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Re: [ATM] astigmatism as seen via ronchi?
Sorry I did not specify the origin correctly it was
the back of the mirror.
The derivation is with no sagitta it would be 1/2 the
thickness. The mass removed by the curve is about
.707 the sagitta. Thus the cog is .707 behind the 1/2
thicnesss.
Dale
--- Mitch_Bisnauth@Cknainc.com wrote:
> 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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