Re: [ATM] convex secondary

[Mike Lockwood <melockwo@uiuc.edu>]

Jonathan,

Jonathan Lavoie wrote:
> I found a really complete wepbage describing many possible tests for convex
> surfaces.
> http://www.users.bigpond.com/PJIFL/page13.html

That's a nice page.  Thanks for the URL.

> Up to now in my quest for a good convex testing method here is what I have:
> 1-Interference testing: Seem really accurate and easy to interpret but added
> difficulty in figuring the hyperbola because in addition of getting the good
> K value, the RoC ALSO needs to match perfectly (if I understant correctly)

In my case, the test plates for the two Cass secondaries that I made 
had conic constants of between -3.5 and -4.  The focal ratio of the 
concave test plates were around F/4 to F/5.  In that range, the 
figuring of the test plates was still quite easy (for 4" and 5" 
diameter mirrors), and was accomplished in a few evenings.  (I figured 
them to the proper hyperbola.)  I used two different Couder masks with 
different zones to test.  When the results agreed for the two masks 
and and the figure looked good, I declared the test plates done.

If the secondary has a much stronger curve, then you will probably 
need to use the caustic test to figure the test plate.  What's the 
secondary diamter and ROC for the F/9 RC system?

My page describing the figuring of a 5" Cass. secondary using a test 
plate is at

   http://bi-staff.beckman.uiuc.edu/~melockwo/telescopes/
                  16in_CUAS_cass/secondary/secondary.html

You can see in the last fringe shot that I didn't match the ROC 
exactly (the fringes curve).  That picture was analyzed with FringeXP 
and most of the error was power (mismatch in ROC).  So, the hyperbola 
shape is quite good, but it just has a slightly different ROC than the 
test plate, and that's no problem at all.

	Mike Lockwood

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