Re: ATM 11.2" F/2 Primary testing questions :)

["James W. Burrows" <burrjaw@earthlink.net>]

At 15:39 2001-12-30 -0500, James Lerch wrote:

>Construction has begun on my 11.2" F/2 Dedicated CCD scope (CCD chip 
>resides where secondary mirror would normally be).
>
>#1  Just where do you measure the Roc on such a beast?  The difference in 
>ROC between zone 1 and Zone 7 will be nearly 0.325" (8.25mm)  on a moving 
>source tester.  With this fast of a primary, +/- 0.325" in ROC makes a 
>fairly substantial difference on the Strehl Ratio.  I "Assume" the roc 
>would be relative the Zone 1, but just wanted to make sure....

In general, you don't care what the ROC "really" is.  Just measure the 
distance from mirror vertex to (moving-source) KE with a tape 
(fixed-source, vertex to source, and vertex to KE) for one zone, then use 
increments for the other zones.  With Sixtests, enter those numbers; for 
other test reduction packages, close enough to call one of those 
measurements the "ROC".  Finding the best-fit parabola involves finding the 
ROC, which will depend on the tape measurement, but the resultant Strehl 
ratio doesn't.  In other words, if you input 11.2*4*25.4 = 1138 mm for the 
tape measurement or 1148, the ROC will be close either of those numbers, 
but the RMS (Strehl ratio) will be pretty much be unchanged.

>#2  With such a Fast Primary, I assume Foucault testing will be 
>unsuccessful.  If so, would the Wire test be more appropriate?  Does 
>anyone have a Web reference to the setup of a wire Tester?  If the Wire 
>test is still inappropriate, I guess that would leave me with the Hartmann 
>test or the Caustic test?  I've done a Hartmann test with good results on 
>more "Normal" mirrors, and I believe my current test platform will be 
>sufficient for the caustic test,  I'm just looking for some guidance on 
>which might be best/easiest ;)

I found while testing my 10"f/2.6 RC cass primary that, as everybody knows, 
Foucault was hopeless, and I don't think caustic was accurate enough.  I 
used FPF (Focal Point Foucault) (before the days of webCams), which 
required a lot of fiddling - mask hole pair readings averaged with single 
hole readings, etc.  It leaves wire test or Hartmann (BTW, my Hartmann 
program now asks for chip and pixel sizes; I'll upload it to my web site 
when I redo the help file).

         -- Jim Burrows
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