That’s true for any procedure, protocol, or technique ever used to measuring anything.
I don’t think everyone considers the possible variability in testing when they are told or promised a certain level of quality. If someone buys a scope and it comes with a printout of an interferometer test saying it is an optic with a Strehl of 0.98 most people will consider it accurate. Maybe it is, maybe it isn’t.
In the case of the AstroTech AT-102EDL claiming a Strehl better than 0.95, I certainly do believe it to be accurate. Why? Because the integrity and reputation of Astronomics is second to none! If it were a document given with a scope sold on eBay I would have less confidence.
My main statement to consider is this...
If the ED and EDL objectives were exactly the same, made with the same glass elements, made from the same batch etc. and so on, one may conclude that the lenses that exhibit a Strehl of 0.95 or greater were used in the EDL scopes, and the remainder, likely less than 0.95 Strehl, went to the ED scopes.
But, the lenses are NOT the same and had to be made independent of each other. They have different types of ED glass. So the conclusion would have to be that the lenses in the less expensive ED version are of a quality only known by the manufacturer. Perhaps they are spot checked as someone suggested. But due to the lesser cost, they are not guaranteed to have a specific level of quality.
One cannot assume, or should not assume that the objectives used in the AT-102ED are automatically noticeably inferior, regarding figure, to the AT-102EDL, simply based on the cost difference. The EDL uses FPL-53 glass. It has a better lens cell. It has a better focuser - I’ve read these things and have no reason to dispute them. So right there along with the guarantee of 0.95 Strehl are reasons for a price difference.
Okay? The EDL version is a better scope than the ED version. No argument. It’s worth the extra money. Would I have spent the extra money for the EDL? Yes, if I had the extra money to spend. But I didn’t. If I had unlimited funds for this hobby, I might have purchased a Takahashi or TeleVue or TEC or a used AstroPhysics in the 100mm class. But it’s a moot point.
All I’m saying, as a result of my experience with observing with telescopes since the 1960s, and as a result of the observations of others who have the ED scope, is that the objective lens is one of quality, giving very good views with a low level of visual chromatic aberration, giving very good views at higher magnifications and giving very good views with a noticeably high level of contrast. That’s all I really care about. For my cost, and I bought the AT-102ED used, I feel I have a really good scope, not lacking in optical performance.
Is it optically as good as an AT-102EDL or like size TeleVue, Tak, Tec, AP scope? I don’t know and I don’t/can’t care. It’s what I can afford and it’s good enough for me.
Furthermore, if I set it up next to another brand 102 size scope and I felt the optical performance in my scope was obviously better than the other scope, I would not say so to the owner or say so in a post. Someone spent their hard earned money on the scope and they don’t want to hear it isn’t as good. It’s just the way I approach things. If they are happy with it and use it with enjoyment I think that’s great!
But here on CN, a wonderful site for astronomy and telescopes, many seem to constantly compare this vs that. “My _______ scope beat up the _________ scope and left it in the dust!” Often if think this site should have been named CN, for Comparisons ad Nauseam.
So that’s my story, gwlee. If you want to comb through all this nonsense, pick out a sentence, quote it and dispute it, that’s fine. I was often told to avoid this vs. that topics especially in the refractor and eyepiece forums. I’m going to try and follow that advice and leave the daunting task of assessing relative quality to others.