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Re: [ATM] convex secondary

At 2005-03-03 08:32 -0500, Jonathan Lavoie wrote:

>Bad news about my idea of shortening the 16" RoC to use it in a Hindle 
>test then grind it back to it's good focal length: I forgot that it's 
>current RoC is twice the F length so if I want to use it as a Hindle there 
>is a lot of glass that will be removed. So this idea becomes impraticable.
>
>I heard of another test for convex but can't remember where. The idea was 
>to test from the back through a liquid of similar refraction index that is 
>in contact with the back surface, and just do standard foucault 
>measurement. Anyone heard of it?

Let me offer two possibilities (since you certainly wouldn't want to hack 
up the primary to make a Hindle sphere):

1)  I'd guess (no experience here) the best way to do a through-the-back 
test is to buy a bi-convex or plano-convex lens of the proper diameter and 
refigure one face to the proper profile.  In order to program Foucault test 
reduction, you'd have to know the radius of the back surface and the lens' 
index of refraction.

2)  I have a 10" coated Hindle sphere with a 62 mm hole and R = 522 mm 
currently being used as a (very nice) shaving mirror.  Run a quicky back of 
the envelope ray trace and see if it would be suitable for testing your 
proposed secondary - if so, I'm willing to loan it to you (not for too long 
- I'm in the process of refiguring my 10" RC cass).  The Hindle test can be 
run non-null, again by fiddling with the Foucault reduction, so the Hindle 
sphere's R doesn't have to match one of the hyperbolic secondary's 
radii.  In fact, I think a non-null test is better for figuring because it 
tells you where the deviations are, while a null test, like a star test, 
just says something ain't quite right.

         -- Jim Burrows
         -- mailto://burrjaw@earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84) 

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