischa DATA FLAT parallel_manager sh process_science_para sh home mischa DATA BIAS FLAT NGC1234 WEI MPGESO Mem 326 MB Disk 81 14866 MBE A Figure 27 The Calibration PG If you decide to calculate a SUPERFLAT from this data then activate the Calculate SU PERFLAT CheckBox The parameter configuration dialogue appears Alternatively you can bring it up with the Configure PushButton Superflat parameters Here you determine how objects in the images are detected and which method is used for image combination The left field takes the detection threshold DT per pixel given in units of sigma of the sky background noise The middle field takes the minimum number of connected pixels DMIN above DT which make up an object The smaller both values are the fainter the objects you mask The right field accepts the size of the convolution kernel for the sky background Its effect is very minor at this

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