Hi I'm not sure if anyone here has used ISIS image subtraction program written by Alard, available online at http://www2.iap.fr/users/alard/package.html, but I'm hoping someone has. I'm currently working on a bit of research that requires PSF matching to work, and this program should easily take care of that. However, I can't find any documentation other than his tutorial on how to use it. That would be fine, as I have it installed, but it fails during the second step of the tutorial (the ref.csh) on my data. Specifically, I get "Run error....Singular matrix in routine LUDCMP Goodbye ! " Here's hoping someone is able to help, it's driving me a bit insane. Thanks in advance!