Separate the stars from the galaxies:

For all weak lensing methods, useful stars need to be identified and separated from galaxies in order to determine the PSF of the atmosphere and telescope at the time the image was taken. 

Stellar selection is probably the most important step of the analysis and you will need to balance two requirements:

1. •You need a large enough number of stars to sample the variations in the PSF across the field of view
   

2. •You don’t want to include too many faint stars which just add noise to your final PSF model
   

Finding the balance is a case of trial and error and depends on the telescope.  If you PSF does not vary rapidly you should opt for fewer bright stars.  However if the telescope is known to have small-scale variations in the PSF, for example from observations of stellar clusters, you need as many stars as possible to model this.

The Size-Magnitude Locus is the most commonly used method and is implemented in KSBf90/findstars.f90.  

Taking your SExtractor catalogue, findstars.f90 plots the half-light radius (flux_radius from SExtractor) vs magnitude.  Use the cursor to zoom in on the stellar branch (type ‘l’ for look) and select the region (type ‘s’ for select).

I find it is more reliable to use two size estimates.  Move on to the FWHM vs Mag size plane (type ‘n’ for next) and select the stellar locus again (type ‘s’ for select).  Don’t forget to save your selection when you exit (type ‘x’ to save and exit, not ‘q’ for quit without saving).

Running findstars.f90 from the command line:

findstars.a -in SExtractor.cat -out star_criteria.dat -param KSBf90.param

-in : Your SExtractor catalogue

-out:  The output file from this code which contains your stellar selection criteria

-param:  The name and location of your parameter file

Page maintained by Catherine Heymans, IfA, University of Edinburgh