M36

I've seen 10 x 10 pixel size and 9.6 x 7.5 from different sources, which doesn't help.

Ahh I see, the values I was using was 9.6 x 7.4 microns

The reason the stars are somewhat bloated is that I have the IR filter removed. In the early days, it was adding too much crud to the image and I hadn't taken any flat frames. That's another next step

That would explain it alright. I'm thinking of getting a minus violet filer to aliminate those bloated stars

Focus is bang on though - if you're a little out of focus with the SCT, you end up with doughnuts.

Fair enough, although I've found with dslr focus and my C9.25 SCT that you can still be a tiny bit out of focus and not see doughnuts, you need to be a little further out of focus to see the doughnut.

With the IR filter, I'd expect even tighter stars. But with me going for nebulas next (I have a Lumicon UHC filter) I'm not sure I can or should use these in tandem....

I dont see why not, they both block different wavelenghts so it shouldnt be a problem. I'd consider a minus violet filter as a better alternative to a standard IR tho!

Anyway keep up the great work!