Re: ATM commercial mirrors -- are they getting better or not?

["B. Pepin" <atmjedit@conceptsnet.com>]

Guy you CAN share the name of that maker on-list I think, since we would all
like to know that a commercial mirror can be that good!


----- Original Message -----
From: Guy Brandenburg <gfbranden@earthlink.net>
To: <atm@shore.net>
Sent: Saturday, October 14, 2000 5:45 PM
Subject: ATM commercial mirrors -- are they getting better or not?


>
> Last night, at the NCA mirror-making club at American U in Washington,
> DC, we tested a commercial 8" f/6 Newtonian mirror that someone brought
> in, part of a brand-new common-name-brand equatorially mounted telescope
> he had just purchased. He wanted to know whether the optics were any
> good or not.
>
> I was surprised to see that it was actually quite good. The Ronchi lines
> were very smooth and uniform, showing no turned edge or other high or
> low zones anywhere. I had a Couder 4-zone mask that I had, and I did a
> Foucault test with it, and ran the numbers through Tex, (didn't copy the
> numbers, sorry) which pronounced it 1/8 wave. Jerry Schnall did a simple
> center-zone to edge-zone test, and found it to be about 60% or 80%
> corrected, I don't recall exactly.
>
> The only other commercial mirror I tested was about 5 or 6 years ago,
> and it was terrible. All sorts of wavey lines in the Ronchi test, and
> lots of zones etc visible in both the Ronchi test and the knife-edge
> test. Jerry said he's tested many dozens of commercial mirrors over the
> years, and the one we looked at last night was the best he'd ever seen.
>
> My question is, did our friend just get lucky, or are the big producers
> of medium-sized, mass-produced optics getting better? Jerry said he
> thinks they might be getting better because of increased competition. I
> don't know. I tend to think he simply got lucky.
>
> But if I could make a mirror that good without a lot of anguish and
> hours of refiguring, I would be very pleased indeed. And if the
> commercial outfits can routinely turn out something as good as the one
> we saw last night, then we amateurs are wasting our time. So -- is it
> luck, or a trend?
>
> Any comments?
>
> Guy Brandenburg
>