Re: Nxt Proj. :SCT, shd be 8"compound.

PFAFF2@delphi.com
Tue, 04 Jul 1995 01:32:42 -0400 (EDT)

>> >Uh - oh ...
>
> >The Schmidt Cassegrain is one of most difficult designs to propely
> >execute, and also among the ones with smallest tolleraces. I'd suggest
> >that you play with a good ray tracing software first and see for
> >yourself how sensitive the thing is before spending more time on
> >it........
>
> >.......I'm not trying to scare anyone here. It _IS_ within the grasp
> >of an amateur to make a SCT. It is only far less likely that you will
> >make a GOOD one.
>
> As usuall, Bratislav, you are so succinct........
>
> I should of said "next project: 8" compound planatary scope with a 9"
> Bk-7 optical window." Since my first post, I've reread my references
> (Texereau and Makintosh), spent a couple of hours after work scanning
> microfilm archive of S&T at the county library, and realize this is not
> going to be easy, but that's part of the point. My use of the TLA SCT
> was only ment in the generic sense, in post respect. But this is what
> I figure I have going:
>
>
> >Figuring a Schmidt corrector plate couldn't be further from "straight
> >forward". If you thinking about vacuum method, I can't comment, I
> >haven't done it myself that way, but even with that you'll have to
> >produce almost perfect optical window before - NOT a trivial task,
> >especially with thin plates involved.
>
> Well, since I didn't necessarily mean Schmidt, maybe this is a mute
> point. But I do have a vaccume pump at work, and it'd be easy enough to
> make a vaccume fixture.
> Believe me, the parellelism is not an issue, we control thickness on
> 12.5" blocks to between .0001 to .0002 over 12".
>
>
> Bill "masochist" Marriott
> Forest Knolls, Ca. USA
> btk@ix.netcom.com

Go to it Bill: I have made two corrector plates ( 11 inch and a 16 inch ) for my Schmidt cameras. They were easier to make then a good paraboloid and a lot more fun. I used the vacuum method with a water filled vacuum pan. As for vacuum lung power will do just fine. As for wedge I have read that anything under .001 is ok. My surplus 11 inch BK7 filter glass had about .0002" wedge and my plate glass 16 inch plate had .0004" wedge out of the box. Dont worry about glass surface flatness if you put 1/2 correction on each side, it will be ground out and polished anyway.

ATM's seem to be afraid to make them. We asked Buchroeder about a Wright-Jones Bird design. He thought were out of our mine. Any ATM with some experience and access to a machine shop to make the vacuum pan should have no problem. I will be happy to answer any question if I can.

So many telescopes so little time Bob Pfaff