The quality of the lens you get will be the determining factor and what you can do with it and how far you can push it.
High powers are definitely not a problem for my classic 6 and 5 inch Jaeger's f5 refractors, other than a small exit pupil and the limited eye relief that usually comes with short fl eyepieces. And CA of course.
Your biggest obstacle is likely to be chromatic aberration. The best solution I found is the Baader LP495 long pass filter. It gives some color, but the sharp cutoff wipes almost all of the Violet more effectively than any other filter I've used. The only exception is one of my Jaegers which has has some red that cleans up better with a Semi APO filter IIRC.
The CA is easiest to see slightly out-of-focus, one side has an obvious violet smearing. It also shows up very easily with daytime testing on distant tree leaves and branches and even power lines. It doesn't really bother me as such but it does definitely reduce contrast and sharpness.
Skywatchers can be very good, particularly in these days of computerized manufacturing. There is no getting away from the CA in achromatic designs, however.
If you can't set up a DPAC, research extra focal star tests (which I find difficult to interpret at times, in part due to a fair amount of unclear information on the web), but if inside and outside focus patterns are a match that's a good sign. And in good seeing, the final test is always on a star in focus.
Oddly enough, I've always found a daytime snap to focus test as pretty telling. The best Scopes do not require any hunting, and the point of focus is very definite and jumps out at you.
I'd pick up a good quality Ronchi tester. The Gerd Neumann looks like the best quality you can buy at the moment, but you may have to look around to get one. Just search the name with Ronchi or tester added. I believe his website in Germany has a reasonable amount of interpretation information IIRC. You can make one, but the last time I checked getting a decent ronchi screen was a little difficult.
Optcorp usually stocks them in the US. I did not check our host, astronomics, for what they sell along this line, and it is absolutely the first stop to make.
A Ronchi is relatively easy to do a basic interpretation on a refractor since you are looking for straight lines of even width, top to bottom. The DPAC doubles the obviousness of any error, but a bare Ronchi still will give you a quick and reasonable test of the objective, enough to decide whether to send it back and get another one.
Swapping OTAs is no one's favorite thing to do, but with a reputable brand you should have a reasonably the chance of getting a decent lens if your testing shows one to be unacceptable.
this is one of the weaknesses of the used market, as many people dump their poor-performing Scopes without warning anyone. I've had two reversed element refractors, and two allegedly excellent performers that were merely very good. I would have been fine, but it was not what I was looking for at the time. A Ronchi test confirmed my visual impression.
It's almost impossible to get a single scope to do anything you want, but a very sharp short tube can get more done than many people might think.
Add to that problems that come with a long tube 6 inch refractor, and a short tube looks better all the time. My f8 was a beast to transport, even within the house, that was a pain to get mounted stably due to the length and torque arm that comes with a heavy cell.
Edited by markb, 30 May 2020 - 01:31 PM.