From: John Swenson 
To: atm list 
Reply-To: John Swenson 


Gary, Bob et al, on MY astroscan when star testing I get severe spherical
aberation and a Ronchi test (scope still in tact, ie going through the
front "window") shows almost straight lines, they are slightly
bent near the edges. This is NOT an 1/8 wave parabolic or a properly done
schmidt newt. Testing the refelctions off the front glass it looks like a
pretty flat piece of glass.

As far as I can tell this is a pretty good f 4.2 spherical mirror with a
  window in front, which to me sounds like a prime candidate for
converting to a Houghton. Which was the question in the first place :)

I've already done parabolic mirrors, I wanted to try a Houghton and this
sounds like it might be an appropriate starting point. So the prime
question is, how thin can I make the lenses and what grinding and polishing
problems will I have if I make them thin?

I'm planning on building a small machine to do these (I'm working on a full
size M-O-M now, but I think I'll try a quick and dirty small machine to try
making these lenses).

How much difference is working with BK7 as opposed to plate glass? I can
get some plate glass blanks to practice on very cheaply and work out the
  techniques but will that apply to BK7?

Thanks for any input on this,

John S.

--- BBBS/NT v4.00 MP