2.1.17 Aperture photometry ¶

The colors we calculated in Working with catalogs (estimating colors) used a different segmentation map for each object. This might not satisfy some science cases that need the flux within a fixed area/aperture. Fortunately Gnuastro’s modular programs make it very easy do this type of measurement (photometry). To do this, we can ignore the labeled images of NoiseChisel of Segment, we can just built our own labeled image! That labeled image can then be given to MakeCatalog

To generate the apertures catalog we will use Gnuastro’s MakeProfiles (see MakeProfiles). But first we need a list of positions (aperture photometry needs a-priori knowledge of your target positions). So we will first read the clump positions from the F160W catalog, then use AWK to set the other parameters of each profile to be a fixed circle of radius 5 pixels (recall that we want all apertures to have an identical size/area in this scenario).

$ rm *.fits *.txt
$ asttable cat/xdf-f160w.fits -hCLUMPS -cRA,DEC \
           | awk '!/^#/{print NR, $1, $2, 5, 5, 0, 0, 1, NR, 1}' \
           > apertures.txt
$ cat apertures.txt

We can now feed this catalog into MakeProfiles using the command below to build the apertures over the image. The most important option for this particular job is --mforflatpix, it tells MakeProfiles that the values in the magnitude column should be used for each pixel of a flat profile. Without it, MakeProfiles would build the profiles such that the sum of the pixels of each profile would have a magnitude (in log-scale) of the value given in that column (what you would expect when simulating a galaxy for example). See Invoking MakeProfiles for details on the options.

$ astmkprof apertures.txt --background=flat-ir/xdf-f160w.fits \
            --clearcanvas --replace --type=int16 --mforflatpix \
            --mode=wcs --output=apertures.fits

Open apertures.fits with a FITS image viewer (like SAO DS9) and look around at the circles placed over the targets. Also open the input image and Segment’s clumps image and compare them with the positions of these circles. Where the apertures overlap, you will notice that one label has replaced the other (because of the --replace option). In the future, MakeCatalog will be able to work with overlapping labels, but currently it does not. If you are interested, please join us in completing Gnuastro with added improvements like this (see task 14750 ⁴¹).

We can now feed the apertures.fits labeled image into MakeCatalog instead of Segment’s output as shown below. In comparison with the previous MakeCatalog call, you will notice that there is no more --clumpscat option, since there is no more separate “clump” image now, each aperture is treated as a separate “object”.

$ astmkcatalog apertures.fits -h1 --zeropoint=26.27 \
               --valuesfile=nc/xdf-f105w.fits \
               --ids --ra --dec --magnitude --sn \
               --output=cat/xdf-f105w-aper.fits

This catalog has the same number of rows as the catalog produced from clumps in Working with catalogs (estimating colors). Therefore similar to how we found colors, you can compare the aperture and clump magnitudes for example.

You can also change the filter name and zero point magnitudes and run this command again to have the fixed aperture magnitude in the F160W filter and measure colors on apertures.