Focussing and Airy Disk

[Mr niall]

Hello all

Can't believe I'm asking this question three years down the road but chances are my stupidity is the problem not the scope.

I've moved from a 12" dob to a 60mm refractor. Its lots of fun. However I am struggling to focus at high powers. Or am I?

I'm using a 60/900 so with a 10mm (off brand plossl) eyepiece I'm getting 90x with an exit pupil of 0.7mm give or take so I'm probably not going to go much higher than that.

Jupiter and Saturn seem fine (well I say fine, they're a bit smushy and at this time of year with them being so low its difficult to know if "best focus is really in focus", Moon seems fine, great actually - but I'm struggling at achieve focus on point sources - like Vega or Deneb for example. I'm getting near focus and then diffraction rings start appearing or the whole "star surrounded by a dancing strobe light" type sensation. Racking the focus in and out reveals a nice array of diffraction rings that my limited knowledge can establish are roughly concentric and look ok. I'd come to the conclusion that I had a spherical aberration that was limiting my experience in what (lets be honest) is a very budget scope. Certainly the criteria for spherical aberration of "fine at low powers, won't focus at high powers" applies to me in this case.

But now I'm wondering if what I'm actually seeing is the Airy Disk and the scope is fine. Having a 12" dob I never really got anywhere near 1mm exit pupil on point sources (why bother?) so I'm wondering if its just something I've never encountered before; especially considering the generally rubbish atmospheric conditions of the last few days. Could that be it? Is this a typical (not feeling like you can focus to a point on bright stars at low exit pupils) experience?

Or is it just indicative of a poor scope?

Many thanks

Niall