I am using SExtractor (Source Extractor) to identify Star-Forming regions in a galaxy. The problem is I am unable to choose the parameters required for it. SExtractor is just a tool to identify the sources in a field. These can be point sources as well as extended diffuse sources. So it is not clear how to go about applying the sextractor and then isolating/identifying only the extended regions as the star-forming regions. Basically, why and how to choose the extent of 'X' pixels?

I need to calculate the Petrosian radius for each source as well. I can draw a histogram of the Petrosian radii for all the sources detected within the PSF of the instrument and use only those sources that have a radius larger than a certain value (say the FWHM or PSF of a bright Point source in the field). I think the histogram will be skewed towards larger radii as there are a large number of SF regions in the image and then set the size, like say 'X' pixels. And then I could do the photometry on the identified SF regions.

How do I find the Petrosian radius and proceed with using the SExtractor? Could someone tell me the inputs for this? Here are the default parameters.

Default configuration file for SExtractor 2.12.4

EB 2010-10-10


CATALOG_NAME FUV_BaF2_comp.txt # name of the output catalog CATALOG_TYPE ASCII_HEAD # NONE,ASCII,ASCII_HEAD, ASCII_SKYCAT, # ASCII_VOTABLE, FITS_1.0 or FITS_LDAC PARAMETERS_NAME default.param # name of the file containing catalog contents


DETECT_TYPE CCD # CCD (linear) or PHOTO (with gamma correction) DETECT_MINAREA 15 # min. # of pixels above threshold

DETECT_THRESH 5 # or , in mag.arcsec-2 ANALYSIS_THRESH 1.5 # or , in mag.arcsec-2

FILTER Y # apply filter for detection (Y or N)? FILTER_NAME gauss_4.0_7x7.conv # name of the file containing the filter

DEBLEND_NTHRESH 30 # Number of deblending sub-thresholds DEBLEND_MINCONT 0.18 # Minimum contrast parameter for deblending

CLEAN Y # Clean spurious detections? (Y or N)? CLEAN_PARAM 1.0 # Cleaning efficiency


WEIGHT_TYPE NONE # type of WEIGHTing: NONE, BACKGROUND, # MAP_RMS, MAP_VAR or MAP_WEIGHT WEIGHT_IMAGE weight.fits # weight-map filename


FLAG_IMAGE flag.fits # filename for an input FLAG-image FLAG_TYPE OR # flag pixel combination: OR, AND, MIN, MAX # or MOST


PHOT_APERTURES 5.0 # MAG_APER aperture diameter(s) in pixels PHOT_AUTOPARAMS 2.5, 1.5 # MAG_AUTO parameters: <Kron_fact>,<min_radius> PHOT_PETROPARAMS 2.0, 3.5 # MAG_PETRO parameters: <Petrosian_fact>, # <min_radius> PHOT_AUTOAPERS 0.0,0.0 # , minimum apertures # for MAG_AUTO and MAG_PETRO

SATUR_LEVEL 55000.0 # level (in ADUs) at which arises saturation SATUR_KEY SATURATE # keyword for saturation level (in ADUs)

MAG_ZEROPOINT 0.0 # magnitude zero-point MAG_GAMMA 4.0 # gamma of emulsion (for photographic scans) GAIN 1.00 # detector gain in e-/ADU GAIN_KEY GAIN # keyword for detector gain in e-/ADU PIXEL_SCALE 1.0 # size of pixel in arcsec (0=use FITS WCS info)

Star/Galaxy Separation

SEEING_FWHM 1.5 # stellar FWHM in arcsec STARNNW_NAME default.nnw # Neural-Network_Weight table filename


BACK_TYPE AUTO # AUTO or MANUAL BACK_VALUE 0.0 # Default background value in MANUAL mode BACK_SIZE 8 # Background mesh: or , BACK_FILTERSIZE 5 # Background filter: or ,

Check Image


Memory (change with caution!)

MEMORY_OBJSTACK 3000 # number of objects in stack MEMORY_PIXSTACK 300000 # number of pixels in stack MEMORY_BUFSIZE 1024 # number of lines in buffer


ASSOC_NAME sky.list # name of the ASCII file to ASSOCiate ASSOC_DATA 2,3,4 # columns of the data to replicate (0=all) ASSOC_PARAMS 2,3,4 # columns of xpos,ypos[,mag] ASSOC_RADIUS 2.0 # cross-matching radius (pixels) ASSOC_TYPE NEAREST # ASSOCiation method: FIRST, NEAREST, MEAN, # MAG_MEAN, SUM, MAG_SUM, MIN or MAX ASSOCSELEC_TYPE MATCHED # ASSOC selection type: ALL, MATCHED or -MATCHED


VERBOSE_TYPE NORMAL # can be QUIET, NORMAL or FULL HEADER_SUFFIX .head # Filename extension for additional headers WRITE_XML N # Write XML file (Y/N)? XML_NAME sex.xml # Filename for XML output XSL_URL file:///usr/local/share/sextractor/sextractor.xsl # Filename for XSL style-sheet

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    $\begingroup$ Why have you chosen/do you think that Sextractor is the right tool for the job? $\endgroup$
    – ProfRob
    Sep 20, 2023 at 17:05
  • $\begingroup$ Could you suggest any other tool which can be used instead of this? I know only this, so I used this. My supervisor also suggested this. Kindly let me know if there is any other better tool. $\endgroup$ Sep 21, 2023 at 15:02


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