I humbly disagree. I think you would be surprised how they are derived and how much they vary from one optician to another. I suspect this is the reason some of the best refuse to put a number on their finished product.
...Well, the mirror values are objective.
The number of texts describing the efforts to scrub boundary layers and rapidly equilibrate and track moving temperatures in these forums alone is enough to fill a text. And that's just dealing with thermals, not correction. There's much left to be written.
Mirror vs ambient? I was told with 2 degrees Fahrenheit was close enough. If the mirror is then close enough to ambient temperature, boundary layers don't form causing issues. And of equal quality, a smaller mirror never outperforms a larger mirror.
Similarly, a smaller, more responsive aperture may indeed outperform a larger aperture struggling with rapid temperature changes. As I mentioned earlier, more resolving power makes larger apertures better detectors of thermal instability.
Once again, it depends on how the data is derived, how it's being measured, how many data points are being used, how the "outlier"s are handled, etc.
...No. Measured data by definition cannot be subjective.
Even Zambuto would tell you it's fluid, and although he doesn't use an interferometer to quantify his work, Wolfgang Rohr (considered by some an expert with an interferometer) has found his work to be better than most, while showing wild disagreement with IF certified mirrors sent for evaluation. Changing the way a large, thin mirror is supported (for example, changing the altitude of the telescope), measurably changes the mirror surface--before cable slings and better edge supports, as much as a wave deformation!
...The Strehl of a mirror is the Strehl of that mirror (given a reasonable margin or error).
I bet most "good" opticians are reticent to have their work evaluated by someone else unless they have thoroughly vetted the tester, the equipment, the method, and the reliability. Even then, I think they would prefer having a "ringside seat" to make sure their metric is either supported, or closely in line with the results of an outside testing facility. If you're looking for a "bureau of standards" reference for mirror testing reliability, you're probably not going to be able to afford it. Similarly, optical flat testing can be performed with a reference sphere or a reference flat, which even then, is dependent on the reference accuracy and the number of data points, and here we go again.
...I don't know what an acceptable margin of error is, but I bet any good optician could tell us.
Don't get me wrong, there's a technological shift occurring with relatively inexpensive IFs (there's a thread in the ATM forum right now). All that data and number crunching can be a good thing or a bad thing...
I have certain "rules of thumb" I adhere to, but, it's all subjective and dependent on many outside factors.
...When it is safe to clean you new coatings should not be subjective. The coater should be able to tell us this. As, I have recently learned, when to *recoat* them is subjective depending on how much image degradation you are okay with. I would just pic your favorite optician or telescope manufacturer and go with what they say is their guideline.
...How do you know my scope has been thermally unsettled for all star tests?
I didn't say "for all star tests", but you described your first tests that way in this thread, IIRC.
Perhaps it's just atmospheric turbulence. As I mentioned in an earlier post, the scintillation you've described in your drawings is indicative of this.
I've had it at 1-2 degrees from ambient temperature the last few nights I've used it after I changed my cool-down routine. I've had similar star test results, slightly easier to read on the inside of focus, since doing so.
When I make a comment based on an assumption, I usually note that it is, indeed an assumption. But when someone calls me out and says they "detest" my commentary, I do my best not to take it personally. I have tried to offer my insight and I've tried to be supportive of your journey. If there was a way I could be there to help, I would very much like that.
Your statement exemplifies what I have come to detest on CN. Experienced (in your case well respected as well) people making statements put forth as a matter of fact which actually contain assumptions...
I don't know. When my 22 is reasonably stable and seeing is as perfect as I can hope for, Sirius at 550X with full aperture is a brilliant pinpoint with steady gray ribbons defining the diffraction pattern (not a flicker). On one occasion I observed the Pup embedded in one of these ribbons, with a dark minima around it. But I'm sorry to tell you that I don't know what the temperature differential was, or just how good the seeing actually was. It was an exceptionally steady night and a serendipitous alignment. I've had similar observing moments over the past several years (sparkling stars embedded in M82 and NGC4449, surface detail on Mars during the last opposition, following Porimma through periastron, both components of the Twin Lensing Quasar, etc.). Those observations are the data points I use to define my scope's performance.
...Is it impossible that I have learned how to cool my big mirror down?