[Fwd: Re: ATM Autocollimation Testing via Flats]

[Roger Ceragioli <rogerc@as.arizona.edu>]

Hi, Mark.

I guess one thing you could do is to test with the flat in several 
different orientations (clocking angles) and look for systematic 
differences in the position of the two foci (tangential/sagittal).  If 
there is power error, the tangential/sagittal foci should remain 
constant in clocking angle on your test stand.  But for pure 3rd order 
astigmatism in the glass, the tangential/sagittal focal positions will 
clock around as you rotate your flat.  If you're diligent, you should be 
able to deduce the constant 1st order defocus term (i.e. power) and 
subtract it from the data.  What's left over--if consistent--should be 
(or could be) the 3rd order astigmatism in the glass.

BTW, so far I haven't seen any noticeable astigmatism in any of the 
dozen or so flats that I've made to date (4-12" dia.), including several 
elliptical diagonals which I've cut from circular disks (a la Tex in his 
chapter on diag. making).  Careful polishing procedures should guard 
against it, I'm guessing, in any reasonably (>16" dia.?) sized flats.

Hope this helps,
Roger Ceragioli

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