Quote:
Originally Posted by ngcles
Hi Bobby,
Yep the AOS mirrors were very frequently excellent performers and even those that weren't, were at least very good. Many were made by Mark Suchting (Satchmo here on IIS). I have never viewed with a Suchting mirror that wasn't an excellent performer. I own one myself -- a 31cm f/5.3.
I've looked through at least a couple of dozen GSO and Skywatcher mass-produced Dobson mounted Newtonians and most have been pretty good -- some were ball-tearers so far as optical quality is concerned. They do both tend to use slightly over-sized secondary mirrors. I did a test report on a Skywatcher 12" over a decade ago for AS&T and couldn't fault the optics on that one. It wasn't a hand-picked one either. I picked the boxes myself at random off a palette-load of similar boxed tube assemblies. Many of the earlier GSOs had plate glass mirrors which is an inferior substrate to Pyrex which is a low-expansion glass. I have seen one GSO so far that had a mirror I'd describe as being poor -- but they're pretty rare so far as I can see. I have visited the Skywatcher factory in China (while I was on an eclipse tour in 2009) and thought it was a pretty professional factory set-up.
A hand-made, individually manufactured mirror is more likely to be a top performer than a mass-produced one, but it likely to cost somewhat more as well. The Chinese made Newtonians, despite some shortcomings (particularly in the mounts and the bearings) are generally very good value for money.
Doppler wrote "Shine a torch down the tube and have a look at the mirror from the rear, you will see any damage to the coating like scratches etc. The coating is very thin so some light will show through even on a good mirror, but a bad coating will be quite noticeable."
Great advice. A few pin-holes are pretty normal but if you see a lot of them, or that the whole surface is gently glowing (a thin coating) the mirror may need a re-coat.
Best,
L.
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Les has given you some great advice. If you can find a Suchting mirror buy it, regardless of the coating. Just use the coating quality as a price bargaining point. Mark's mirrors are all exceptional and well worth getting re coated. The mass produced Taiwanese / Chinese mirrors from GSO and Synta are generally pretty decent. I have looked through well over 50 of these scopes over the years and I would assess them as follows. Of the 50 plus scopes I have looked through I have seen 2 ashtrays that I wouldn't waste my time looking through. About 1 out of 10 I would rate as ordinary, but it's going to have to do. About 6 out of 10 are pretty decent and happy to use them, 2 out of 10 are very good and 1 out of 10 are exceptionally good. I have a 10" F5 GSO dob that I have had for about 18 years that I leave at a friends rural property so I can use it when I go there without having to transport a scope. I also have a premium 10"/F5.3 SDM with Suching mirror, which is as good as it gets for a 10" scope. I got very lucky with the 10" GSO. It has an exceptional mirror for a mass produced scope. David Collis Bird bench tested it for me many years ago and his only comment was "I don't know how they can make a mirror this good for that price". Is it as good as my Suchting mirror ? Not quite, it's not quite as smooth so contrast under really good skies is slightly down, it has a touch more scatter and it is thicker and takes longer to cool. However, it takes good conditions with dark steady skies, good eyepieces and an experienced observer to pick a difference. The defining factor however is that 1 in 10 mass produced mirrors are this good and all of Mark's perform at the same level.
The other thing I should add is that generally what you observe as coating defects either by day, or under torchlight, don't have as noticeable an effect on optical performance under the stars as you would think they might have. My 14" Zambuto has plenty of pinholes in the coating and it still performs flawlessly. My 18" Obsession has essentially no coating left on the outside 1 cm of its entire circumference to go with its multitude of pinholes and it still performs fine. Its probably 5% to 8% or so down in its limiting magnitude threshhold and in its object brightness presentation, but because it deteriorates very slowly over time you don't notice it. In addition to that, laboratory tests have proven that the human eye cannot detect brightness changes <=5%
Cheers,
John B