EXTRACTOR An Astronomical Source Detection Program
Contents
1. for astrometry A slightly modified version of SExtractor 2 19 5 is also included in the distribution of EXTRACTOR You can find details of SExtractor s operation and how to configure it in the User s Guide which is issued as a Starlink Miscellaneous User Document MUD 163 configure EXTRACTOR in much the same way although with see Section 6 SExtractor home page http terapix iap fr soft sextractor offers more information such as a link to the SExtractor for Dummies In the Starlink release both EXTRACTOR and SExtractor do not offer the galaxy model fitting features that the full SExtractor builds do This has been done to reduce the external package dependencies required also it isn t clear how useful these abilities would be without also creating an NDF version of PSFEx This document describes how to run EXTRACTOR and includesjinstructions for running SExtractor from the Starlink distribution 2 Running EXTRACTOR Like any Starlink program EXTRACTOR may be run from the Unix shell or from a user interface such as see SG 5 A useful summary of the rich variety of methods of specifying parameter values is given in SUN 95 C shell users should also consult SC 4 the C Shell CookBook The examples below assume you are running from the Unix shell but the commands and parameters are exactly the same from ICL After normal Starlink startup at the shell prompt type extractorsetup to initialise the package and th2. want measurements that are not available by default you must take a copy of this file and edit it Remember to also change default sex to use this file otherwise you will continue to use the system wide defaults One off modifications of parameters can be made using the KEYWORDS NAME and VALUE parameters EXTRACTOR_DIR config default sex KEYWORDS _LOGICAL Read Whether you want to enter a series of parameter names and values interactively If TRUE then the parameters NAME and VALUE are used to cyclically prompt for program parameters and the values you want to use To end the cycle respond with a null symbol FALSE NAME LITERAL Read The name of a preferences parameter that you want to set interactively Respond with when you have no more to enter VALUE LITERAL Read The value of the parameter you have just specified using the NAME prompt
3. HRESH value is not given then a default step of 0 75 magnitudes per square arcsec is used The actual formula used to generate the thresholds is SUN 226 13 References 6 I Ax 1070 4 start stepxi zp i 0 15 where J is the ith threshold A is the area of an image pixel in arcseconds start step and zp are as described above If RAD_TYPE is INT then the correct RAD_THRESH format is RAD_THRESH step start step being the interval between thresholds in magnitudes and start being the threshold used for the first level If start is not given then the analysis threshold is used The formula used to generate these thresholds is I start x 10704 step i j 0 15 If no RAD_THRESH values are given this time then the APM PISA see SUN 109 analysis thresh olds are used I start x 20 2 1 15 This gives approx 0 75 magnitude steps 2 5 x log 2 The first threshold is set to the analysis threshold Notes before any measurements will be made at least one of the catalogue parameters RADO through RAD15 must be present in the default param file The minimum starting threshold that can be used is the analysis one no information about values below this is available For completeness the existing IS00 IS07 areas in SExtractor are based on an optimal sampling of each object profile Under this scheme each threshold is I start x 2 i 0 7 where I peak intensity So you get a range of levels for each object spa
4. SUN 226 13 Starlink Project Starlink User Note 226 13 A J Chipperfield P W Draper 19th September 2014 EXTRACTOR An Astronomical Source Detection Program User Manual O ws y E ae io a a o OO O SUN 226 13 Abstract ii EXTRACTOR will detect sources in an astronomical image and build a catalogue listing them It is based on the popular SExtractor program has very flexible configuration facilities and can handle images and catalogues in a variety of formats iii SUN 226 13 Contents Contents 1 2 Running EXTRACTOR 1 pidas y Dad ada n A A A 2 3 4 5 Using GAIA and EXTRACTOR 4 Implementation notes 4 Isophotal radii 5 References 6 gt QO N WN ho O A hell o Eh A o er hel o 5 o E P lt H kev gt Q H o R 7 EXTRACTOR xe ala ale ae A e A el a ag we a ee et 8 1 SUN 226 13 Running EXTRACTOR 1 Introduction EXTRACTOR is a program for automatically detecting objects on an astronomical image and building a catalogue of their properties It is particularly suited for the reduction of large scale galaxy survey data but also performs well on other astronomical images EXTRACTOR is Emmanuel Bertin s SExtractor Source Extractor program re packaged for use in the Starlink Software Environment see SG 4 This means that it uses the Starlink parameter system accepts images in format see SUN 33 and uses the library SUN 210
5. TOR_DIR config default sex or by keyword extractor config EXTRACTOR_DIR config default sex In this case you would be prompted for IMAGE but not for CONFIG The additional parameters KEYWORDS NAME and VALUE allow configuration parameters specified in the preferences file to be overridden without editing the file For example extractor keywords true NAME Parameter name gt catalog_name VALUE Parameter value gt sky cat NAME Parameter name gt catalog_type VALUE Parameter value gt ascii_skycat NAME Parameter name gt CONFIG Configuration file EXTRACTOR_DIR config default sex gt IMAGE Input image image gt would change the name and type of the catalogue produced for this run only The list of changes is terminated by replying with the NULL response to the prompt The SExtractor User s Guide MUD 165 gives a full list of possible parameter names and values but it is only sensible to change a few in this way 2 1 Running EXTRACTOR from scripts If you d like to run EXTRACTOR from a script just say for instance sampling different populations of objects at different thresholds you can do this using the NAME VALUE parameters avoiding the need to have multiple configuration files but you need to adopt a slightly different strategy to normal programs Here s one example script 3 SUN 226 13 Using images in non NDF formats Example batch script bi
6. ased on the SExtractor program which is described in the SExtractor s User Guide MUD 165 Consult this about all the various options that are available and for the rationale behind the program Usage extract image config keywords name value Parameters IMAGE LITERAL Read The name of the image which contains the objects you wanted detected and parameterised If you have initialised the CONVERT package see SUN 55 then you may process foreign formats such as FITS and IRAF Using this parameter you may give two image files The first image will be used for detection and parameterising and the second will be used to actually measure the data values Using this method allows you to measure the same objects many images or to use a high signal to noise image to determine the measurement regions on a low signal to noise image global_data_file CONFIG LITERAL Read The name of the file that contains the many program parameters things like the threshold for object detection This is initially a file named default sex that can be found in the directory EXTRACTOR_DIR config To modify the parameters used by this program you must take a copy of this file and edit it Guidance about the values that parameters can take may be found in this file as well as in the associated SExtractor documentation see MUD 165 The measurements made are determined by a list of parameters in the file EXTRACTOR_DIR config default param Again if you
7. en extractor to run the program You will now be prompted for the values of parameters CONFIG and IMAGE in turn IBERTIN E and ARNOUTS S 1996 A amp AS 117 393 Note this document is still in draft consequently you should read this in conjunction with the A amp AS paper and thelearlier SExtractor user document SUN 226 13 Running EXTRACTOR 2 CONFIG Configuration file EXTRACTOR_DIR config default sex gt IMAGE Input image image gt where CONFIG is the name of the preferences file and IMAGE is the name of the image file to be processed Suggested values are given between in the prompt You can accept the suggested values by just typing lt RETURN gt If you accept the suggested values shown above EXTRACTOR will process the NDF image using the installed default configuration files to produce a catalogue named test cat this name is specified in the preferences file CONFIG has a default value of EXTRACTOR_DIR config default sex This file has some sensible defaults but you will probably want to copy it to your own directories and modify it to your taste as you would for the native SExtractor program Similarly with the other configuration files The environment variable EXTRACTOR_DIR is defined to point to the directory containing the EXTRACTOR program and the config directory You can also provide parameter values on the command line Either positionally extractor image EXTRAC
8. he exe cutable image is installed in EXTRACTOR_DIR and is ran as described in the SExtractor User s Guide MUD 165 To use this version either put EXTRACTOR_DIR on your PATH or define an alias alias sex EXTRACTOR_DIR sex then type sex image c configuration file Parameter1 Value1 Parameter2 Value2 Note that if the c option is omitted file default sex in the current working directory will be used if you want to use the installed default file you will need to specify it explicitly 5 Using GAIA and EXTRACTOR The Graphical Astronomy and Image Analysis Tool GAIA SUN 214 has an interactive tool box facility that uses the EXTRACTOR and SExtractor programs This allows you to interactively adjust the detection preferences and identifies the objects that have been detected by drawing suitable ellipses over your image in fact GAIA produced the image used on the title page of this document GAIA also displays the results catalogue so you can view the measurements associated with each object and vice versa i e you can select one of the displayed ellipses and view the associated measurements or select a row of measurements and view the associated object The catalogue interface also allows you to sort and select objects on the basis of their measurements 6 Implementation notes EXTRACTOR will read data in floating point format It will also read NDFs that contain BAD pixels These will appear as missi
9. n csh Initialize EXTRACTOR extractorsetup Extract all objects above 1 sigma extractor keywords true image image config default sex lt lt E0OF catalog_name threshi cat detect_thresh 1 0 EOF Extract all objects above 2 sigma and measure on a different image extractor keywords image detect measure config default sex lt lt EOF catalog_name thresh2 cat detect_thresh 20 EOF Using the C shell lt lt EOF mechanism allows you to send information to the program as if you re typing it in 3 Using images in non NDF formats If you want to read an image in one of the formats handled by the CONVERT package just start up CONVERT before running EXTRACTOR and specify the image with an appropriate extension For example convert CONVERT commands are now available Defaults for automatic NDF conversion are set Type conhelp for help on CONVERT commands Type showme sun55 to browse the hypertext documentation extractor CONFIG Configuration file EXTRACTOR_DIR config default sex gt IMAGE Input image image gt image fit will use the FITS file image fit automatically converting it to a temporary NDF file and then reading the temporary file SUN 226 13 Implementation notes 4 4 Running SExtractor A native FITS only version of SExtractor is also distributed with the EXTRACTOR package for convenience and use in the object detection toolbox in the GAIA application T
10. ng regions so any objects that are intersected will be split into parts Since EXTRACTOR uses the AST SUN 211 library to perform astrometry it can be used with im ages that contain DSS calibrations as well as FITS WCS and AST native ones native SExtractor will only use images that have FITS WCS calibrations The GAIA display tool is a convenient way to add such astrometrical calibrations Two current limitations are that no support is provided for the use of NDF variance arrays although these would probably map onto the weight image concept of SExtractor and 5 SUN 226 13 Isophotal radii that the propagation of the input NDF to the check image is not done so WCS calibration for instance is lost NDF pixel coordinates i e ones that include the NDF origin can be obtained using the X_PIXEL and Y_PIXEL parameters The SExtractor program has a very large range of processing options Consequently many of these options have not been tried extensively in the EXTRACTOR incarnation so some problems may arise when using non default configurations A SExtractor facility that has not been implemented is the piping of catalogues to standard output The use of weight and flag images are not supported Finally as noted earlier the galaxy fitting parameters are not available If you come across any problems with EXTRACTOR please notify Starlink Software Support starlink jiscmail ac uk 7 Isophotal radii SExtracto
11. nning the range from its analysis threshold to just below the peak intensity 8 References Bailey J A SG 5 ICL The Interactive Command Language for ADAM Bertin E MUD 165 SExtractor User s Guide DRAFT Bertin E and Arnouts S 1996 A amp AS 117 393 Currie M J amp Berry D S KAPPA Kernel Application Package Currie MJJ et al SUN 55 CONVERT A Format conversion Package Davenhall A C CURSA Catalogue and Table Manipulation Applications Draper P W amp Gray N GAIA Graphical Astronomy and Image Analysis Tool Lawden M D SG 4 ADAM The Starlink Software Environment Warren Smith R F SSN 20 Adding Format Conversion Facilities to the NDF Data Access Library Warren Smith R F SUN 33 NDF Routines for Accessing the Extensible N Dimensional Data Format 7 SUN 226 13 Specification of EXTRACTOR SUN 226 13 Specification of EXTRACTOR 8 EXTRACTOR EXTRACTOR Extracts sources from astronomical images Description EXTRACTOR is a program for detecting and measuring the properties of all the sources on an astronomical image It offers a large range of configuration options for controlling the way that objects are detected and the measurements that are made of them The source measurements are written to a catalogue which can be of several different formats so that they can be analysed possibly by a catalogue handling package like CURSA SUN 190 EXTRACTOR is b
12. r can measure the areas of objects at 8 optimally selected thresholds such information is often used to estimate object profiles These thresholds are consequently different for each object Traditionally this has not been the case and isophotal areas radii have been measured at fixed magnitude intervals above the detection threshold Consequently for convenience and compatibility with existing data and analysis methods the EXTRACTOR and Starlink SExtractor programs have been extended to provide a flexible configuration scheme that allows this traditional behaviour to be recovered and naturally extended The configuration options that control the chosen isophotal thresholds are e RAD_TYPE and e RAD_THRESH RAD_TYPE can be either SB or INT which indicate that the levels will be defined in terms of surface brightnesses or intensities i e magnitudes and data counts respectively The RAD_THRESH option can have up to three qualifying values depending on the value of RAD_TYPE If RAD_TYPE is SB then you should enter a line consisting of RAD_THRESH step start zp in your default sex file The value step being the required interval between levels start being the value used as the first threshold and zp the data zero point all in magnitudes per square arc second If only one value is given then the starting point is assumed to be the analysis threshold and the zero point is derived from the photometric value MAG_ZEROPOINT If a RAD_T
Download Pdf Manuals
Related Search