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3. 3 3 Analyzing HST Images This section describes methods for using STSDAS and IRAF to work with two dimensional image data from HST Subjects include e Relating your image to sky coordinates e Examining and manipulating your image e Working with STIS and NICMOS imsets e Converting counts to fluxes Analyzing HST Images W 3 9 3 3 1 Basic Astrometry This section describes how to determine the orientation of an HST image and the RA and Dec of any pixel or source within it including e Tasks that supply positional information about HST images e Methods for improving your absolute astrometric accuracy Positional Information The header of every calibrated HST two dimensional image contains a linear astrometric plate solution written in terms of the standard FITS astrometry header keywords CRPIX1 CRPIX2 CRVAL1 CRVAL2 and the CD matrix CD1_1 CD1_2 CD2_1 and CD2_2 IRAF STSDAS tasks can use this information to convert between pixel coordinates and RA and Dec Two simple tasks that draw on these keywords to relate your image to sky coordinates are e disconlab Displays your image with a superimposed RA and Dec grid Simply open an SAOimage window and type for example sd gt disconlab n3tcOla5Sr_cal fits 1 e xy2rd Translates x and y pixel coordinates to RA and Dec The task rd2xy
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5. and error Constructing tables is a necessary skill if you plan to use certain tasks such as those in the STSDAS fitting package that do not currently recognize the multispec format WCS header information Tabulating your spectra is also the best option if you want to join two or more spectra taken with different gratings into a single spectrum covering the complete wavelength range Because the data are stored as individual wavelength flux pairs you do not need to resample and therefore degrade the individual spectra to a common linear dispersion scale before joining them Instead you could create separate tables for the spectra from different gratings and then combine the two tables using for example the tmerge task cl gt tmerge n5548_h13 tab n5548_h19 tab n5548 tab append Note that you will first have to edit out any regions of overlapping wavelength from one or the other of the input tables so that the output table will be monotoni cally increasing or decreasing in wavelength 3 5 2 Preparing STIS Spectra for Analysis Calibrated STIS spectra emerge from the pipeline either as two dimensional images _x2d files or as one dimensional spectra in tabular form _x1d files You can analyze calibrated two dimensional STIS spectra in IRAF as you would any other long slit spectral image because their headers already contain the necessary wavelength information Tabulated STIS spectra can be analyzed directly using STSDAS
6. re running IRAF cl gt images im gt tv Displaying HST Images W 3 5 Several different display servers including SAOimage SAOtng the next genera tion of SAOimage and Ximtool can be used with IRAF SAOtng may be retrieved via anonymous FTP from sao ftp harvard edu in the directory ftp pub rd Ximtool may be retrieved via anonymous FTP from iraf noao edu in the directory pub v2103 beta Ximtool is particularly handy if you want to blink images 3 Display the image with the IRAF display task using the syntax appropriate for the file format Chapter 2 explains how to specify GEIS groups and FITS extensions tv gt display fname cOh 2 1 GEIS group 2 tv gt display fname fits 11 1 FITS extension 11 tv gt display fname fits sci 3 1 FITS extension sci 3 Note that when using display or any other task on GEIS images you do not need to specify a group the first group is the default However when working with FITS files you must specify an extension unless the FITS file contains only a sin gle image in the primary data unit and has no extensions Figure 3 2 shows how to display group two of a WF PC 1 image If you want to display all four chips of a WF PC 1 or WFPC2 image simulta neously you can create a mosaic with the STSDAS wmosaic task in the hst_calib wfpe package Type help wmosaic for details ap e am 3 6 JH Chapter 3 STSDAS Basics Figure 3 2 Displaying an Image 7 ETDAN Run d
7. see page 3 29 poffsets stsdas hst_calib ctools GEIS image Determine pixel offsets between shifted spectra rapidlook stsdas hst_calib ctools GEIS image Create and display a 2 D image of stacked 1 D images recombine stsdas hst_calib ctools GEIS image Combine sum or average GEIS groups in a 1 D image with option of propagating errors and data quality values resample stsdas hst_calib ctools GEIS image Resample FOS and GHRS data to a linear wavelength scale see page 3 21 sarith noao onedspec Multispec image Spectrum arithmetic scombine noao onedspec Multispec image Combine spectra sfit noao onedspec Multispec image Fit spectra with polynomial function sgraph stsdas graphics stplot Image table Plot spectra and image lines allows overplotting of error bars and access to wavelength array see page 3 17 specalign stsdas hst_calib ctools GEIS image Align and combine shifted spectra see poffsets specplot noao onedspec Multispec image Stack and plot multiple spectra splot noao onedspec Multispec image Plot and analyze spectra amp image lines see page 3 27 Analyzing HST Spectra W 3 27 splot The splot task in the IRAF noao onedspec package is a good general analysis tool that can be used to examine smooth fit and perform simple arithmetic Operations on spectra Because it looks in the header for WCS wavelength information your file must be suitably prepared Like all IRAF tasks splot can work on only one group at a time from a m
8. IRAF tools for studying the characteristics of an image and Table 3 3 lists some useful IRAF STSDAS tasks for manipulating image data implot The IRAF implot task in the plot package allows you to examine an image interactively by plotting data along a given line x axis or column y axis When you run the task a large number of commands are available in addition to the usual cursor mode commands common to most IRAF plotting tasks A complete listing of commands is found in the on line help but the most commonly used are listed in Table 3 2 Figure 3 4 shows an example of how to use the implot task Analyzing HST Images W 3 11 Table 3 2 Basic implot Commands oe Keystroke Command O o ai pe Display on line help E Plot a line Plot a column Quit implot Move down Move up Space Display coordinates and pixel values Figure 3 4 Plotting Image Data with implot STSDAS calcomp gkidir imdkern phistogram sgidecode surface contour gkiextract implot pradprof sgikern velvect ertpict gkimosaic nsppkern prow showcap gdevices graph pcol prows stdgraph gkidecode hafton pcols pvector stdplot pl gt dir wk wOmWO502t cOd wOmw0502t cOh Pl gt implot wOmw0502t cOh 200 wOmw0502t dOh tektronix Tek NOAO IRAF V2 10EXPORT stevens lager stsci edu Fri 16 07 15 20 Aug 93 Line 200 of wOmw0502t ch WOMWOS02TL1 4I T T T Plot line 200 of a WF
9. absorption line profiles and mean galactic extinction 3 6 References 3 6 1 Available from STScl e STSDAS Users Guide version 1 3 3 September 1994 e STSDAS Installation and Site Managers Guide version 2 0 August 1997 e Synphot Users Guide August 1997 e IGI Reference Manual version 3 0 November 1997 3 6 2 Available from NOAO e A Beginners Guide to Using IRAF 1994 J Barnes e User Manual for SAOimage 1991 M Van Hilst 4 Additional information is available in the Astronomical Data Analysis Software and Systems IIIT ASP Conference Series Vol 61 page 437 1994 References Ef 3 33 e Photometry Using IRAF 1994 L Wells e A User s Guide to Stellar CCD Photometry with IRAF 1992 P Massey and L Davis ise e am 3 6 3 Other References Cited in This Chapter e Horne K 1988 in New Directions in Spectrophotometry A G D Philip D S Hayes and S J Adelman eds L Davis Press Schenectady NY p 145 e Koorneef J R Bohlin R Buser K Horne and D Turnshek 1986 in Highlights of Astronomy Vol 7 J P Swinds ed Reidel Dordrecht p 833 e Kriss G 1994 in Astronomical Data Analysis Software and Systems III PASP Conference Series Vol 61 p 437 3 34 JH Chapter 3 STSDAS Basics o e o Z
10. allows you to create PostScript files of your IRAF STSDAS plots For example setting the device parameter in a plotting task equal to psi_port or psi_land invokes psikern and directs your plot to either a portrait mode or a landscape mode PostScript file For example ae e am st gt fwplot y3b10104t clh 19 device psi_land st gt gflush tmp pskxxxx The above commands would write a plot of flux vs wavelength in landscape mode into a temporary PostScript file named tmp pskxxxx by a UNIX system See the online help for more about psikern including plotting in color and incorporating PostScript fonts into your plots IgI As your plotting needs grow more sophisticated and especially as you try preparing presentations or publication quality plots you should investigate the Interactive Graphics Interpreter or igi This task in the STSDAS stplot package can be used with images as well as two and three dimensional tables and can draw axes error bars labels and a variety of other features on plots Different line weights font styles and feature shapes are available enabling you to create complex plots Figure 3 5 shows a sample plot created in igi however because igi is a complete graphics environment in itself it is well beyond the scope of this document You can learn more about igi in the IGI Reference Manual available through the STSDAS Web pages Figure 3 5 Sample igi Plot Observed versus Theoreti
11. dimensional binary tables For example suppose the first extension of the FITS file data fits contains a STIS echelle spectrum and you want to extract only the wavelength and flux arrays corresponding to spectral order 68 You could then type tt gt txtable data fits 1 c WAVELENGTH FLUX r sporder 68 gt gt gt out_table This command would write the wavelength and flux arrays to the columns of the output table out_table To specify multiple rows in a tabulated echelle spectrum you would type tt gt txtable data fits 1 c WAVELENGTH FLUX r row 10 12 gt gt gt echl This command would generate three separate output files named echl_r0010 tab echl_r0011 tab and echl_r0012 tab See the online help for more details on txtable and the selectors syntax and remember to include the double quotation marks The similar tximage task can be used to generate single group GEIS files from STIS data which can then be used as input to tasks such as resample tt gt tximage data fits 1 c WAVELENGTH r row 4 wave hhh tt gt tximage data fits 1 c FLUX r row 4 flux hhh 3 5 3 General Tasks for Spectra IRAF has many tasks for analyzing both one and two dimensional spectral data Many observers will already be familiar with noao onedspec and noao twodspec packages and those who are not should consult the online help Table 3 5 lists some of the more commonly used IRAF STSDAS sp
12. operation of mssplit it assembles separate imsets into a single data file There are additional tasks in this package for deleting and sorting imsets as well as tasks for addressing a specific image class within an imset Analyzing HST Images W 3 15 3 3 4 Photometry Included in this section are e A list of IRAF STSDAS tasks useful for determining source counts e Instructions on how to use header keyword information to convert HST counts to fluxes or magnitudes e A brief description of synphot the STSDAS synthetic photometry package IRAF and STSDAS Photometry Tasks The following are some useful IRAF STSDAS packages and tasks for performing photometry on HST images e apphot aperture photometry package e daophot stellar photometry package useful for crowded fields e isophote package for fitting elliptical isophotes e imexamine performs simple photometry measurements e imstat computes image pixel statistics e iments sums counts over a specified region subtracting background e plcreate creates pixel masks Consult the online help for more details on these tasks and packages The document Photometry using IRAF by Lisa A Wells provides a general guide to performing photometry with IRAF it is available through the IRAF web page http iraf noao edu The apphot package allows you to measure fluxes within a series of concentric apertures This technique can be used to determine the flux in the win
13. section for plotting To plot STIS spectra in BINTABLE extensions you first need to understand how STIS spectra are stored as binary arrays in FITS table cells Chapter 2 page 2 9 discusses this format and describes the selectors syntax used to specify these data arrays Each row of a STIS echelle table contains a separate spectral order and each column contains data of a certain type such as WAVELENGTH data or FLUX data To specify a particular array you must first type the file name then the extension containing the BINTABLE then the column selector then the Displaying HST Spectra W 3 19 row selector For example to select the WAVELENGTH array corresponding to spectral order 80 of the echelle spectrum in extension 4 of stis fits you would specify the file as stis fits 4 c WAVELENGTH r sporder 80 oe o The sgraph task and the igi plotting package to be discussed below both understand the selectors syntax In particular if you wanted to plot the flux versus wavelength in STIS echelle order 80 you could type st gt sgraph stis fits 4 r sporder 80 WAVELENGTH FLUX Remember to include the quotation marks Otherwise sgraph will complain about too many positional arguments Note also that sgraph understands only row selector syntax columns are chosen by name The STIS specific echplot task is particularly useful for browsing STIS echelle spectra It can plot single spectral ord
14. to an ST magnitude plug it into the following equation m 2 5 x log10 F PHOTZPT where the value of the PHOTZPT keyword is the zero point of the ST magnitude scale The zero point of the ST magnitude system has always been and probably always will be equal to 21 10 a value chosen so that Vega has an ST magnitude of zero for the Johnson V passband see Koornneef et al 1986 Horne 1988 and the Synphot Users Guide synphot The STSDAS synthetic photometry package called synphot can simulate HST observations of astronomical targets with known spectra It contains throughput curves for the components of all HST instruments such as mirrors filters gratings apertures and detectors and can generate passband shapes for Displaying HST Spectra W 3 17 any combination of these elements It can also generate synthetic spectra of many different types including stellar blackbody power law and H II region spectra and can convolve these spectra with the throughputs of HST s instruments You can therefore use it to compare results in many different bands to cross calibrate one instrument with another or to relate your observations to theoretical models oe o One useful application of synphot is to recalculate the value of PHOTFLAM for a given observation using the latest calibration files For example to recalculate PHOTFLAM for an FOC observation you could use the calephot task in synphot as follows s
15. PC 1 image To Print This Plot 0 Press Type gflush to flush the buffer imexamine The IRAF imexamine task in the images tv package is a powerful task that integrates image display with various types of plotting capabilities Commands can be passed to the task using the image display cursor and the graphics cursor A complete description of the task and its usage are provided in the online help available from within the IRAF environment by typing help imexamine INTRO 3 3 12 JB Chapter 3 STSDAS Basics Table 3 3 Image Manipulation Tasks Task Package Purpose boxcar gcombine gcopy geomap geotran grlist gstatistics imcale imedit imexamine implot magnify msarith mscombine msstatistics newcont pixcoord plcreate rotate saodump siaper images imfilter stsdas toolbox imgtools stsdas toolbox imgtools images immatch images immatch stsdas graphics stplot stsdas toolbox imgtools stsdas toolbox imgtools images tv images tv plot images imgeom stsdas toolbox mstools stsdas toolbox mstools stsdas toolbox mstools stsdas graphics stplot stsdas hst_calib wfpc xray ximages images imgeom stsdas graphics sdisplay stsdas graphics stplot Boxcar smooth a list of images Combine images using various algorithms and rejection schemes Copy GEIS multigroup images Compute a coordinate transformation Resample an image based on geomap output List of file names of all groups o
16. STSDAS Basics In This Chapter Navigating STSDAS 3 1 Displaying HST Images 3 4 Analyzing HST Images 3 8 Displaying HST Spectra 3 17 Analyzing HST Spectra 3 21 References 3 32 The Space Telescope Science Data Analysis System STSDAS is the software system for calibrating and analyzing data from the Hubble Space Telescope The package contains programs called tasks that perform a wide range of functions supporting the entire data analysis process from reading tapes through reduction and analysis to producing final plots and images This chapter introduces the basics of STSDAS showing you how to display your data leading you through some simple data manipulations and pointing you towards more sophisticated tasks some of which are described in the instrument sections of this handbook STSDAS is layered on top of the Image Reduction and Analysis Facility IRAF software developed at the National Optical Astronomy Observatory NOAO Any task in IRAF can be used in STSDAS and the software is portable across a number of platforms and operating systems To exploit the power of STSDAS effectively you need to know the basics of IRAF If you are not already familiar with IRAF consult the IRAF Primer in Appendix A before reading further 3 1 Navigating STSDAS The tasks in STSDAS are far too numerous and complicated to describe comprehensively in this volume Instead we will show you where to find the STSDAS tasks ap
17. alue associated with each pixel in the spectrum into the flux header and is selected by setting the parameter function table To minimize header size set the parameter format toa suitable value For example using format 8 7g will retain the original seven digits of precision of the wavelength values while not consuming too much space in the flux header file Be aware that there is a physical limit to the number of header lines that can be used to store the wavelength array approximately 1000 lines This limit cannot be overridden Under ordinary circumstances this limitation is not an issue How ever if many spectral orders have been spliced together it may not be possible to store the actual wavelength array in the header and a fit must be done instead imtab Another way to combine wavelengths with fluxes is to create an STSDAS table from your spectrum The imtab task in the STSDAS ttools package reads a GEIS format spectral image and writes the list of data values to a column of an STSDAS table creating a new output table if necessary The following example shows how Analyzing HST Spectra If 3 23 to create a flux wavelength and error table from group eight of a GEIS format FOS dataset cl gt imtab yOcy0108t cOh 8 yOcy0108t tab wavelength cl gt imtab yOcy0108t clh 8 yOcy0108t tab flux cl gt imtab yOcy0108t c2h 8 yOcy0108t tab error The last word on each command line labels the three columns wavelength flux
18. cal Data ape oo Sample Number Analyzing HST Spectra W 3 21 3 5 Analyzing HST Spectra This section describes some IRAF STSDAS tasks that can be used for analyzing and manipulating spectral data Certain of these tasks operate directly on HST data files created by the pipeline However a number of the most useful IRAF tasks such as splot require special preparations of data other than STIS two dimensional spectra Before discussing these tasks we will first show how to recast your data into forms that are more generally accessible oe o 3 5 1 Preparing FOS and GHRS Data The FOS and GHRS data reduction pipelines store fluxes and wavelengths in separate files In GEIS format the c1h file contains the flux information and the cOh file contains the wavelength information Because IRAF tasks generally require both the flux and wavelength information to reside in the same file you will probably want to create a new file that combines these quantities Several options for combining flux and wavelength information are available e resample This simple task resamples your flux data onto a linear wave length scale creating a new flux file containing the starting wavelength of the new grid in the CRVALI keyword and the wavelength increment per pixel in the CD1_1 keyword Encoding the wavelength information into these standard FITS header keywords makes this format quite portable but the resampling proces
19. ceived from the Archive into GEIS format see Converting FITS to GEIS on page 2 11 for instructions After conversion the clh file will hold the calibrated flux values for each pixel the cOh file will hold the corresponding wavelengths and the c2h file will hold the propagated statistical errors Each group of an FOS or GHRS GEIS file contains the results of a separate subintegration FOS readouts taken in ACCUM mode are cumulative so the last group contains the results of the entire integration In contrast GHRS readouts and FOS readouts in RAPID mode are independent If you want to see the results 3 18 JH Chapter 3 STSDAS Basics of an entire GHRS FP SPLIT integration you will need to align and coadd the spectra in the groups of the GHRS file as described in Volume 2 of this handbook You can also combine all the groups in an FOS or GHRS data file without wavelength alignment using the recombine task in the hst_calib ctools package See online help for details Oo 2 am The STSDAS task sgraph in the graphics stplot package can plot the contents of a single GEIS group For example if you want to see group 19 of the calibrated FOS spectrum with rootname y3b10104t you can type st gt sgraph y3b10104t clh 19 Given an input flux image c1h the task fwplot in the hst_calib ctools package will look for the corresponding wavelength cOh file and plot flux versus wavelength If requested it will also loo
20. ectral analysis tasks and below we briefly describe splot one of the most versatile and useful Remember that many of these tasks expect to find WCS wavelength information oe o INTRO 3 3 26 EH Chapter 3 STSDAS Basics in the header so you should first run mkmultispec or tomultispec on your data if necessary Table 3 5 Tasks for Working with Spectra Task Package Input Format Purpose boxcar images imfilter Image Boxcar smooth a list of images bplot noao onedspec Multispec image Plot spectra non interactively continuum noao onedspec Image Continuum normalize spectra fitprofs noao onedspec Image Non interactive Gaussian profile fitting to features in spectra and image lines gcopy stsdas toolbox imgtools GEIS image Copy multigroup images grlist stsdas graphics stplot GEIS image List file names for all groups in a GEIS image used to make lists for tasks that do not use group syntax grplot stsdas graphics stplot GEIS image Plot arbitrary lines from 1 D image overplots multiple GEIS groups no error or wavelength information is used grspec stsdas graphics stplot Multispec GEIS image Plot arbitrary lines from 1 D image stack GEIS groups magnify images imgeom Image Interpolate spectrum on finer or coarser pixel scale nfitld stsdas analysis fitting Image table Interactive 1 D non linear curve fitting see page 3 29 ngaussfit stsdas analysis fitting Image table Interactive 1 D multiple Gaussian fitting
21. ers overplot multiple orders on a single plot or plot up to four orders in separate panels on the same page For example to overplot the orders contained in rows two through four and row six on a single page cl gt echplot stis_xld fits 1 r row 2 4 6 output igi gt gt gt plot_style m Note that the plot_style parameter governs how the spectral orders are plot ted The plot_style values s m and p plot one order per page several orders on a single plot and one order per panel respectively The default brightness unit is calibrated FLUX although you can specify other quantities e g NET counts using the lux_col parameter See the online help for details 3 4 3 Producing Hardcopy This section shows how to generate hardcopies of plots directly and describes igi the Interactive Graphics Interpreter available in STSDAS Direct Hardcopies To print a quick copy of the displayed plot 1 Type gcur in the command window where your CL prompt is located 2 Move the cursor to any location in the graphics window 3 Press E to write the plot to the graphics buffer 4 Type q to exit graphics mode 5 At the cl prompt type gflush A Plots will be printed on the printer defined by the IRAF environment variable stdplot Type show stdplot to see the current default printer use set stdplot printer_name to set the default printer PEE 3 20 EB Chapter 3 STSDAS Basics The PostScript kernel psikern
22. f a GEIS image to make lists Compute image statistics Perform general arithmetic on GEIS images Fill in regions of an image by interpolation Examine images using display plots and text see page 3 11 Plot lines and columns of images see page 3 10 Magnify an image Performs basic arithmetic on STIS and NICMOS imsets Extension of gcombine for STIS and NICMOS imsets Extension of gstatistics for STIS and NICMOS imsets Draw contours of two dimensional data Compute pixel coordinates of stars in a GEIS image Create a pixel list from a region file e g from SAOimage Rotate an image Make image and colormap files from SAOimage display Plot science instrument apertures of HST a Will process all groups of a multigroup GEIS file 3 3 3 Working with STIS and NICMOS Imsets STIS and NICMOS data files contain groups of images called imsets associated with each individual exposure A STIS imset comprises SCI ERR and DQ images which hold science error and data quality information A NICMOS imset in addition to its SCI ERR and DQ images also contains TIME and SAMP images recording the integration time and number of samples corresponding to each pixel of the SCI image See the STIS and NICMOS Data Structures chapters for more details on imsets Analyzing HST Images W 3 13 Here we describe several new STSDAS tasks located in the stsdas toolbox imgtools mstools package that have been designed to help you work with with im
23. gs of the PSF which is useful if you wish to estimate the flux of a saturated star by scaling the flux in the wings of the PSF to an unsaturated PSF Converting Counts to Flux or Magnitude All calibrated HST images record signal in units of counts or Data Numbers DN NICMOS data is DN s The pipeline calibration tasks do not alter the units of the pixels in the image Instead they calculate and write the inverse sensitivity conversion factor PHOTFLAM and the ST magnitude scale zero point PHOTZPT into header keywords in the calibrated data WF PC 1 and WFPC2 observers should note that the four chips are calibrated individually so these photometry keywords belong to the group parameters for each chip 2 Except for 2 D rectified STIS images which are in units of I see Chapter 23 oe O o Oo e am 3 16 JH Chapter 3 STSDAS Basics For all instruments other than NICMOS PHOTFLAM is defined to be the mean flux density F in units of erg cm s A that produces 1 count per second in the HST observing mode PHOTMODE used for the observation If the F spectrum of your source is significantly sloped across the bandpass or contains prominent features such as strong emission lines you may wish to recalculate the inverse sensitivity using synphot described below WF PC 1 observers should note that the PHOTFLAM value calculated during pipeline processing does not include a correction for temporal va
24. he input SAMP values minus the values discarded by masking or rejection Oo 2 am msstatistics This tool is an extension of gstatistics in the STSDAS package which is in turn an extension of imstatistics The main novelty is the inclusion of the error and data quality information included with STIS and NICMOS images in computing statistical quantities In addition to the standard statistical quantities min max sum mean standard deviation median mode skewness kurtosis two additional quantities have been added to take advantage of the error information the weighted mean and the weighted variance of the pixel distribution If x is the value at the i th pixel with associated error the weighted mean and variance used in the task are Xi i O X O w 1 6 X 0 and 2 1 O o i O X O The data quality information carried by the STIS or NICMOS file is used to reject pixels in the statistical computation Users can supply additional masks to reject objects or regions from the science arrays mssplit and msjoin The mssplit task extracts user specified imsets from a STIS or NICMOS data file and copies them into separate files Each output file contains a single imset along with the primary header of the original file You might find this task useful for reducing the size of a STIS or NICMOS file containing many imsets or for performing analysis on a specific imset The msjoin task inverts the
25. he pixels in the SCI images are in counts but msarith can also operate on count rates mscombine This task allows you to run the STSDAS task geombine on STIS and NICMOS data files It divides each imset into its basic components SCI ERR and DQ plus SAMP and TIME for NICMOS to make them digestible for gcombine The SCI extensions become the inputs proper to the underlying gcombine task and the ERR extensions become the error maps The DQ extensions are first combined with a user specified Boolean mask allowing selective pixel masking and then fed ino the data quality maps If scaling by exposure time is requested the exposure times of each imset are read from the header keyword PIX VALUE in the TIME extensions Once gcombine finishes mscombine reassembles the individual output images into imsets and outputs them as a STIS or NICMOS data file The output images and error maps from gcombine form the SCI and ERR extensions of the 3 14 JH Chapter 3 STSDAS Basics output imset The DQ extension will be a combination of the masking operations and the rejection algorithms executed by gcombine For NICMOS the TIME extension will be the sum of the TIME values from the input files minus the rejected values divided on a pixel by pixel basis by the number of valid pixels in the output image The final TIME array will be consistent with the output SCI image average or median of the science data The SAMP extension for NICMOS is built from all t
26. inverts this operation SAOimage displays the current x y pixel location of the cursor in the upper left corner of the window To find the RA and Dec of the current pixel you supply these coordinates to xy2rd by typing sd gt xy2rd n3tcOla5r_cal fits 1 x y Table 3 1 lists some additional tasks that draw on the standard astrometry key words Observers should be aware that these tasks do not correct for geometric distortion Only FOC images currently undergo geometric correction during standard pipeline processing the cOh cOd and clh cld FOC images have been geometrically corrected STIS images will be geometrically corrected in the pipeline once suitable calibration files are in hand If you need precise relative astrometry you should use an instrument specific task that accounts for image distortion such as the metric task for WF PC 1 and WFPC2 images described on page 28 18 Table 3 1 Additional IRAF and STSDAS Astrometry Tasks Task Purpose compass Plot north and east arrows on an image north Display the orientation of an image based on keywords rimcursor Determine RA and Dec of a pixel in an image wescoords Use WCS to convert between IRAF coordinate systems weslab Produce sky projection grids for images a World Coordinate System WCS Type help specwcs at the IRAF prompt for details oe o Oo e am 3 10 JM Chapter 3 STSDAS Basics Do not use tasks like ri
27. is range For example to specify a pixel range from 101 to 200 in the x direction and all pixels in the y direction from group three of a GEIS format image you would use a command such as tv gt display image hhh 3 101 200 1 To specify the same pixel range in the second SCI extension of a NICMOS FITS image you would use a command such as tv gt display image fits sci 2 101 200 1 E If you specify both a group and an image section of a GEIS file the group number must come first When displaying sections of STIS and NICMOS FITS images you must specify the extension and the extension designation must come first Figure 3 3 shows examples of displaying an image and an image section 1 Type help display within IRAF to obtain more information about these parameters o e am INTRO 3 3 8 JH Chapter 3 STSDAS Basics Figure 3 3 Displaying Sections and Groups of an Image Display only a section of group 2 of the image STSDAS Display group 2 of 8 Fi st gt display wOmw0502t cOh 2 503450 503450 entire image frame to be written into 1 4 1 zi 5 630896 z2 17 13277 se J STSDAS SAOimage st gt display wOmw0S02t cOh 2 frame to be written into 134 1 wOmw9502t cOhL2 WOMWOSO2T 2 4 zi 13 14832 z2 13 61855 CIRAF s J amp SAOimage wOmw0S02t cOhL2 WOMWOSO2T 2 4 lt IRAF
28. isplay task ETE limi MAMEET from IRAF window 4 i a im F cl Ar el ioe e am z1 and z2 are image intensity range Image appears in SAOimage window To print hardcopy Click etc 2 Click print Modifying the Display There are two ways to adjust how your image is displayed e Use the SAOimage command buttons that control zooming panning etc e Reset the display task parameters Once an image appears in your SAOimage window you can use the SAOimage commands displayed near the top of the image window to manipulate or print your image The SAOimage Users Guide describes these commands although most are fairly intuitive Just click on the buttons to scale pan or print the image or to perform other commonly used functions On line help is also available at the system level type man saoimage in Unix or help saoimage in VMS The example in Figure 3 2 shows how you should display an image for a first look By default display automatically scales the image intensity using a sampling of pixels throughout the image During your first look you may want to experiment with the scaling using the zscale zrange z1 and z2 parameters The zscale parameter toggles the autoscaling Setting zscale and zranget tells the task to use minimum and maximum values from the image as the minimum and maximum intensity values To customize your minimum and Displaying HST Images W 3 7 maximum inte
29. it Print contents of fit tables created by fitting task When using tasks such as ngaussfit and nfitld you must provide initial guesses for the function coefficients as input to the fitting algorithms You can either specify these initial guesses via parameter settings in the task s parameter sets psets or enter them interactively For example suppose you want to fit several features using the ngaussfit task Using the default parameter settings you can start the task by typing fi gt ngaussfit n4449 hhh linefits tab This command reads spectral data from the image n4449 hhh and stores the results of the line fits in the STSDAS table linefits tab After you start the task your spectrum should appear in a plot window and the task will be left in cursor input mode You can use the standard IRAF cursor mode commands to rewindow the plot resticting your display to the region around a particular feature or features that you want to fit You may then want to e Define a sample region using the cursor mode f s command over which the fit will be computed so that the task will not try to fit the entire spec trum oe o 3 30 JH Chapter 3 STSDAS Basics e Define an initial guess for the baseline coefficients by placing the cursor at two baseline locations one on either side of the feature to be fitted using the e keystroke e Use the R keystroke to redraw the screen and see the baseline that you ve just defi
30. k for the error c2h file and plot the error bars To see a plot of the same spectrum as above but with a wavelength scale and error bars type st gt fwplot y3b10104t clh 19 plterr If you ever need to plot the contents of multiple groups offset from one another on the same graph you can use the grspec task in the graphics stplot package For example to plot groups 1 10 and 19 of a given flux file you can type st gt grspec y3b10104t clh 1 10 19 Note that grspec expects group numbers to be listed as a separate parameter rather than enclosed in the standard square brackets 3 4 2 STIS Spectra STIS data files retrieved from the Archive can contain spectra in two different forms as long slit spectral images in FITS IMAGE extensions or as extracted echelle spectra in FITS BINTABLE extensions Currently only echelle spectra emerge from the pipeline in tabular form while long slit spectra emerge as images You can use sgraph to plot STIS long slit spectra by specifying the image section that contains the spectrum For example to plot the entire x range of the calibrated two dimensional spectrum in the first extension of the file o43balbnm_x2d fits averaging rows 100 through 1000 you would type st gt sgraph o43balbnm_x2d fits 1 100 1000 Displaying the long slit spectral image using the display task see page 3 4 will allow you to see the range of your spectrum in x and y pixel space so you can choose a suitable image
31. late tables For example e tread displays a table allowing you to move through it with the arrow keys oe e am e tprint displays a table e tcopy copies tables e tedit allows you to edit a table Many other tasks in ttools perform a variety of other functions See the online help for details 3 2 Displaying HST Images This section will be of interest primarily to observers whose datasets contain two dimensional images as it explains e How to display images in IRAF using the display task e How to display subsections of images Observers viewing WF PC 1 and WFPC2 data may wish to remove cosmic rays before displaying their data see page 26 20 The FOC photon counting hard ware does not detect cosmic rays at easily as CCDs the NICMOS pipeline auto matically removes cosmic rays from MULTIACCUM observations and the STIS pipeline automatically removes cosmic rays from CR SPLIT association prod ucts 3 2 1 The display Task The most general IRAF task for displaying image data is the display task the best choice for a first look at HST imaging data To display an image you need to 1 Start an image display server such as SAOimage in a separate window from your IRAF session either from a different xterm window or as a back ground job before starting IRAF To start SAOimage type the following in any xterm or other system window saoimage amp 2 Load the images tv package from the window where you
32. mcursor or xy2rd directly on WF PC 1 or WFPC2 images if you require accurate relative positions Calibrated WF PC 1 and WFPC2 images retain a residual distortion which will affect the accuracy of relative posi tions Both wmosaic and metric found in the stsdas hst_calib wfpe package correct for this distortion Improving Astrometric Accuracy Differential astrometry measuring a position of one object relative to another in an image is easy and relatively accurate for HST images while absolute astrometry is more difficult owing to uncertainties in the locations of the instrument apertures relative to the Optical Telescope Assembly OTA or V1 axis and the inherent uncertainty in the Guide Star positions However if you can determine an accurate position for any single star in your HST image then your absolute astrometric accuracy will be limited only by the accuracy with which you know that star s location and the image orientation If there is a star on your image suitable for astrometry you may wish to extract an image of the sky around this star from the Digitized Sky Survey and measure the position of that star using for example the GASP software described in the STSDAS User s Guide These tools can provide an absolute positional accuracy of approximately 0 7 Contact the Help Desk for assistance send E mail to help stsci edu 3 3 2 Examining and Manipulating Image Data This section describes implot and imexamine two basic
33. n also select rows based upon values in some other column For example to select all rows whose spectral order lies in the range 270 to 272 type cl gt tomultispec myfile_xld fits r sporder 270 272 gt gt gt new_ms imh The calibrated flux is extracted by default However other intensity data can be specified by setting the 1lux_col parameter Be careful not to restrict the search for matching rows too heavily Column selectors cannot be used with tomultispec y Analyzing HST Spectra W 3 25 Choose the type of fitting function for the tomultispec dispersion solution with care Using the table option which writes the entire wavelength array to the image header for each order will fail if more than about three orders are selected This restriction results from a limit to the number of keywords that can be used to store the dispersion relation D K i txtable Tabulated STIS spectra are stored as data arrays within individual cells of FITS binary tables see Chapter 2 page 2 9 These tables are effectively three dimensional with each column holding a particular type of quantity e g wavelengths fluxes each row holding a different spectral order and each cell holding a one dimensional array of values spanning the wavelength space of the order The txtable in the tables ttools package extracts these data arrays from the cells specified with the selectors syntax and stores them in the columns of conventional two
34. n parentheses are shown in Figure 3 7 3 See the online help for details and a complete listing of cursor mode colon com mands type help cursor Figure 3 6 Fitting HB and OIll Emission Features in NGC 4449 6 00E 14 5 00E 14 n4449 hhh 3 00E 14 2 00E 14 STSOUIRAE V2 10EXPORT deer aN chat stsci edt Mon ve 26 04 21 Feb 94 func Gaussians low_rej 0 high_r total 3240 sample 354 rejecte E reai RMS row 55 8E 16 Analyzing HST Spectra W 3 31 4 00E 14 INTRO 3 4800 4900 X Figure 3 7 Coefficients and Error Estimates 5050 function Gaussians coeff1 8 838438E 14 coeff2 1 435682E 17 coeff3 1 854658E 14 coeff4 4866 511 coeff5 5 725897 coeff6 1 516265E 14 coeff7 4963 262 coeff8 6 448922 coeff9 4 350271E 14 coeff10 5011 731 coeffll 6 415922 rms grow 0 naverage low_reject high_reject niterate sample 5 837914E HOOH 513048E 16 03789007 0905327 740680E 16 06048062 116878 903318E 16 01856957 03769293 4800 132 5061 308 Go EDD DONS Fap p Baseline zeropoint fix Base tine 5Lope fix amplitude var center var FWHM var amplitude var center var FWHM var amplitude var center var FWHM var 3 32 JH Chapter 3 STSDAS Basics 3 5 5 specfit The specfit task in the STSDAS contrib package is another powerful interactive facility f
35. name and deleting imdelete image files These tasks operate on both the header and data portions of the image The package also contains a number of general purpose tasks for operations such as rotating and magni fying images e stsdas toolbox imgtools This package contains general tools for working with multigroup GEIS images including tasks for working with masks and general purpose tasks for working with the pixel data such as an interactive pixel editor pixedit e stsdas toolbox imgtools mstools This package contains tools for working with FITS image extensions in particular STIS and NICMOS image sets imsets e stsdas analysis This package contains general tasks for image analysis Tables Several of the analysis packages in STSDAS including calibration pipeline tasks create output files in STSDAS table format which is a binary row column format or in FITS binary table format ASCI format tables are also supported for input only The STSDAS User s Guide describes the STSDAS table format in Figure 3 1 STSDAS Version 2 0 Package Structure wfpc J fos hrs y ce oe convfile jito Ego Machine Dependent focutility E Implicitly Loaded INTRO 3 BB SVGSLS bunebien 3 4 JH Chapter 3 STSDAS Basics detail Tasks in the ttools package or in the external tables package can be used to read edit create and manipu
36. ned o e aa Z e Set the initial guesses for the Gaussian centers and heights by placing the cursor at the peak of each feature and typing P y e Press F to compute the fit once you ve marked all the features you want to fit The results will automatically be displayed You can use the show command to see the coefficient values Note that when the ngaussfit task is used in this way i e starting with all default values the initial guess for the FWHM of the features will be set to a value of one Furthermore this coefficient and the coefficients defining the baseline are held fixed by default during the computation of the fit unless you explicitly tell the task through cursor colon commands to allow these coefficients to vary It is sometimes best to leave these coefficients fixed during an initial fit and then to allow them to vary during a second iteration This rule of thumb also applies to the setting of the errors parameter which controls whether or not the task will estimate error values for the derived coefficients Because the process of error estimation is very CPU intensive it is most efficient to leave the error estimation turned off until you ve got a good fit and then turn the error estimation on for one last iteration Figure 3 6 shows the results of fitting the HB 4861A and OII 4959 and 5007 A emission features in the spectrum of NGC 4449 The resulting coefficients and error estimates i
37. nnot be used for navigation See the online help for details oe o INTRO 3 3 28 EB Chapter 3 STSDAS Basics Table 3 6 Useful splot Cursor Commands Command Purpose Manipulating spectra f Arithmetic mode add and subtract spectra 1 Convert spectrum from fy to f invert transformation with n n Convert spectrum from f to fy s Smooth with a boxcar u Define linear wavelength scale using two cursor markings Fitting spectra d Mark two continuum points amp de blend multiple Gaussian line profiles e Measure equivalent width by marking points around target line h Measure equivalent width assuming Gaussian profile k Mark two continuum points and fit a single Gaussian line profile m Compute the mean RMS and S N over marked region t Enter interactive curve fit function usually used for continuum fitting Displaying and redrawing spectra a b Z Expand and autoscale data range between cursor positions Set plot base level to zero Clear all windowing and redraw full current spectrum Redraw spectrum with current windowing Window the graph Etch a sketch mode connects two cursor positions Overplot standard star values from calibration file Zoom graph by a factor of two in X direction Switch between physical pixel coordinates and world coordinates General file manipulation commands Display help Get another spectrum Write current spectrum to new
38. nsity display values set zscale zrange z1 to the minimum value and z2 to the maximum value that you want displayed For example im gt disp wOmw0507v cOh 1 zrange zscale z21 2 78 z22 15 27 Notice in Figure 3 2 that when you run display the task shows you the z1 and z2 values that it calculates You can use these starting points in estimating reasonable values for the minimum and maximum intensity display parameters If you want to display an image with high dynamic range you may prefer to use logarithmic scaling However the log scaling function in SAOimage divides the selected intensity range into 200 linearly spaced levels before taking the log The resulting intensity levels are rendered in a linear rather than logarithmic sense You can often obtain better results if you create a separate logarithmic image to display One way to create a logarithmic image is with the imcalc task im gt imcalc x2ce0502t clh x2ce0502t hhh logl0 im1 1 0 If the peak pixel in your original image contained 2000 counts for example you would then display the logarithmic image with z1 0 and z2 3 3 3 2 2 Working with Image Sections Sometimes you may want to display only a portion of an image using the syntax for specifying image sections discussed in Chapter 2 Your specified pixel range should give the starting point and ending point with a colon separating the two List the horizontal x axis range first followed by the vertical y ax
39. numbers are written to the corresponding beam numbers in the multispec image the aperture numbers are indexed sequentially starting from one You can choose to fit the dispersion solution interactively but the default fourth order Chebyshev polynomial will likely suffice for all STIS spectral orders except for prism dispersed spectra However you cannot use the interactive option if you are selecting more than one order from the input file For example if you want to write all spectral orders from the STIS file myfile_xld fits to a multispec file you can type cl gt tomultispec myfile_xld fits new_ms imh Note that the imh suffix on the output file specifies that the output file is to be an OIF file This format is similar to GEIS format in that it consists of two files a header file imh and a binary data file pix The output format for tomulti spec will always be OIF If you want to select particular spectral orders rather than writing all the orders to the multispec file you will need to use the selectors syntax To select only the spectrum stored in row nine of the input table the previous example would change to cl gt tomultispec myfile_xld fits r row 9 new_ms imh Note that the double quote marks around the file name and row selector are neces sary to avoid syntax errors To select a range of rows say rows nine through eleven you would type cl gt tomultispec myfile_xld fits r row 9 11 new_ms imh You ca
40. or existing image Quit and go on to next input spectrum Analyzing HST Spectra W 3 29 3 5 4 STSDAS fitting Package The STSDAS fitting package contains several powerful and flexible tasks listed in Table 3 7 for fitting and analyzing spectra The ngaussfit and nfitld tasks in particular are very good for interactively fitting multiple Gaussians and nonlinear functions respectively to spectral data These tasks do not currently recognize the multispec WCS method of storing wavelength information They recognize the simple sets of dispersion keywords such as WO WPC and CRPIX CRVAL and CDELT but these forms apply only to linear coordinate systems and therefore would require resampling of your data onto a linear wavelength scale before being used However these tasks do accept input from STSDAS tables in which you can store the wavelength and flux data value pairs or wavelength flux error value triples imtab on page 3 22 Table 3 7 Tasks in the STSDAS fitting Package Task Purpose convert Convert ASCII data base format to STSDAS table format function Generate functions as images tables or lists gfitld Interactive 1 d linear curve fit to images tables or lists i2gaussfit Iterative 2 d Gaussian fit to noisy images script nfitld Interactive 1 d non linear curve fit to images tables or lists ngaussfit Interactive 1 d multiple Gaussian fit to images tables or lists n2gaussfit 2 d Gaussian fit to images prf
41. or fitting a wide variety of emission line absorption line and continuum models to a spectrum This task was written by Gerard Kriss at Johns Hopkins University Extensive online help is available to guide you through the task although because it is a contributed task little to no support is provided by the STSDAS group oe o The input spectrum to specfit can be either an IRAF image file or an ASCII file with a simple three column wavelength flux and error format If the input file is an IRAF image the wavelength scale is set using values of WO and WPC or CRVAL1 and CDELT1 Hence for image input the spectral data must be on a linear wavelength scale In order to retain data on a non linear wavelength scale it is necessary to provide the input spectrum in an ASCII file so that you can explicitly specify the wavelength values associated with each data value The online help explains a few pieces of additional information that must be included as header lines in an input text file By selecting a combination of functional forms for various components you can fit complex spectra with multiple continuum components blended emission and absorption lines absorption edges and extinction Available functional forms include linear power law broken power law blackbody and optically thin recombination continua various forms of Gaussian emission and absorption lines absorption edge models Lorentzian line profiles damped
42. propriate for handling certain jobs You can refer to online help or the STSDAS User s Guide for details on how to use these tasks Some useful online help commands are 3 2 JE Chapter 3 STSDAS Basics e help task provides detailed descriptions and examples of each task e help package lists the tasks in a given package and their functions e describe task provides a detailed description of each task o e aa Z e example task provides examples of each task apropos word searches the online help database for tasks relating to the specified word see Figure A 4 in Appendix A 3 1 1 STSDAS Structure STSDAS version 2 0 is structured so that related tasks are logically grouped In many cases you can find a task that performs whatever function you need simply by looking in the appropriate package For example all of the tasks that are used in the calibration process can be found in the hst_calib package and all tasks used for image display and plotting can be found in the graphics package Figure 3 1 shows the STSDAS package structure Note that IRAF version 2 11 must be installed on your system in order for you to use STSDAS 2 0 and TABLES version 2 0 3 1 2 Packages of General Interest Images Both IRAF and STSDAS contain a large number of tasks that work with HST images Some of the packages you should investigate are e images This package includes general tasks for copying imcopy moving imre
43. riations in throughput owing to contamination buildup Likewise FOC observers should note that PHOTFLAM values determined by the pipeline before May 18 1994 do not account for sensitivity differences in formats other than 512 x 512 see Format Dependent Sensitivity on page 7 10 To convert from counts or DN to flux in units of erg em s AT multiply the total number of counts by the value of the PHOTFLAM header keyword and divide by the value of the EXPTIME keyword exposure time You can use the STSDAS task imeale to convert an entire image from counts to flux units For example to create a flux calibrated output image outimg fits from an input image inimg fits 1 with header keywords PHOTFLAM 2 5E 18 and EXPTIME 1000 0 you could type st gt imcalc inimg fits 1 outimg fits im1 2 5E 18 1000 0 Calibrated NICMOS data are in units of DN sl so the PHOTFLAM values in their headers are in units of erg cm A You can simply multiply these images by the value of PHOTFLAM to obtain fluxes in units of erg em s AT NICMOS headers also contain the keyword PHOTFNU in units of Jy s Multiplying your image by the PHOTFNU value will therefore yield fluxes in Janskys eu If your HST image contains a source whose flux you know from ground based measurements you may choose to determine the final photometry of your HST image from the counts observed for this source To convert a measured flux F in units of erg cm s Al
44. s loses some of the original flux information In addi tion the error c2h and data quality cqh files cannot be similarly res ampled limiting the usefulness of this technique e mkmultispec This task writes wavelength information into the header of a flux file while preserving all the original information It is therefore a better choice than resample for most applications and we describe it in more detail below e imtab An alternative to writing wavelength information into the header is to use the imtab task to create a table recording the wavelength flux and if desired the error data corresponding to each pixel Many STSDAS tasks such as those in the STSDAS fitting package can access data in tabular form so we describe this approach in more detail as well mkmultispec The most convenient method of combining wavelength and flux information and one that has no effect on the flux data at all is to use the mkmultispec task This task places wavelength information into the headers of your flux files according to the IRAF multispec format World Coordinate System WCS The multispec coordinate system is intended to be used with spectra having nonlinear dispersions or with images containing multiple spectra and the format is recognized by many tasks in IRAF V2 10 or later For a detailed discussion of the multispec WCS type help specwcs at the IRAF prompt Oo e 3 22 JE Chapter 3 STSDAS Basics The mkm
45. sets as units and to deconstruct and rebuild them oe o msarith This tool is an extension of the IRAF task imarith to include error and data quality propagation The msarith task supports the four basic arithmetic operations and can operate on individual or multiple imsets The input operands can be either files or numerical constants the latter can appear with an associated error which will propagate into the error array s of the output file Table 3 4 below shows how this task operates on the SCI ERR and DQ images in a STIS or NICMOS imset as well as the additional TIME and SAMP images belonging to NICMOS imsets Table 3 4 Task msarith Operations Operation Operand2 SCI ERR DQ TIME SAMP ADD file opl op2 Jol 02 OR T1 T2 S1 S2 SUB file opl op2 do 02 OR Ti SI MULT file op1 op2 opl x op2 a1 op1 62 0p2 OR Tl Sl DIV file opl op2 op1 op2 o1 op1 62 0p2 OR Tl S1 ADD constant opl op2 lol ae o2 SUB constant op1l op2 lol Zei 02 MULT constant opl op2 opl x op2 o1 op1 02 op2 T1 op2 DIV constant op1 op2 op1 0p2 01 0p1 62 0p2 i T1 op2 In Table 3 4 the first operand op1 is always a file and the second operand op2 can be either a constant or a file The ERR arrays of the input files o1 and 02 are added in quadrature If the constant is given with an error 02 the latter is added in quadrature to the input ERR array Note that in Table 3 4 t
46. tasks that understand the selectors syntax described on page 2 9 However to use IRAF tasks such as splot that rely on the multispec WCS or to use STSDAS tasks that do not understand three dimensional tables you will have to prepare your data appropriately This section describes two useful tasks for putting your data in the proper form e tomultispec This task is the STIS analog to mkmultispec described above it extracts STIS spectra from tables and writes them as IRAF spec tral images with wavelength information in the header e txtable This task extracts specified data arrays from STIS table cells and places them in conventional two dimensional tables for easier access e tximage Extracts specified data arrays from STIS table cells and places them into 1 D images This task can write single group GEIS files tomultispec The tomultispec task in the stsdas hst_calib ctools package extracts one or more spectral orders from a STIS table fits a polynomial dispersion solution to oe o Oo e am 3 24 JE Chapter 3 STSDAS Basics each wavelength array and stores the spectra in an output file in original IRAF format OIF using the multispec WCS This task is layered upon the mkmultispee task which performs a similar operation for FOS and GHRS calibrated spectra see page 3 21 Most of the parameters for tomultispec echo those for mkmultispec As a helpful navigational aid the STIS spectral order
47. ultigroup GEIS file You can specify which GEIS group you want to operate on by using the square bracket notation for example cl gt splot yOcy0108t clh 8 If you don t specify a group in brackets splot will assume you want the first group In order to use splot to analyze your FOS or GHRS spectrum you will first need to write the wavelength information from your cOh file to the header of your c1h files in WCS using the mkmultispec task see page 3 21 The splot task is complex with many available options described in detail in the online help Table 3 6 summarizes a few of the more useful cursor commands for quick reference When you are using splot a log file saves results produced by the equivalent width or de blending functions To specify a file name for this log file you can set the save_file parameter by typing for example cl gt splot yOcy0108t clh 8 save_file results log If you have used tomultispec to transform a STIS echelle spectrum into imh pix OIF files with WCS wavelength information see page 3 23 you can step through the spectral orders stored in image lines using the and keys To start with the first entry in your OIF file type cl gt splot new_ms imh 1 You can then switch to any order for analysis using the key to increment the line number the key to decrement and the key to switch to a specified image line Note the beam label that gives the spectral order ca
48. ultispec task can put wavelength information into the flux header files in two different ways The first involves reading the wavelength data from the cOh file fitting the wavelength array with a polynomial function and then storing the derived function coefficients in the flux header file c1h in multispec format Legendre Chebyshev or cubic spline spline3 fitting functions of fourth order or larger produce essentially identical results all having rms residuals less than 10 much smaller than the uncertainty of the original wavelength information Because these fits are so accurate it is usually unnecessary to run the task in interactive mode to examine them If there are discontinuities in the wavelengths which could arise due to the splicing of different gratings you should run mkmultispec in interactive mode to verify the fits Because mkmultispec can fit only simple types of polynomial functions to wave length data this method will not work well with FOS prism data because of the different functional form of the prism mode dispersion solution For prism spec tra use the header table mode of mkmultispec see below or create an STSDAS table using imtab The other method by which mkmultispec can incorporate wavelength information into a flux file is simply to read the wavelength data from the cOh file and place the entire data array directly into the header of the flux c1h file This method simply dumps the wavelength v
49. y gt calcphot foc f 96 x96zlrg f501n unit 1 flam counts The first argument to calcphot gives the instrument and its configuration in this case the FOC 96 camera in full zoomed format with the F501 filter See the obsmode task in synphot and the Synphot User s Guide for help with these observation mode keywords The second tells the task to model a flat Fy spectrum having unit flux and the third tells the task to produce output in units of counts per second After you run calephot its result parameter will contain the count rate expected from the FOC given this configuration and spectrum The PHOTFLAM keyword defined to be the flux required to produce one count per second simply equals the reciprocal of this value which you can print to the screen by typing 1 calcphot result at the IRAF prompt Please see the Synphot User s Guide for more details on this multipurpose package and see Appendix A for information on getting the synphot dataset which is not included with STSDAS Information about retrieving the synphot dataset can be found in Appendix A 3 4 Displaying HST Spectra This section shows how to plot your HST spectra for a quick first look and how to generate hardcopies of your plots Because the STIS data format differs from that of FOS and GHRS we will discuss STIS data separately 3 4 1 FOS and GHRS Spectra Before you work with FOS and GHRS data within STSDAS you will want to convert the FITS files you re
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