Thank you so much-I've been looking for something that would do EXACTLY what this does for a long time now. Incredibly useful as I technically could do it by hand, but have not the stomach for such calculations, nor indeed the implementation.
EDIT: Oh, and as for the PS-GIMP changeover, I did it myself four or five years ago. I used to be a page editor on my high school newspaper and we used adobe products there, so I was used to PS (and InDesign, mostly). When I graduated I had to transition to GIMP because I'm a poor college student, and the transition was remarkably smooth. I won't say that I like the GIMP any better than PS, but I've certainly become very comfy with it. I've actually used GIMP to touch up some pretty pictures I've taken on big research telescopes after my science observing is over, and it plays nicely with .fits files (special astronomy image format), so I was extremely pleased with that bit of extra functionality. Really what I'm trying to say is that there's no reason to be scared of it. Also, it's free, so that's good.