Bob
):-{])) <---- madden@netcom.com madden@svpal.org Remember amateur astronomers: "keep looking for the next Universe"
---------- Forwarded message ---------- Date: Fri, 12 May 1995 16:27:20 +0700 From: Rik Hill <rhill@LPL.Arizona.EDU> To: madden@netcom.com Subject: Re: flat subjects
Hi Bob,
I would, and have made tools out of disks of glass (some plate retrieved from the local window shop) aluminum, iron, wood, plaster, and cement patio tiles. All worked provided they were sealed (except for the metals). For grinding I would usually get some plate disks the same size or old mirror blanks or lacking these, hard ceramic tiles on the tool substrate. Basically, the same thing you would do for a concave mirror. Just think of this as a concave mirror with a real-REAL long focal length...like thousands of feet.
I don't know what kind of glass you are starting with so I can't advise a grit. When I started with a piece of polished plate and used a machine I would only a 15 micron grit. But when starting with a rough Pyrex blank by hand, I would think about #200 carbo. It also depends on your rush. With my machine, which was set up so it could run for hours unattended, I would always start with a lesser grit to avoid micro- fractures. Time was not important.
I would check for flatness with several of the spherometers I have but actually, you would to just as well to get a quality straight edge and a bright light source and keep things so there are no visible gaps when the straght edge is placed on edge on the optical surface. This testing should go on with every grit size and you should not go on to the next grit unless the surface is flat by these tests. Finish with your 3-5 micron.
Make the pitch lap just like you would with a mirror. All the motions and strokes and speeds are individual to the machine. I ran my machines slow with weight at first and then as I neared completion I would reduce the weight. The speed I would not change but the stroke I would.
I picked up my knowledge from the ATM series (read them religiously when I got out of the Navy in '71), the old Maksutov Circulars (now in book form as Telescope Making Techniques), the book by Twyman already referenced to you and other atm books (Neal Howard, Allyn Thompson, etc.) Then I got a job where I was trained as a professional optician and now astronomy. I've had a damned good time!
Enjoy the weekend...I'm otta here.
-Rik