Quote:
Originally Posted by Satchmo
Sounds like you got lucky with the GSO , John . The point is that an individually crafted mirror ( hopefully) alleviates any doubt about the mirror quality. Mass produced mirrors are a bit of a lucky dip- more so on the larger sizes.
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Hi Mark,
Yes I did get very lucky with that 10"/F5 GSO mirror. What is sad is that I tried for ages to sell that scope with Argo Navis and a heap of other modifications for less than $1,000 and couldnt get a buyer. The Argo Navis, encoders and mounting hardware are worth nearly $1,000. David Collis-Bird actually said to me one night we were using the scope, "I don't know how the Chinese can make a mirror this good for this price". Star testing indicates that the mirror is probably a 1/7th to 1/8th wave mirror at the wavefront, it is a tiny fraction undercorrected which isn't such a bad thing. It has a good edge and is reasonably smooth. It doesn't quite have the smoothness level of my 3 premium mirrors, but its pretty good. It also doesn't cool as well as a thinner pyrex mirror, so it takes a bit longer to stabilise and deliver its best images.
As you correctly point out these mass produced mirrors are a bit of a lucky dip, but so are mirrors from a couple of supposed premium US mirror makers, that cost infinitely more money. I won't go there but you and I both know who we are talking about.
I have used quite a few of these mass produced mirrors and generally in the 6",8",10" and 12" sizes they are pretty decent, with a few being outstanding and a few lemons slipping through the cracks. I have found in the larger 14" and 16" sizes they are a lot more hit and miss. I have seen a few dud 16" GSO's and a couple of dud Synta 14" mirrors. That having been said Rick Petrie's 14" Synta mirror is excellent and as you know I have a good 14" mirror in my own stable to compare it to.
Cheers,
John B