Thanks Dave and Matt.
Yes I see what you mean Matt. I still prefer No 1 though. I wish I understood more about how these programs determine frame quality. Avistack, as far as I can see, doesn't give me as much opportunity to select sharp frames.

The stacked image from Avistack (before sharpening) was noticeably soft compared to what I got out of registax.
I could be totally wrong, but my impression is that Registax allows better selection of sharp detail but doesn't handle the variation in position due to seeing as well as Avistack, and Avistack handles the variation in position due to seeing quite well, but not the change in sharpness.

Then again it might just be that I don't know how to drive Avistack...
Al.