Posts Made By: Rod Kaufman

I usually clean my mirror once a year in the springtime and to so so, I follow the method recommended by optic waves lab:
https://www.youtube.com/watch?v=9Y8xFnXFVGQ
Of the various methods I've tried in the past, this works the best. My mirror is a 16" pyrex f4.5 and the enhanced coating by Nova optical is now 8 years old and shows no signs of deterioration despite many hours under the stars. I inspect the coating before and after cleaning by holding it up to the sun and by looking thru the back of the mirror for pin holes. Shinning a bright light in front of the mirror didn't reveal any defects, either.
I've enjoyed the views thru the scope which is an astrosystems structure. The mirror was refigured prior to the Nova coating by Ed Jones, a highly experienced optical engineer and I certainly recommend him for testing/refiguring optics:
https://opticaleds.com/
Happy viewing!

Thomas,
That's interesting. If you read that other amateur website, and I wouldn't fault you if you didn't, you'd see the head optics guru who can do no wrong chiming in on a thread on supposed mirror coating failure and stating he uses the same stripping agent as the guy in the video. I think the guy in the video has been doing it longer than he has...

If you watched the optic wave lab video on how to clean your mirror, you'll realize that a lot of the stuff you've read and heard about the "dangers" of cleaning your mirror are a whole lot of BS.
Now, over on that other amateur website, you'll read about the dangers of stripping it, too. Well, here's another optician, showing you how it's done:
https://www.youtube.com/watch?v=PdmwqrdDOMs
It's not rocket science, either...

Thomas,
Your "doll-house" concept is quite innovative. The design is highly efficient and user-friendly.
If I recall correctly, I seem to remember an article I read years ago wherein a technician used a "hand held reflectometer" from Cal-Tech to measure reflectivity of a new recoating of the 200" mirror at Mt Palomar. The image in the article showed a tech in a clean suit taking the small gizmo out of his pocket and taking a reading on the mirror surface. I'd sure like to have one of those but I don't even know where you'd find one and I'm assuming they cost a lot, too...

Measuring mirror coating reflectivity can be an expensive proposition especially if you use a commercially manufactured instrument.
I found this ultra-low cost portable method on the web. What do you think of it? Thomas, your input would be quite helpful here:
https://www.sites.google.com/site/photonadroplet/telescope-mirror-reflectivity-meter 

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

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 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

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