I am always sceptical when people brag about magnifications greater than 300x.***
Why does this bragging claim often come from people who own refractors?
***It is a fact of live that for us mere mortals atmospheric seeing for most of the time is stacked against us.
The exit pupil at high powers also works against us. We can certainly go well beyond 300x on some objects, like double stars, some small bright planetary nebula, and the moon. Mars, too. I've taken my scope up to about 100x per inch, but only looking at Arcturus in very good seeing. Not much to see up that high except a huge bright diffraction artifact. But, we can go up that high if we want, just not going to see much.
I ran across this interesting quote from Suiter, not sure what to make of it.
"Angular errors in the instrument are magnified until they are bigger than errors in the eye. Once the separation between the finest possible details have been magnified beyond 5 arcminutes (i.e., 1/6 the diameter of the Moon), aberrations in the telescope begin to dominate aberrations in the eye...Ironically, some people boast about telescopes that can "withstand more than 100 power/inch" (40 per cm). What they don't realize is that they're not bragging about the telescopes. They are inadvertently admitting the poor quality of their own visual acuities. When using extremely high magnifications beyond 100/inch, the diffraction disk appears bigger than two-thirds the angular diameter of the full Moon." Ref: 1st Edition, chapter 13.3. Medium-Scale Roughness, or Primary Ripple page 241
I've certainly been that high, but only on a star. Not viewing generally. But 0.5mm exit pupil (50x/inch) is very common (in my MCT) and not unheard of when seeing permits. Interestingly, in my 8" Newt under seeing fluctuating around Pickering 5, I was able to hold the moon at 300x with blurring in only the worst moments. I was stunned at how sharp the image remained most of the time even though the tiniest detail for the aperture was not resolved due to seeing. I did not have to back off magnification in what I consider mediocre seeing for the tropics. That's cool.
I have long given up upon the intra- and extra- focal (Suiter) star test. It cannot be done (and everyone who thinks he can star test his telescope in the field and derive some numbers is under an illusion). It is only useful for rough collimation checks or maybe giving a first guess of a possible coma.
What counts is the focused star and this can only be tested interferometrically.
You're right we cannot put a number to it with any confidence, especially PV error or Strehl. You need data and computers for that. I would not be too hard on the star test, though, it does take practice, good seeing, and some understanding. You can make some educated guesses as to whether it's good or bad and approximately this or that level of presumably smooth SA (and be dead wrong about the actual PV error). I might agree, sometimes simulations are difficult to read and compare. (The MCT star test confused me for a long time, and still kind of does. It really scared me when I removed the baffle.) But, we can overcome that by understanding what's going on with what we see. But, you're right, it is hard if not impossible to quantify it accurately. At best, it's a wag.
For example, I am quite pleased with my Newt in that the obstruction shadows do not differ significantly either side of focus with a 33% obstruction at 10 waves defocus. There is some obvious under correction seen at 8 and 10 waves defocus, but it does not /look/ severe. Especially taking the shadows into account. Best I can tell, no significant roughness (when cooled in reasonably good seeing) or at least not worse than the seeing at the time. Thankfully, no indications of a significant turned edge, even though it may well have a slight one - up or down. I could not discern astigmatism at higher power even though there is likely some amount, and I see no glaring zones even though they likely exist to some degree. There is some coma, but it is a f/6 Newt after all and fairly well collimated. Not concerned about it.
I do not need to know the PV wavefront error or Strehl, I just have to see something that is not obviously concerning. The mirrors are certainly not premium, but they /appear/ pretty good. Good enough, best I can tell as a caveat. It approximates something around 1/6 LSA (according to Suiter's LSA approximation of the shadow ratio and the /perceived/ ring brightness) even though SA is certainly is not the only aberration present. The rest, outside of coma, are just not readily visible - to me, anyway. So, I do not concern myself with them. As I said above, the moon looks surprisingly good. I'm happy...so far. That's all I need to know.
Sorry for rambling, just chatting...scope is cooling and the Pup awaits.
Edited by Asbytec, 20 January 2019 - 07:39 AM.