Re: [ATM] astigmatism as seen via ronchi?

[Mike Lockwood <melockwo@uiuc.edu>]

Guy,

Guy Brandenburg wrote:
> If you see the lines on a more-or-less spherical mirror in a ronchi
>  view as looking more or less like this:  //////   when you are 
> outside of the center of curvature, and then more or less like 
> this:  \\\\\ when you are inside the center of curvature (or vice 
> versa), does that mean you have really bad astigmatism?

Maybe.  It means that, as the mirror sits on the test stand, the test 
is showing astigmatism on an axis that is not parallel or 
perpendicular to the knife edge.

This could be due to the mirror's figure, cooling of the mirror during 
testing, the test stand's support of the mirror, or possibly even a 
widely separated knife-edge and source.

If the mirror is thick/small, the most likely culprit is the mirror. 
If not, it could be any of those causes.

The first thing to do is rotate the mirror on the test stand and see 
if the lines respond as function of the mirror's rotation angle.  That 
is, when the mirror is rotated to angle A, do the lines always look 
the same?  If this is true for two or more trials (rotating the mirror 
on the stand repeatedly), there is a good chance that the mirror is at 
fault.

HOWEVER, if the edge of the mirror blank is not machined, or the back 
is not flat, those physical imperfections could cause uneven support 
as a function of mirror rotation, because gravity causes the mirror 
and stand to interact.  This could cause the appearance that he 
astigmatism is in the mirror.

Isn't testing fun?

	Mike Lockwood

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