SExtractor Draft - METU Astrophysics
SExtractor Draft - METU Astrophysics SExtractor Draft - METU Astrophysics
47 Weighting 227.1 Weight-map formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227.2 Weight threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237.3 Effect of weighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237.4 Combining weight maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247.5 Interpolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 Flags 248.1 Internal flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258.2 External flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Measurements 259.1 Positional parameters derived from the isophotal profile . . . . . . . . . . . . . . 269.1.1 Limits: XMIN, YMIN, XMAX, YMAX . . . . . . . . . . . . . . . . . . . . . . . . 269.1.2 Barycenter: X, Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269.1.3 Position of the peak: XPEAK, YPEAK . . . . . . . . . . . . . . . . . . . . . . 279.1.4 2nd order moments: X2, Y2, XY . . . . . . . . . . . . . . . . . . . . . . . . 279.1.5 Basic shape parameters: A, B, THETA . . . . . . . . . . . . . . . . . . . . . 279.1.6 Ellipse parameters: CXX, CYY, CXY . . . . . . . . . . . . . . . . . . . . . . . 289.1.7 By-products of shape parameters: ELONGATION, ELLIPTICITY . . . . . . . 299.1.8 Position errors: ERRX2, ERRY2, ERRXY, ERRA, ERRB, ERRTHETA, ERRCXX,ERRCYY, ERRCXY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299.1.9 Handling of “infinitely thin” detections . . . . . . . . . . . . . . . . . . . 309.2 Astrometry and WORLD coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . 319.2.1 Angular coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319.2.2 Use of the FITS keywords for astrometry . . . . . . . . . . . . . . . . . . 329.3 Cross-identification within SExtractor . . . . . . . . . . . . . . . . . . . . . . . . 329.3.1 The ASSOC list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329.3.2 Controlling the ASSOC process . . . . . . . . . . . . . . . . . . . . . . . . . 329.3.3 Output from ASSOC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33A Appendices 35A.1 FAQ (Frequently Asked Questions) . . . . . . . . . . . . . . . . . . . . . . . . . . 35
51 What is SExtractor?SExtractor (Source-Extractor) is a program that builds a catalogue of objects from an astronomicalimage. It is particularly oriented towards reduction of large scale galaxy-survey data,but it also performs well on moderately crowded star fields. Its main features are:• Simplicity of usage and configuration.• Speed: typically 500 kpixel/s with a Pentium2@450MHz.• Ability to work with very large images (up to 65k × 65k pixels on 32bit machines, or2G × 2G pixels on 64bit machines), thanks to buffered image access.• Robust deblending of overlapping extended objects.• Real-time filtering of images to improve detectability.• Neural-Network-based star/galaxy classifier.• Flexible catalogue output of desired parameters only.• Pixel-to-pixel photometry in dual-image mode.• Handling of weight-maps and flag-maps.• Optimum handling of images with variable S/N.• Special mode for photographic scans.• Modularity of the code that enables one to implement new parameters.The purpose of SExtractor was to find a compromise between refinement in both detection andmeasurements, and computational speed.2 Installing the software2.1 Software and hardware requirementsSince the beginning in 1993, the development of SExtractor was always made on Unix systems(successively: SUN-OS, HP/UX, SUN-Solaris, Digital Unix and GNU/Linux). Successful portsby external contributors have been reported on non-Unix OSes such as AMIGA-OS, DEC-VMSand even MS-DOS Windows95 1 and NT ; ). They are however not currently supported by theauthor, and Unix remains the recommended system for running SExtractor. The software isgenerally run in (ANSI) text-mode from a shell. A window system is therefore unnecessary withpresent versions.On the hardware side, memory requirements obviously depend on the size of the images to beprocessed. But to give an idea, a typical processing of 1024 × 1024 pixel images should requireno more than 8 MB of silicon memory. For very large images, (32000 × 32000 pixels or more), aminimum of 200MB is recommended. Swap-space can of course be put to contribution, althougha strong performance hit is to be expected.1 Binaries are available on the WWW, see e.g. http://www.tass-survey.org/tass/software/software.html#sextract
- Page 1: 1DraftSExtractorv2.1.3User’s guid
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- Page 8 and 9: 8BACK SIZE — integers (n ≤ 2) S
- Page 10 and 11: 10INTERP TYPE ALL keywords (n ≤ 2
- Page 12 and 13: 12VIGNET(15,15), yield arrays of nu
- Page 14 and 15: 14Segmentation in SExtractor is ach
- Page 16 and 17: 166.2 Filtering6.2.1 ConvolutionDet
- Page 18 and 19: 186.2.4 Image boundaries and bad pi
- Page 20 and 21: 206.4 DeblendingEach time an object
- Page 22 and 23: 22are generally useless, except per
- Page 24: 243. The CLEANing process (§??) ta
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- Page 29 and 30: 29¡ ¡ ¢ £ ¤ ¥ ¦ § ¨ © ©
- Page 31 and 32: 31Positional errors are more diffic
- Page 33 and 34: 33barycenter of the current SExtrac
- Page 35: 35AAppendicesA.1 FAQ (Frequently As
47 Weighting 227.1 Weight-map formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227.2 Weight threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237.3 Effect of weighting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237.4 Combining weight maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247.5 Interpolation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 Flags 248.1 Internal flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258.2 External flags . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 Measurements 259.1 Positional parameters derived from the isophotal profile . . . . . . . . . . . . . . 269.1.1 Limits: XMIN, YMIN, XMAX, YMAX . . . . . . . . . . . . . . . . . . . . . . . . 269.1.2 Barycenter: X, Y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269.1.3 Position of the peak: XPEAK, YPEAK . . . . . . . . . . . . . . . . . . . . . . 279.1.4 2nd order moments: X2, Y2, XY . . . . . . . . . . . . . . . . . . . . . . . . 279.1.5 Basic shape parameters: A, B, THETA . . . . . . . . . . . . . . . . . . . . . 279.1.6 Ellipse parameters: CXX, CYY, CXY . . . . . . . . . . . . . . . . . . . . . . . 289.1.7 By-products of shape parameters: ELONGATION, ELLIPTICITY . . . . . . . 299.1.8 Position errors: ERRX2, ERRY2, ERRXY, ERRA, ERRB, ERRTHETA, ERRCXX,ERRCYY, ERRCXY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299.1.9 Handling of “infinitely thin” detections . . . . . . . . . . . . . . . . . . . 309.2 Astrometry and WORLD coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . 319.2.1 Angular coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319.2.2 Use of the FITS keywords for astrometry . . . . . . . . . . . . . . . . . . 329.3 Cross-identification within <strong>SExtractor</strong> . . . . . . . . . . . . . . . . . . . . . . . . 329.3.1 The ASSOC list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329.3.2 Controlling the ASSOC process . . . . . . . . . . . . . . . . . . . . . . . . . 329.3.3 Output from ASSOC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33A Appendices 35A.1 FAQ (Frequently Asked Questions) . . . . . . . . . . . . . . . . . . . . . . . . . . 35
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