Refractor focus issue / max magnification.

[Mr niall]

I'm struggling to reach focus on both my 60/700 and 60/900. 

Witha 10mm eyepiece I'm getting 70x and 90x respectively. If this is too much for the scope then that's no problem - but I'm used to bigger apertures so was slightly surprised that sharp focus isn't really achievable. 

Whether I'm using the bundled 10mm MA or a slightly better 10mm Plossl I'm experiencing a situation where it approaches focus then starts to drift away again as it gets close, usually with a fuzzy airy disk or - on planets, like a smush smear almost around the edges. 

Have replaced the diagonal but not experienced an improvement. I'll admit the diffraction test confused me a little: if I reach focus and keep going I get a series of perfect sharply defined concentric circles, however when I come back from focus and keep unwinding I don't really get anything but a big old circle, it's such a different experience on both sides of focus its really thrown me. 

This has been repeated on several stars over several nights.

Also - anything over 100x gives some fairly serious CA which is a big surprise for long focal length scopes. But I appreciate that I'm pushing the envelope a bit. 

Any insight appreciated. 

[Rob Sellent]

The 'smush smear' you saw could just boil down to the scopes having to push through more atmosphere on a night of less than good seeing.

You were also using 0.8mm and 0.6mm exit pupil respectively and in my experience that kind of power often requires decent seeing. That's another possible reason to what you experienced .

Regarding the star test, as Roland Christen of Astro-Physics put it, "Every Apo lens I have ever star tested, even ones that tested close to 1/20 wave P-V, shows different inside and outside patterns of interference. This is normal. By contrast, I have an 8" SCT that shows perfectly identical inside vs. outside diffraction patterns, but tests only 1/4 wave."

I figure, then, that if you can get some decent views working around a general maximum of x1 mag per mm aperture, your scopes are doing just fine.  

 

Edited September 7, 2019 by Rob Sellent
didn't even inspire a response of any kind, so I thought perhaps the original post was a bit long winded :-)