Re: [ATM] astigmatism as seen via ronchi?

[Guy Brandenburg <gfbrandenburg@yahoo.com>]

The edges were machined, and the back is polished, and it's held in a beautiful belt that touches exactly 50% of the back when it's on the stand. The source is an LED that shoots out its rays about 5 mm below where I look through the ronchi screen; passing through the same screen.
I will try rotating the mirror. 
It's quite thin and large (3/4" and 16.5" D).
It's been cooling to ambient for months.


Don R Surles <Don.R.Surles-1@usa.dupont.com> wrote: You may also want to check the ronchi screen for plumbness...if the ronchi  screen is not very close to vertical then the lines will not be vertical  and they will change angles from inside to outside of focus.
Don...
Mike Lockwood  
Sent by: atm-bounces@atmlist.net
03/03/2008 12:43 PM
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Subject
Re: [ATM] astigmatism as seen via ronchi?
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



Guy Brandenburg, Washington, DC
My home page on astronomy, mathematics, education:
http://home.earthlink.net/~gfbranden/GFB_Home_Page.html
or else 
http://tinyurl.com/r6fh2

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