Re: [ATM] astigmatism as seen via ronchi?

[Mitch_Bisnauth@Cknainc.com]

ummmm....COG of the mirror from front edge = (t/2) - (0.707S) .... ??  any 
available derivation laying around that can be shared....my 
geometry/calculus is rusty...

MitchB




Dale Eason <atmpob@yahoo.com> 
Sent by: atm-bounces@atmlist.net
03/03/2008 07:36 PM

To
Guy Brandenburg <gfbrandenburg@yahoo.com>, atmlist <atm@atmlist.net>
cc

Subject
Re: [ATM] astigmatism as seen via ronchi?






As Mike already pointed out.  Yes that is a sign of
astig but next you have to figure out if it in the
mirror or caused by the test stand.

Belts and slings can cause a lot of astig when testing
on the bench.  There are many things that need to be
adjusted correctly for a sling to work well holding a
thin mirror. Touching at only 50% is not enought.  One
other is that the center of the belt must be on the
Center of Gravity of the edge of the mirror and it
must be there within a few millimeters.  The COG of
the mirror from the front edge can be calculated as
thickness/2 - .707 the depth of the sagitta. 

That mirror is thin enough to bend a lot (several
waves) on the test stand.  Mirrors like that are very
hard to support correctly on the test stand. 

As Mike suggested you need to rotate the mirror and
see how that changes the astig you can see with the
ronchi.  The problem with that is that you must get
the mirror support consistant each rotation.  I found
that very hard to do with a sling.

Dale Eason

--- Guy Brandenburg <gfbrandenburg@yahoo.com> wrote:

> 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
> To
> atmlist 
> cc
> 
> 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
> 
> =============================
> "Education isn't rocket science. It's much, much
> harder."
> (Author unknown)
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



 
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