III. 3. C. Picking the PSF stars, and making the PSF: pickem3var.cl

After much fiddling, we concluded that for the 4-m Mosaic data that the PSF is variable even within the boundary of a single chip. Tests, using a large aperture as the reference, convinced us that using a variable PSF is the right then to do. So, we need a lot of good PSF stars, and we need them well distributed over the chip.

We found the psfselect routine to be inadequate for several reasons. First, and foremost, it has no way to distinguish two blended stars that were not detected as such by daofind. So, instead we use the aperture corrections to determine if a single star is really single.

The script begins by dumping the ids, x and y positions, magnitude and error from the *.mag.1 file into a tempoary file, "temppsf". The FORTRAN program phil2select then goes through this file, selecting only stars which are truely well isolated based upon the photometry list. These are put in the temporary file "pospst". After exiting from the FORTRAN code, the script then calls fwhmed.f, which compares the fwhm's of these stars to that determined earlier by character3.cl, and throws out any that are 15% larger or smaller. The script then runs phot on these stars, using both a small and large aperture (3.5 and 15 pixels, with a sky annulus extending from 18 to 26 pixels. A second FORTRAN mode2pick finds the modal aperture correction in this bunch, and leaves the value sitting in a file "temppst4". The FORTRAN code then selects stars which have aperture corrections within 0.03 mag of the mode. Upon exiting the FORTRAN code, the script then reads the aperture correction out of temppst4 and stores it in the header as "apcor", and then proceeds to build a point spread function using the "pst-like" list "temppst3", generated by mode2pick. Up to 100 stars are used. Since the list has been sorted by magnitude, stars from all around the frame are hopefully used.

In late-breaking news, if not enough stars are found for a good variable PSF a constant PSF is determined. If there's not enough stars for that, then the file is listed in "FAILED" and the routine moves on....