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What's on this secondary?

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#51 Vic Menard

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Posted 17 June 2013 - 07:03 PM

...Well, the mirror values are objective.

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.

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.

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.

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.

...No. Measured data by definition cannot be subjective.

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.

...The Strehl of a mirror is the Strehl of that mirror (given a reasonable margin or error).

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!

...I don't know what an acceptable margin of error is, but I bet any good optician could tell us.

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.

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...

...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.

I have certain "rules of thumb" I adhere to, but, it's all subjective and dependent on many outside factors.

...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.

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.

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.

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...

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.

...Is it impossible that I have learned how to cool my big mirror down?

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.

#52 robininni

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Posted 17 June 2013 - 07:32 PM

Okay, Vic, thanks for hanging in with me. I appreciate you. I also changed the 'strong' word detest to dislike, but you were too quick on the draw and quote-replied before I submitted my change. :bow:

#53 Vic Menard

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Posted 17 June 2013 - 07:56 PM

Rob,
I do appreciate your dilemma.
Hopefully, your solution will provide a clear path from this unfortunate situation.
As Tippy D'Auria would say, "Soon it will be time, to commit astronomy!"
And then there's Plato, "He was a wise man who invented beer."

My best,

Vic Menard

#54 Joe G

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Posted 17 June 2013 - 09:02 PM

Rob,

I have been following your various threads from the beginning. As many have pointed out, there is not always a "perfect" answer to your questions. With all dew (astro joke) respect, please consider that you are getting many answers from some of the most respected amateur/professional telescope gurus in this field. Vic, Jason, Howie, Mike, Don, etc have posted literally thousands of helpful replies for many of us asking similar questions over and over.

As you are finding out, there are a myriad of factors that affect telescopes and their performance in the real world. As many have suggested enjoy your telescope(s) and keep asking questions. But lighten up a little. The folks responding in this thread (and your many others) mean no harm and have been very helpful in ramping you up the learning curve. Imagine what your learning curve would be like if there was no Cloudy Nights?

Also, there have been relatively few vendors that have not been given a hard time on CNs. There is no plot to protect any vendor at the expense of sharing knowledge, both good and bad. Personally, I believe that the majority of the folks responding in this thread are highly objective. Learn from them.

Three nights ago, I set up my 16 inch reflector for some friends on a weekend get away. I wanted to show them Saturn. My mirror is a refigured 16 inch Lightbridge. It performed much better after the refigure (based upon a side-by-side comparison with my 12 inch Orion reflector). Much to my surprise, Saturn looked okay. No one complained about it. But stars were oblong rectangles. Of course I was frustrated and tweaked the collimation with Catseye tools, my Glatter laser, etc. Nothing changed. I had a fan blowing on the mirror for hours.

The next day I loosened the mirror clips. Voila, later that evening it made a major difference. Without CNs, I would never have known what to do or what to expect. I would not have known where or how to get my mirror refigured or recoated. I would not have known that a Glatter laser is a huge improvement over an Orion laser. I would not have purchased Vic's fine book on collimation. Nor seen Jason's many descriptions and charts and how he improved the Catseye autocollimator.

Keep asking questions. But remember these people are just trying to help.

Joe

#55 okieav8r

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Posted 17 June 2013 - 10:01 PM

Rob,

Keep asking questions. But remember these people are just trying to help.

Joe


Also remember, that at some point, you just have to let go and have fun with it. After all, it was in interest in astronomy that got you into this endeavor in the first place.

#56 frito

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Posted 17 June 2013 - 10:23 PM

man you don't want to know about the pinholes you see in coatings on mirrors when you do this to them.

its normal, carry on enjoy the views.

optics are never perfect, glass is never perfect. we live in an imperfect world, scrutinizing things to this level will just drive you crazy.

#57 frito

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Posted 17 June 2013 - 10:40 PM

also Rob, i work in the glass industry.

specifically i run a ceramic ink jet printer that prints directly onto glass, we have the same problem with ceramic inks, a substance that is compatible with glass as its the same material and it is fused into the glass surface via heating the glass to high temperature via our tempering furnace. its extremely rare that i get a print on even small sample sized glass (12" x 12") that is completely free of pinholes.

the coating process used to put aluminum and many other materials onto the surface of glass can and will suffer the same problems if not worse ones than our process simply because those coating processes are not actually putting anything into the glass surface, they are simply layering on top of the surface via vacuum deposition

see http://en.wikipedia....tter_deposition for more info.

this process allows the coating of otherwise incompatible materials but the problem with it is durability. its durability is very low compared to a process that is fused into the glass like say our ceramic inks or pyrolytic process used in hard coat low-e's and other coating types like reflective coatings on architectural flat glass.

http://www.pilkingto...ucts/trade/t...

if it helps you at all to know this, minor surface defects like these effect the image extremely little, in total its probably a fraction of a percentage of light lost due to it and i would wager that one could search the world and never find a aluminum coated mirror that is free of a defect. its not worth loosing sleep over. just like in your brand new multi-thousand dollar heavy glass shower door you just bought has tons of surface defects known as pitts from the tempering process they must go through for safety requirements that the average person will never even see but if one is as picky as you are they will see them and raise holy hell over it and it has no real impact on anything ever.

#58 Vic Menard

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Posted 18 June 2013 - 09:30 AM

The advice that Rob was/is seeking was not unsolicited, and while some responses may have seemed unwarranted, in retrospect, his enthusiastic curiosity (and subsequent angst) is both genuine and "normal" considering the step up in aperture (10 to 25) and the unforeseen detours along the way.

Many of us have encouraged Rob to get in some serious observing time to get a better idea of his scope's performance. (Sometimes, not "knowing" can make a big difference, but in this case, Pandora's box has already been opened.)

Tom Clark is a good friend and an old observing buddy from Chiefland, Florida (he's since moved to New Mexico for even darker skies). I often describe Tom's observing method as "brute force" astronomy. His 42-inch f/4 GoTo Dobsonian is the largest scope I've looked through, and for the 10 years he had the scope in Chiefland, he invited an awful lot of folks to climb the movable stairwell to the eyepiece for the brilliant views of many deep sky "showcase" objects. For reference, "low" power (31mm Nagler and Paracorr = about 7mm exit pupil) on this scope is 158X (about 0.5-degree tfov). IIRC, Tom's pretty fond of his new ES20 (245X at 0.4-degree tfov--still less than 6X per inch of aperture). On their way to New Mexico, his 42-inch mirror and 7-inch m.a. secondary spent some time at Mike Lockwood's shop. The story is here.

This isn't intended to admonish Rob for expecting to receive what he thought he purchased! While Tom was pretty sure his optics weren't Mike Lockwood class, I think if he had known how bad they really were it could have spoiled those 10 years of observing fun in Chiefland.

Of course, now Tom's optics are Mike Lockwood class, and soon, the comparisons will begin. But the optics have a new home now too, so will Mike get all the credit, or will Tom attribute part of the performance to New Mexico skies? I know--we'll ask Jeannie! :waytogo:

#59 dave brock

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Posted 19 June 2013 - 01:15 AM

This isn't intended to admonish Rob for expecting to receive what he thought he purchased! While Tom was pretty sure his optics weren't Mike Lockwood class, I think if he had known how bad they really were it could have spoiled those 10 years of observing fun in Chiefland.


Maybe if he had done what Rob is doing he would have had the optics refigured ~ 10 years ago and would have enjoyed his scope even more over that time? :question:

Dave

#60 Vic Menard

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Posted 19 June 2013 - 08:00 AM

...Maybe if he had done what Rob is doing he would have had the optics refigured ~ 10 years ago and would have enjoyed his scope even more over that time? :question:

I'm not so sure it was possible back then--42-inch aperture optical work had limited takers--and that's not even considering pickup and delivery!

But as I commented above, let the comparisons begin...

#61 davidpitre

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Posted 19 June 2013 - 04:15 PM


Maybe if he had done what Rob is doing he would have had the optics refigured ~ 10 years ago and would have enjoyed his scope even more over that time? :question:

Dave

I'm guessing he would not have enjoyed his time spent any more than he did. I have a couple of Zambuto mirror scopes and a couple of so-so Chinese mirrors and some in between (Russian, Japanese). I generally enjoy my time with my lesser quality scopes just as much as the top notch.

#62 dave brock

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Posted 20 June 2013 - 12:09 AM

I generally enjoy my time with my lesser quality scopes just as much as the top notch.


So if for some reason you had to sell one of them, you'd toss a coin? :poke: :step:

Dave

#63 jpcannavo

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Posted 20 June 2013 - 05:32 AM

Rob
There is one more point worth making here. I understand that you are looking for some sort reasonably definitive input/data/content to judge the quality of said coating. Quite understandable. But part of the problem here is that while we bench test, star test - and "interferometize" mirrors to get analogous handles on their figures, we don't routinely make use of a parallel methodology to get a handle on the spectral reflectance of their coatings. As such, we judge by a combination of hard won experience, inference and leap of faith. There are however methods of direct measurement of "specular reflectance" out there: reflectance

Now to be clear, I am not saying that this is what you, I or anyone should/should not be looking into. I am just offering an observation about the state of the art. BTW, if you want to open another can of worms (that I myself have spent too much time with) start wondering about how much scattering there is from a coating. BTW, and as you likely know, the harder it is to see the laser on the secondary, the less light (ceteris paribus) it is scattering.






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