Using SExtractor for JWST data

Question

I am new to this tool for catalogue creation(SExtractor). I want to know about how to set parameters for the extraction for JWST data.

The parameter file looks like this.

#-------------------------------- Catalog ------------------------------------
 
CATALOG_NAME     SMACS0723_jwst.cat       # 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   5              # min. # of pixels above threshold
DETECT_THRESH    1.5            # <sigmas> or <threshold>,<ZP> in mag.arcsec-2
ANALYSIS_THRESH  1.5            # <sigmas> or <threshold>,<ZP> in mag.arcsec-2
 
FILTER           N              # apply filter for detection (Y or N)?
FILTER_NAME      default.conv   # name of the file containing the filter
 
DEBLEND_NTHRESH  32             # Number of deblending sub-thresholds
DEBLEND_MINCONT  0.005          # Minimum contrast parameter for deblending
 
CLEAN            Y              # Clean spurious detections? (Y or N)?
CLEAN_PARAM      1.0            # Cleaning efficiency
 
MASK_TYPE        CORRECT        # type of detection MASKing: can be one of
                                # NONE, BLANK or CORRECT
 
#------------------------------ Photometry -----------------------------------
 
PHOT_APERTURES   5              # MAG_APER aperture diameter(s) in pixels
PHOT_AUTOPARAMS  2.5, 3.5       # MAG_AUTO parameters: <Kron_fact>,<min_radius>
PHOT_PETROPARAMS 2.0, 3.5       # MAG_PETRO parameters: <Petrosian_fact>,
                                # <min_radius>
 
SATUR_LEVEL      50000.0        # 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             0.0            # 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      1.2            # stellar FWHM in arcsec
STARNNW_NAME     default.nnw    # Neural-Network_Weight table filename

I want to know how could i get these extraction values, specifically DETECT_THRESH and GAIN. Also suggest me some platforms where I could get interact with peers working on JWST.

Answer 1

I'm going to work up the config file from the bottom and suggest settings that need to be changed generally and then finish up with discussing DETECT_THRESH as that depends on what you're trying to do. First off, you should bear in mind that SExtractor was designed for optical CCD data from the ground through a blurring atmosphere (and that's where I'm familiar with it) and so JWST NIR data from space will be different.

First off, you will want to set SEEING_FWHM to something more appropriate for NIR space data. [This page] discuss the JWST NIRCam Point Spread Function (PSF) and Table 1 has expected FWHMs for the different filters; for the SMACS0723 data I looked at (MAST query results) and the F150W filter I downloaded, the FWHM value from Table 1 was 0.050 arcsec so this should go in SEEING_FWHM; adjust for the filter of your data as this is much more wavelength dependent than it will be from the ground. Note that the SEEING_FWHM is only used for the star/galaxy separation - things that look round to the size of the SEEING_FWHM are more likely to be classified as stars, extended objects that are larger than SEEING_FWHM and which are more elongated are more likely to be classified as a galaxy (it's a 0 (star) ... 1.0 (galaxy) scale). If you don't care about this, then this value isn't critical.

Next, PIXEL_SCALE should be set to 0 so it will read the WCS from the FITS header; the JWST instruments have complex and variable orientation and distortions and you're much better off having SExtractor figure this out itself. Now GAIN: this should be set to 1.0 as calibrated JWST data are in surface brightness units of MegaJanskys/steradian (MJy/sr as indicated by BUNIT1 in the FITS header) and not the counts, electrons or ADUs from CCDs that GAIN applies to. More information on the units and how to convert from MJy/sr to AB or Vega magnitudes is given here.

Finally, the actual source detection part... SExtractor works by running a filter (given by FILTER_NAME) over the image after a background mesh has been fitted looking for cluster of pixels that are larger than DETECT_MINAREA pixels and which are DETECT_THRESH times greater than the standard deviation of the "sky" in the fitted background mesh. For ground-based data, where the PSF is blurred to a near-Gaussian by the atmosphere, this filter makes sense. Given the multi-segment mirror and 6 spider spikes of the JWST PSF, this sort of Gaussian filter may not make sense (you can substitute different filters (see the SExtractor documentation) so a more WebbPSF-like may work better). The detection parameters of DETECT_MINAREA and DETECT_THRESH (always set ANALYSIS_THRESH to the same value as DETECT_THRESH otherwise Bad Things will happen) need to be tweaked and adjusted, SExtractor run and then the results overplotted over the image to get a good tradeoff between picking up the things you want and are interested in and junk/artifacts from the image. Since every dataset and science case is different, these will need to be experimented with and adjusted for the particular case - there is no "magic set" of parameters that will work for all data and science.