Creates a DAOPHOT style PSF from specified stars in buffer im. The program will prompt for stars to use unless they are specified with the STARS= keyword. If this keyword is used, the stars will automatically be accepted (no user final verification) unless the INT keyword is also specified. The program will prompt for an input aperture magnitude file and an output PSF file name unless these have been previously specified with the DAOFILES MAG=magfile PSF=psffile command.
A DAOPHOT PSF consists of two parts: an analytic function and a lookup table of residuals to this function. Splitting the PSF in this fashion allows for more accurate interpolation and representation of the PSF to arbitary pixel centering. The analytic part of the PSF is numerically integrated to high accuracy at any desired pixel centering, and the residuals are numerically interpolated and then added to the analytic function.
The choice of analytic function is set using the IPSFMODE option. The default mode, IP=1, uses a Gaussian aligned along rows and columns of the image (but with different FWHM in rows and columns). Other values of IPSFMODE are still in test stage.
For use the the PSF fitting routines, it is often advantageous from the point of computing speed to precompute the pixel-integrated PSF at a grid of subpixel locations, and then interpolate within this grid. Such a grid can be created using the DAOLIB command, and then referenced during the PSF-fitting routine using the IPSFMODE=0 option.