Mirrors Wave-Rated

I always take interest when someone wave-rates mirrors from different makers and when searching the web, I found a source of note. Alan Rahill runs the clear sky clock which a very valuable source of information of sky conditions in many locations and it turns out he's also a mirror maker as well.
Best he's tested were Nova and Zambuto with the latter at better than 1/10 wave. Swayze at 1/5 wave "with micro-ripples". Galaxy at 1/3 wave. See link:
http://www.lcas-astronomy.org/articles/display.php?filename=quality_of_todays_newtonian_optics&category=general

Hi, Rod, et al.! For decades, most of my work responsibilities were Optical Testing. Our (very large) group was even called "Test and Technology Center", serving commercial and government programs.

Mostly interferometry, radiometry, alignments, scattered light, baffling, coatings, mirror supports, materials, and field support of instrument operations.

Right around 1/20-wave, the design, build and calibration of the Test Sets DOMINATED the difficulty of the task. Even factory-fresh interferometers (Zygo, WYKO, etc.) were certified to "only" 1/20-wave. They (very reasonably) assumed that the user would either use as-is (with that uncertainty affecting knowledge of the reported test wavefront), OR that he would calibrate the test cavity...

The Later, highly-desirable option, meant either having NBS-traceable references (typically concave spherical mirrors and flats), or doing absolute-cal in-house... an extraordinarily laborious procedure, developed by one of our own seasoned scientists (my boss!) So, you go to the NBS heritage... and they reference my boss!

Consequently, when I see "interferometrically tested" and superb numbers... I can't help but wonder about the traceability of the equipment. The computer will dutifully spit out all kinds of PV, RMS, Strehl, Zernikes, energy within a circle, impulse response, more. But that is always the combo of the tested article (e.g. telescope mirror), AND interferometer, nulling optics, mounting, etc. Even with laborious "back-outs" of the test-set, the residual uncertainties can be quite significant.

Yet, most houses seem to just USE the interferometer, it they're lucky enough to have one, and report the output. If asked, they proudly say they use a ~Brand Name~ interferometer.

Enter the challenge of parsing mirrors A, B, C... At small fractional-wave shoot-outs, the test environment dominates. Yet, many/most suppliers just flatly/proudly state 1/20-wave, ~diffraction-limited~ etc.; I can't help but wonder.

Even Hubble was delivered as... the most perfect telescope ever made.  Tom

Tom,
I like reading Wolfgang Rohr's interferometry site from time to time but I'm not privy to understanding all of the intricacies of his testing methodology.
Here's one of the latest reports on the relatively new skywatcher 20" synscan units. It appears to have a good primary at 1/7 wave with a strehl of 0.932:
https://translate.googleusercontent.com/translate_c?depth=1&hl=en&rev=search&rurl=translate.google.com&sl=de&sp=nmt4&u=http://r2.astro-foren.com/index.php/de/berichte/03-newton-systeme-und-verwandte-fragen/11-beitraege/03-newton-systeme-und-verwandte-fragen/771-c061b-skywatcher-stargate-500p-synscan-goto-dobson-508-1988-r-3976&xid=25657,15700021,15700124,15700149,15700168,15700186,15700189,15700201&usg=ALkJrhhqaagMvYXF-MMS2I8YAovDOzkU_A
Additionally, here are some older reports of what was manufactured  as a so-called "premium" mirror by its maker. I'll let the reader make up their own minds after reading Wolfgang's reports:
https://translate.googleusercontent.com/translate_c?depth=1&hl=en&rev=search&rurl=translate.google.com&sl=de&sp=nmt4&u=http://r2.astro-foren.com/index.php/de/11-beitraege/03-newton-systeme-und-verwandte-fragen/267-c070-peagsus-newton-320-1500&xid=25657,15700021,15700124,15700149,15700168,15700186,15700189,15700201&usg=ALkJrhg8xTq8i1ouNi5HZyDCWRp6bWG3sg

Hi All,

Anyone who takes the time to look through old Sky and Telescope magazines can witness the "wave wars" of multiple mirror producers. The producers came up with many creative ways to claim optical quality. One of my favorites is the +/- 1/8th wave claim. As in all industry, there are the good people who are honest with their claims and produce a good product and then there are the cheats, like those who sell flawed or average optics at premium prices hoping the end user will not have enough knowledge to notice.

As once stated decades ago, "there are good optics and there are cheap optics, but there are no good cheap optics." The Chinese seem to be bringing this statement into question with the latest quality I have witnessed. I have had some very good Chinese mirrors. As all experienced telescope users should know, price does not necessarily guarantee quality. I once had a guy get very angry (not at me) when his expensive 130mm APO refractor couldn't compete with my "inexpensive" home made 8" F/9 achromatic refractor on Saturn (at 1/3rd the price).

I have owned "certified" SCT's that were only average, some that were outright dogs, and some that were clearly exceptional yet some would state that they are all lunar/planetary dogs (like me in my ignorant, early, telescope years). What I see in the eyepiece will always tell me what I have in an optic set no mater what the claim. Interferometer not needed.

Some of my best optics:

Orion 8" F4.7 (Chinese)
Orion 10" F/5 (Chinese)
Orion 12" F/5 (Chinese)
Royce 10" F/8 (incredible optic)
Unknown 12.5" F/8 
Old Meade 6" F/8
Current C-14
Current C-8
Intes 7" F/15 Mak 
Intes 8" F/15 Mak
Cave 8" Cassegrain
Meade 10" F/6 (chipped back, picked up in their seconds sale at RTMC for 25.00)
Meade 178 and 127 ED Apo's
Cave 6" F/15 refractor

Keith (Scopejunkie)

Keith,
I agree the Chinese-made optics have generally improved overall. I've got an 8" f-6 and 10" f-5 that were professionally tested at about 1/6 wave with good edges. I bought these about ten or so years ago and they work quite well. 
Here's an interesting comparo between a 180mm Mewlon and a 180mm skywatcher mak and the later slightly outclasses it in the numbers game:
https://translate.googleusercontent.com/translate_c?depth=1&hl=en&rev=search&rurl=translate.google.com&sl=de&sp=nmt4&u=http://r2.astro-foren.com/index.php/de/12-beitraege/04-zweispiegel-systeme-astrofotografie/789-d069b-vergleich-180-mewlon-dall-kirkham-und-180-skywatcher-maksutov&xid=25657,15700021,15700124,15700149,15700168,15700186,15700191,15700201,15700205&usg=ALkJrhjCNwSdJRHATp6cmliYBDDdDWGAmg
Also, check out Rohr's personal scope, an USA Orion 12" with good optics:
https://translate.googleusercontent.com/translate_c?depth=1&hl=en&rev=search&rurl=translate.google.com&sl=de&sp=nmt4&u=http://r2.astro-foren.com/index.php/de/11-beitraege/03-newton-systeme-und-verwandte-fragen/747-c091-12-inch-orion-rohr-dobson-optisch-perfekt-mechanisch-fast-perfekt&xid=25657,15700021,15700124,15700149,15700168,15700186,15700191,15700201,15700205&usg=ALkJrhj0VsL_Ch0klw9DYibW5PZo3n083Q
So they can mass produce some good optics and if these and your examples are any indication, the trend appears to be in the positive direction. 
If I were in the market for optics, I'd consider purchasing a Chinese made mirror from a reputable vendor with a reasonable return policy with the understanding that I would have it professionally tested, preferably with interferometry, and subject for a refund if less than expected as per the tests.

Originally Posted by Rod Kaufman

I always take interest when someone wave-rates mirrors from different makers and when searching the web, I found a source of note. Alan Rahill runs the clear sky clock which a very valuable source of information of sky conditions in many locations and it turns out he's also a mirror maker as well.
Best he's tested were Nova and Zambuto with the latter at better than 1/10 wave. Swayze at 1/5 wave "with micro-ripples". Galaxy at 1/3 wave. See link:
http://www.lcas-astronomy.org/articles/display.php?filename=quality_of_todays_newtonian_optics&category=general

Thanks for posting that link Rod......I was pleasantly surprised to see what was said about Nova since my 20" f/5 is a Nova from back in the day when Obsession used Nova mirrors. Over the years I had come to the realization that I likely have an exceptionally good one. It star tests as a system very well and on those nights where the moon, planets, stars and atmosphere all seem to align and the mirror fully equalizes, the views and the power it will take seems almost unlimited ie crisp clear views upto 1000x. They don't happen often but when they do the Nova seems to rise to the occasion for some Ohhhh WOW experiences. 

Sadly as I recall from back then Obsession eventually stopped using Nova due to some consistency issues. I don't believe they have offered any mirrors commercially in a few years....just coating.

Originally Posted by Greg Shaffer

Thanks for posting that link Rod......I was pleasantly surprised to see what was said about Nova since my 20" f/5 is a Nova from back in the day when Obsession used Nova mirrors. Over the years I had come to the realization that I likely have an exceptionally good one. It star tests as a system very well and on those nights where the moon, planets, stars and atmosphere all seem to align and the mirror fully equalizes, the views and the power it will take seems almost unlimited ie crisp clear views upto 1000x. They don't happen often but when they do the Nova seems to rise to the occasion for some Ohhhh WOW experiences. 

Sadly as I recall from back then Obsession eventually stopped using Nova due to some consistency issues. I don't believe they have offered any mirrors commercially in a few years....just coating.

Greg,
I had an experienced observing buddy who used Nova optics in his 12.5" F6 scope and they were superior to my Discovery 12.5" f-5 truss unit. The Discovery model was good and another experienced observing buddy of mine bought it from me when I moved up to my 16" because it thru up a highly contrasty view of the planets. It may have been that the longer f-ratio and somewhat smaller central obstruction from the secondary factored into it but the Nova optics were top notch. I've observed thru other Nova-equipped scopes and they were excellent as well.
i don't see Nova offering new mirrors these days but the owner now provides refiguring services and very competitive pricing, especially if you don't require a regrind:
http://www.nova-optical.com/refigure.htm
I'd consider Nova or Ed Jones at optical Ed's for a redo of a mirror, if necessary:
https://opticaleds.com/optical-testing-and-mirror-refiguring-2/
I think if one has an underperforming Chinese-sourced mirror and the glass was good, either one would do quite well for a refigure...

For those who want an in-depth discussion of interferometry and an analysis of a sample of a commercially made 16"f4.5 mirror, most likely GSO, see here:
http://www.starryridge.com/interferometry/BathAnalysisV2.1.pdf
An additional description and synopsis of interferometry and other commonly utilized optical tests will be found here:
http://www.telescope-optics.net/testing_optical_quality.htm