Re: [ATM] Roddiers' curvature sensing

[Michael Peck <mpeck1@ix.netcom.com>]

At 13:17 5/8/05, matt wrote:


>Mike, while the theory clearly indicates what you said is doable, the French
>Project Roddier software is not doing what you are trying to do. It seems to
>assume both inside focus and outside focus of an object situated at infinity
>not center of curvature. Whatever results you get from the Roddier software
>you would need to further process to remove the error you introduced by
>testing at ROC.

Matt:

Dealing with your points in reverse order:

>Why not evaluate the software by using it in the manner it was intended,
>which would not introduce additional errors that are not clearly
>quantifiable at this stage.

As I said in my first post, I don't have a working telescope with known 
optical quality at the moment whereas I do have an uncoated mirror that's 
been thoroughly tested. I could test the method in the way it was intended, 
but I'd still end up with a disconnect between test results and reality. Of 
course I could always recoat my mirror, put it in a tube, and test it 
outside but that would take more time and effort than I want to expend 
right now. Also, I've idly contemplated the possibility of actually trying 
to make a *good* mirror out of it for the past few years, since it's one 
good figuring session away from being a decent mirror. I think I may need a 
few more years to think about that option.

As for your first point, the analysis presented by the Roddiers' is done in 
(exit) "pupil" space. With a source at the center of curvature of a mirror 
the exit pupil is the mirror itself, and the distance to the pupil is the 
radius of curvature. So I'm not sure the "cheat" of entering the RC in the 
focal length field is even a cheat. Now, if I thought it were worth the 
effort I'd spend the time to understand the math in detail and write my own 
software accounting properly for test geometry. I haven't convinced myself 
it's worth the effort yet.

Mike Peck


------
Michael Peck
mpeck1@ix.netcom.com

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