Re: ATM Astroscan Houghton

[John Swenson <johnswenson1@attbi.com>]

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.