<p>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?</p>
<p>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.</p>
<p>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.</p>
<h2>Default configuration file for SExtractor 2.12.4</h2>
<h2>EB 2010-10-10</h2>
<h1></h1>
<h2>Catalog</h2>
<p>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</p>
<h2>Extraction</h2>
<p>DETECT_TYPE      CCD            # CCD (linear) or PHOTO (with gamma correction)
DETECT_MINAREA   15              # min. # of pixels above threshold</p>
<p>DETECT_THRESH    5            #  or , in mag.arcsec-2
ANALYSIS_THRESH  1.5            #  or , in mag.arcsec-2</p>
<p>FILTER           Y              # apply filter for detection (Y or N)?
FILTER_NAME      gauss_4.0_7x7.conv   # name of the file containing the filter</p>
<p>DEBLEND_NTHRESH  30             # Number of deblending sub-thresholds
DEBLEND_MINCONT  0.18          # Minimum contrast parameter for deblending</p>
<p>CLEAN            Y              # Clean spurious detections? (Y or N)?
CLEAN_PARAM      1.0            # Cleaning efficiency</p>
<h2>WEIGHTing</h2>
<p>WEIGHT_TYPE      NONE           # type of WEIGHTing: NONE, BACKGROUND,
# MAP_RMS, MAP_VAR or MAP_WEIGHT
WEIGHT_IMAGE     weight.fits    # weight-map filename</p>
<h2>FLAGging</h2>
<p>FLAG_IMAGE       flag.fits      # filename for an input FLAG-image
FLAG_TYPE        OR             # flag pixel combination: OR, AND, MIN, MAX
# or MOST</p>
<h2>Photometry</h2>
<p>PHOT_APERTURES   5.0              # MAG_APER aperture diameter(s) in pixels
PHOT_AUTOPARAMS  2.5, 1.5       # MAG_AUTO parameters: &lt;Kron_fact&gt;,&lt;min_radius&gt;
PHOT_PETROPARAMS 2.0, 3.5       # MAG_PETRO parameters: &lt;Petrosian_fact&gt;,
# &lt;min_radius&gt;
PHOT_AUTOAPERS   0.0,0.0        # , minimum apertures
# for MAG_AUTO and MAG_PETRO</p>
<p>SATUR_LEVEL      55000.0        # level (in ADUs) at which arises saturation
SATUR_KEY        SATURATE       # keyword for saturation level (in ADUs)</p>
<p>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)</p>
<h2>Star/Galaxy Separation</h2>
<p>SEEING_FWHM      1.5            # stellar FWHM in arcsec
STARNNW_NAME     default.nnw    # Neural-Network_Weight table filename</p>
<h2>Background</h2>
<p>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 ,</p>
<h2>Check Image</h2>
<p>CHECKIMAGE_TYPE  BACKGROUND,-OBJECTS           # can be NONE, BACKGROUND, BACKGROUND_RMS,
# MINIBACKGROUND, MINIBACK_RMS, -BACKGROUND,
# FILTERED, OBJECTS, -OBJECTS, SEGMENTATION,
# or APERTURES
CHECKIMAGE_NAME  Background_FUV_BaF2_comp.fits, Subtracted_FUV_BaF2_comp.fits     # Filename for the check-image</p>
<h2>Memory (change with caution!)</h2>
<p>MEMORY_OBJSTACK  3000           # number of objects in stack
MEMORY_PIXSTACK  300000         # number of pixels in stack
MEMORY_BUFSIZE   1024           # number of lines in buffer</p>
<h2>ASSOCiation</h2>
<p>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</p>
<h2>Miscellaneous</h2>
<p>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</p>

CC BY-SA 4.0
2023-09-20T17:05:51.850Z
Why have you chosen/do you think that Sextractor is the right tool for the job?
ProfRob
CC BY-SA 4.0
2023-09-21T15:02:57.573Z
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.
Abhinna Sundar
<observational-astronomy><galaxy><star-formation><photometry>
SExtractor parameters for identifying Star Forming regions
Abhinna Sundar
# SExtractor parameters for identifying Star Forming regions

(Asked by: username_0 on 2023-09-20)

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

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

## Extraction

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

## WEIGHTing

WEIGHT\_TYPE NONE # type of WEIGHTing: NONE, BACKGROUND,
# MAP\_RMS, MAP\_VAR or MAP\_WEIGHT
WEIGHT\_IMAGE weight.fits # weight-map filename

## FLAGging

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

## Photometry

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

## Background

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

CHECKIMAGE\_TYPE BACKGROUND,-OBJECTS # can be NONE, BACKGROUND, BACKGROUND\_RMS,
# MINIBACKGROUND, MINIBACK\_RMS, -BACKGROUND,
# FILTERED, OBJECTS, -OBJECTS, SEGMENTATION,
# or APERTURES
CHECKIMAGE\_NAME Background\_FUV\_BaF2\_comp.fits, Subtracted\_FUV\_BaF2\_comp.fits # Filename for the 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

## ASSOCiation

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

## Miscellaneous

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

**Tags:** observational-astronomy, galaxy, star-formation, photometry

> **Comments:**
> **username_1**: Why have you chosen/do you think that Sextractor is the right tool for the job?  
> **username_0**: 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.  

---

## 0 answers

