Re: [ATM] Re:How much of cell induced deformation is too much?

["john sherman" <atm@mail.johnspics.com>]

Hi Vlad!

>I haven't noticed that what I wrote implies that one can't see the 
>difference of nearly 10% in the Strehl. Maybe you can enlighten me? 

Sure, I'll try. You said:

>In regard to cell induced surface error, it is important, but shouldn't 
>be taken to the extreme. Taking into account error contributions from the 
>atmosphere, local turbulence, thermal imbalances, miscollimation, 
>c.obstruction, TE, roughness, etc. 0.80 Strehl seems to be out of the 
>reach. For apertures ~16" and larger, seeing alone will take care of it 
>nearly 100% of the time. Considering this, an attempt to preserve 
>non-existent near-perfect optical quality by nearly elliminating cell 
>induced error seems pointless. 

Which I (no doubt wrongly) interpreted to mean: Any amount of quality way over 80% Strehl ratio is not worth going for in a large scope, because it is unobservable. It would be a waste of time and effort to improve the scope much past this point, since the atmosphere etc ruins the image anyway. You mentioned that a final Strehl of 70% would be a very good result. Is that about what you meant?

Unless I'm somehow mistaken (it has happened), Nils Olof has been saying for a long time now that the Rayleigh 1/4-wave/front* criterion (~80%Sr), according to Rayleigh himself, is just barely good enough! It is not the difference between perfect and not-perfect. It is the difference between okay and junk! Based on my twenty years of experience, I agree!

I think a telescope needs to be way better than 80% Strehl ratio to produce a great image. After all, an 80% Sr means that one-fifth of the light goes away and destroys the image. That's a lot of badly placed light. And the other four-fifths is no longer concentrated into the center of the Airy disc, it is now spread out, making the disc look bigger. You can see the differences between large mirrors even when the seeing does not allow a view of the Airy pattern. Even though we live in an atmosphere, it is possible to see the difference between a great scope and a superb scope on a regular basis. I say it IS worthwhile to strive for a superb surface and figure on a large mirror, and to mount it as well as you can. It is possible to buy or make a large mirror with a Strehl in the high 90's. It is very desirable to own one of these mirrors. Because you can see the difference. We should address those problems you mentioned and minimize them, as you said. But I think that we can !
 achieve a final wavefront error much better than 80%, and I'm sure we are achieving well over 90%. 

I have been saying these things for lots of years, and no one has been able to change my mind. Of course, I cannot prove anything, so I haven't been able to change many minds, either. I have been waiting a long time for a mirror-maker like Kennedy Optics to come along and make large-aperture mirrors good enough to prove my point. And now that he's here, I expect that he will be a legend among y'all in just a couple of years. Please remember, you heard it here on the atm-list first.

But hey, it is common for folks to not agree with what I say. I have stuck my neck out many times and stated things contrary to experts, marketing gurus, and common book knowledge. Many times I have asked people to come over and compare scopes, but most decline. Some do come over, and so far no one has been able to convince me that I'm wrong. Folks often go through life thinking this or that, based on years of experience. But then one night a single view through a superb scope rocks their world, and changes their perception. It only takes one view. I know, cuz it happened to me.

>"non-existent near-perfect optical quality" 

Near-perfection does exist in large apertures, as I pointed out yesterday. And I'm sure that many people on this list have made some awesome big mirrors. Perhaps you are referring to some other scale or aspect?


You also said:
>In order to meaningfully star-test this size telescopes, you need to use 
>800x-1000x magnification.

Star testing a large aperture does NOT always require 800X to 1000X to be meaningful. Do you know, at less than 200X you can tell the difference between a good 22" and a not-so-good 22" mirror? It is true, and I know that from personal experience, and because I have given demonstrations which convinced other people. For example, I could PROVE my old 22" mirror (which I lived with for 5 years) was a bad mirror with only 185X! (in and out of focus) And from there you could raise the magnification as much as you liked, in the best of conditions, and the result did not change. It just looked worse.

Not all star tests are done extra-focal. You don't NEED high-power extra-focal patterns; IN-focus is plenty good enough. Just try to focus Vega, for instance.  My new 22" mirror focuses it down into a MUCH smaller knot than my old 22" mirror ever did. And that is the decisive point!  IN-focus. And the OLD 22" was supposedly better than 85% Strehl ratio, according to Hudek's interferometry. Yet it was a mushy mirror, stars were blobs. I don't care WHAT propaganda he says on his website.. 

The medium-power in-focus test is especially apparent on very faint stars. For example, how many of the faint stars can you see around the Trapezium? A slightly out-of-focus star will not even be seen when it's that faint (especially in front of the bright nebula). You have to have a mirror that does a real good job of focusing to see the maximum number of stars. I will be glad to compare anybody's scope to mine <he he he>. Does your scope show red and blue in the Trifid Nebula? And does it show color on the moon? Mine does. And M-13 in a large, sharp mirror is something to behold.

Yes, I believe in high-power extra-focal diffraction patterns; but I USE my scope IN focus. So that is what is important to me. People are always talking about the "star test, this and that". I say, "Hey, put it IN focus and see what it does!"


>As soon as you do some reading.  ....
>
>, you wouldn't take it as an insult to your telescope when I mention 
>facts about its actual field quality. 

I am tired of reading books, but which ones do you recommend? I will eventually try to get to them. Anyways, I would MUCH rather have you do your field tests of my scope right now. I promise that neither I nor my scope will be insulted by any honest evaluations. The scope itself is not precision-made, and will suffer a lot of criticism. But the image (I firmly believe) will blow you away. I think you would come away from the experience as a believer in very high Strehl ratios on large mirrors (of course, on small mirrors the figure is even more important).

I am getting a small group together for a September new-moon observing session. Would that be a convenient time for you? I know New Jersey is far away, but at least you could drive or take a train and not have to worry about any international borders. I have a secret, personal site here in south/central California which I will be bringing some observers to this year. There should be enough room for you, and a couple of other dedicated, serious people on this atm list with large scopes to compare. (any takers out there?) September provides two potential weekends, so if the first one dies the other might be good. The monsoon determines. May be October.

If your beliefs are cherished, and you do not want them shattered, then I recommend you stay home. But if you are willing to risk crashing headlong into a new reality, then please come over in September and try to convince me that I'm wrong. (and pray to your personal deity that the seeing happens to justify the trip..)


Folks, I'm sorry about the ranting and raving. But I know many of you BUY your mirrors (arrrrghh, I can hear the moaning and groaning already!! But how many of you purists make your own glass,  aluminum, plywood, secondaries, and so forth? [Stan, Jerry, James...]).  After 5 years of living with a bad mirror, and finally having a good mirror again, I can't help the temptation to spread the news. Could you?

Please realize, Vladimir, I do not say these things because I think you are dumb. I do it because I think you are worth it!! I have been reading, enjoying, and learning from your posts since you began making them. Most of them are over my head, but I try...



[Flames humbly accepted.]


John




 *   I often write "wave/front" as an abbreviation for "wave error at the wavefront". Most of you will think I'm silly....





 . . . Richard -- where are you?? Are you 

the only person on earth busy making his own pitch?








And who is mining his own cerium oxide?









































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