Well, Dom, I personally would find it quite relevant.
You see, my take since the beginning has been that enlarging a prime focus image X times (with an eyepiece, with a barlow, with whatever), does not produce an image identical (size apart) to the one natively that large at prime focus.
Your (and others) simulation -- trying to kindly explain that what I and others see is plainly wrong and/or nonexistent -- is based on formulas which calculates the Airy disks, and asserts that the two instruments should behave identically inasmuch as their respective Airy disks (both angular and linear) are calculated as perfectly matching after the enlargement.
Now, formulas are to be both descriptive and predictive. And, if valid, they are to be valid always, not only on odd days.
Mr James Smith, on the base of the results of your calculations -- supposedly describing what is exactly happening at the focus --, and given that the resulting linear Airy disks are calculated as being perfectly equal, is legitimately entitled to expect that -- given that the final Airy dimensions are now such to include not only the C-F defocus of the native f/30, but also the one of the native f/6 * 5x = f/30 equivalent -- his formerly-f/6 now-f/30 instrument should have the exact chromatic aberration level of the native f/30. Not similar, exact.
While nothing of this happens, the CA remaining exactly the one of the original f/6. Only with an image 5 times larger.
And this happens both in even and odd days; and even on Sundays.
Therefore, if the description of what happens at the focus is not accurate (neither descriptive nor predictive) as per the C-F defocus, I am sorry but Mr Smith is understandably entitled to think that such a description is equally not accurate (neither in the description nor in the prediction) as per the behaviour of the seeing-induced defocus, no matter how nice the formula looks.
And, given that your humble servant (and not only him) has also repeated constant empirical evidences that a highly corrected 5" f/6 (triplet apo) is showing a severely more nervous and prone to defocus image than the nearby equally highly-corrected 5" f/31 (achro), which instead remains almost motionless and undisturbed, he is also understandably entitled to agree with Mr Smith perplexity.
And you cannot dismiss Mr Smith simply by telling him that "the inability of a barlow to correct chromatic aberration in an achromat is common knowledge", because:
a) this should not happen, IF f/30 native and f/6 * 5x = f/30 equivalent have the same linear Airy disk at the focal plane, AND what happened at prime focus has no importance; and
b) Mr Smith could equally well reply you that "the insensitivity of a high f/ratio instrument vis-à-vis a lower f/ratio one is also common knowledge"...
Forgive me, but here there seems to be a little impasse: on the one hand we have a heuristic theory that seems to accomodate two repeatedly constant empirical evidences (seeing-induced defocus and C-F defocus as being different in different f/ratio instruments); and on the other hand, we have a formula that does not seem to accomodate neither.
So, maybe, instead of systematically obliterating and dismissing repeated empirical evidences because they do not match with a formula, we may think of a better descriptive formula and/or a more accurate and less simplistic modelling of the seeing behaviour vis-à-vis what is happening at a focal plane of the telescope? Just a thought.
I hope I was able to explain why I would consider my remark as relevant.
Max, first of all, no one is saying what you are seeing is wrong. We are saying the apparent insensitivity to seeing that you observe in long focus instruments is due to all of those reasons you checked in post #1, including one that you did not check (namely that the greater distance of the lens from the ground in long focus scopes helps improve the local seeing). We are saying that the apparent insensitivity to the seeing is not due to the greater depth of focus of the long focus instrument.
Also my "calculations" as you call them, where I stated that a Barlow will reproduce the variations in focus of the native focal length by an amount equal to the magnification of the Barlow can hardly be disputed. Please note that that variation is both spatial and temporal. That is, the magnitude of the focus variation is increased by the effective power of the Barlow and the focus variations occur, obviously, at the same rate, which means that the observed scintillations will be equally rapid.
Furthermore my "calculation" does not imply in any way that the use of the Barlow will correct the chromatic aberration to that of the new focal ratio provided by the Barlow. That apparently is something you believe, not I.
What you fail to realize is that not only will the Barlow magnify the Airy disc, it will also magnify the color blur. The color blur in a 6" f/6 achromatic refractor is 9 times the Airy disc, and it will be 9X the Airy disc with a Barlow inserted into the light path to magnify the image. Your claim asserts that the Barlow will magnify the Airy disc, but not the out-of-focus light, which, clearly is absurd. So Mr. Smith (who, I assume, is not a real person.) is not entitled to "legitimately expect that ... his formerly-f/6 now-f/30 instrument should have the exact chromatic aberration level of the native f/30." Quite frankly, Mr. Smith's expectation is just plain wrong.
Edited by daquad, 08 April 2021 - 08:37 AM.