For example, suppose you have 2 scopes of identical aperture (and preferable same brand and model). One tested .94 while the other tested .98 (again, just an example)
What I want to know is:
1. How much magnification was needed to see the difference? For example, did it take 400x to even begin to see the difference or was it obvious at just 150x?
2. Any difference in Lunar/planetary or deep-sky contrast?
3. Are the differences detectable by the casual observer or did it take an experienced amateur to see it?
4. Anything else?
The Strehl ratio is the ratio of the aberrated image intensity of a point source compared to the image intensity of a hypothetical perfect optic, with no aberrations. The math does not care what kind of aberrations are considered, since it is a comparison of measured vs ideal. SO, in a controlled laboratory setting the assumption is the that the only contribution to deviations from the ideal case are due to aberrations within the optics. This is why this measurement is PRIMARILY used by opticians when they are figuring the optics. However, in the field, atmospheric disturbances, tube currents etc.. will all conspire to drop your "observed" Strehl on a given night. If you have two scopes side by side that are different but close in optical quality, the quality difference will become more evident undergo better seeing conditions. Strehl is ALSO a function of wavelength. It is important to understand that Strehl is a SINGLE measurement that is a bit of a black box. It is possible to have a lens that has a >.9 Strehl in the green that drops off precipitously as you go into the red and blue. Most measurements provided by manufacturers are for 1 wavelength in the green part of the spectrum.
Personally, side by side, I can tell the difference between a scope with a measured Strehl in the green of ~0.94 from a mass produced scope of the same design (likely between 0.85 and 0.9 at a guess) on an poor day of seeing(~4-5 on the Pickering scale). The key difference is just a bit more pop in the contrast between the ice cap, planet and a dust storm on Mars. The 0.94 scope was also easier to get into focus. These differences were subtle in these conditions and I probably would not have noticed them if I had not had the scopes side by side. This of course may have been different on a better night of seeing, or if the difference was between say 0.94 and 0.98. It is all dependent on the conditions and the observer.
Whether or not you would detect a difference between a 0.94 and a 0.98 Strehl optic depends on several things including:
1. your experience as an observer
2. the quality of your seeing conditions
3. the target you are looking at
4. the magnification that you are using.
ASSUMING that the Strehl measurement for a single wavelength accurately captures the general optical quality for both optics across all variables, the differences would likely be subtle, best observed on targets like Jupiter and close double stars, and most noticeable in a side by side comparison.
I think that summarizes my own experience and what a number of very knowledgeable folks here have already said. Multiple times.
Sadly, a discussion like this ALWAYS gets convolved with economic considerations. I am sympathetic to the point of view that newbies spend WAY too much time worrying about getting the last % of optical quality in their purchase instead of purchasing an affordable telescope with the confidence that they will have fun. MOST refractors today using ED glass (and even a lot of achromats) are quite good and will show you much more than your eyes alone. Part of the issue is that people seeking information get into the middle of conversations between very experienced observers, who do not always provide detailed context for their choices and are debating very fine differences. That leaves many folks with the impression that they have to spend thousands of dollars on a refractor in order to not have huge flares of color or to be able to get a pleasing image. This is of course is silly.
On the other side, there is a vocal group of people who imply or directly state that anyone that pays the price for a Tak, AP telescope or a Televue, Nikon, Zeiss eyepiece are either foolish, or pretentious or have more money than sense since there is no discernible difference between one of those and mass production telescope or eyepiece. This is also clearly not true. Whether or not the differences are enough to justify a purchase is subjective and varies from individual to individual. People who are accomplished observers, appreciate the differences that they see, and have the cash to purchase premium brand optics are not fanatics or deluded. Honest folks can come to completely different conclusions regarding relative value based on their own expectations. The simple fact is that a high end refractor costs less than a motorcycle, a nice Bass boat, snowmobile or some hunting rifles. ALL hobbies can get expensive, and dollar for dollar, astronomy is actually quite cheap in my experience. If Astronomy is your main hobby and you want to have the best, go forward and God Bless.
In the end, the best way to determine relative value is to look through the scope and see if you like it. Barring that, price is probably the best indicator of quality for optics. That is not a satisfying answer, but it is what we have got to work with. If you can afford a Tak or an AP, you are curious about how good, good can be, and you will USE the scope consistently, then buy one and you will very likely not be disappointed. If you cannot, then buy what you can, use it and have fun knowing that your views and/or images exceed what was possible for the professional during most of Gallileo, Newton and Einstein's lives.
Finally, saying that there is a subtle difference between what you would see in optics with 0.94 and 0.98 strehl under good conditions is NOT the same thing as saying there is no meaningful difference and claiming otherwise oversimplifies things and can be misleading. I can acknowledge and appreciate the quality of views I have been privileged to observe from a modern Astrophysics scope and still be completely satisfied using my 80 mm Astrotech EDT that costs almost 1/5th the price for a comparable aperture. In the end, the hobby is about what is in the sky, not the gear. The universe is infinite and life is too short to spend much time worrying about these things.
Edited by Wildetelescope, 07 May 2021 - 12:26 PM.