Re: [ATM] convex secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Johnathan,

You might also consider making a 4" diameter concave test plate, and 
figuring the secondary to match it using interference testing.  This 
way you can avoid grinding/polishing the 16" mirror twice.

If the secondary's ROC is not too short, you can test the concave test 
plate very easily with the Focault test.  If it is too short, you may 
need to use the Caustic test.

I'm not sure which method would be easier, but I wanted to mention an 
alternative.  An RC will be challenging.

	Mike Lockwood


Jonathan Lavoie wrote:
> Thanks to Jim and Richard, I decided to refigure the 16 inch and go for 
> the Ritchey-Chrétien design. Since I have to repolish the big mirror, I 
> will be able to use it as a Hindle sphere for testing the secondary. I 
> know it will not illuminate the edge of the secondary but I have no 
> problem to shorten its RoC a bit to make the Hindle test then grind back 
> to the good RoC.
> 
> But as I understand it right now, the Hindle null test will give the 
> good K of eccentricity for a cass secondary that use a primary of K=-1 
> (parabola). In the RC design the primary will be corrected at K= -1.1
> 
> Now my question is: If I want to null test the secondary with the Hindle 
> test, can I just figure the Hindle mirror at K= -0.1 to compensate, then 
> do like I was making a cass secondary? If not (considering that the RoC 
> of the big mirror will be a tad shorter during the test than when it 
> will go back in the scope) is it possible to compute a good K of 
> deformation to get a perfect null ?

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