Hi, I'm a bit new to the forums, this is my first post in fact, but I didn't know where else to go for help on this kind of matter. 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 ! " I think it has something to do with the stamping process, since it says the sigma and median of my image are both zero before this, and that the number of stamps is 0. Here's hoping someone is able to help, it's driving me a bit insane. Thanks in advance!