Hi Bobby,
Quote:
Originally Posted by Bobbyoutback
I liked what NGC-LES had to say , top information .
The best scope was an old Astro Optical 8" F/7 , I pushed it to the theoretical max on those few nights when the seeing close to perfect , saw the pup next to Sirius for the first time & doubles at there limit .
Cheers
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.