Re: [ATM] Large Thin Mirror

["Bob May" <rmay@nethere.com>]

Give you an indication of how much a thin piece of glass bents.
Case in point is a pair of lenses that want a ROC to a few
fringes on their surfaces.  Glass is .75" thick at the thickest,
have a diameter of 13" and have a ROC of about 95" with one
concave and the other convex.  Doing a fringe test, the lower
piece of glass is supported at the 50% zone while the top glass
is supported at the edge.  There is a definite trefoil shape to
the "bullseye" with about 2 waves of apparent error.  Putting the
two pieces of glass together without the upper support produces
nice round fringes which will vary from about 1 wave with a foam
pad on the bottom to about 3 waves with either a central support
of the bottom glass or an edge support.  The 50% zone support
produces an interesting shape where there is a little bit of
trefoil to the fringes (more so than with the edge support) and
the fringes tend to indicate that there is a high zone in the 50%
zone.
Have fun!  You're going to have to really watch how you suport
the glass when you do testing or you will definitely be getting
bad results!
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: Mike Lockwood <melockwo@uiuc.edu>
To: <atm@atmlist.net>
Sent: Friday, May 09, 2008 2:04 PM
Subject: Re: [ATM] Large Thin Mirror


> Hi,
>
> Ted Cohen wrote:
> > Rotation of the glass during grinding is not in question. The
> > question you are raising addresses whether one can grind,
polish,
> > and figure on one type of surface (perhaps plywood, rubber,
foam
> > backing, carpeting, or pitch) and then take the finished
mirror,
> > mount it in a floatation mirror cell, with its different
dynamic
> > qualities, and expect the figure to be the same in both
cases. It
> > would seem that when grinding and figuring you would want the
glass
> > to have the same support characteristics as when the glass is
> > finally mounted.
>  > Am I wrong about this?
>
> Yes, you are wrong.
>
> The mirror needs to have the same figure when it is being
tested and
> when it is in its cell.
>
> The figure it has while sitting on the polishing machine really
> doesn't matter, and it is continually due to polishing and
heating of
> the glass.  The glass will bend while it is being polished, so
we
> require that a) the blank has the same physical characteristics
> (stiffness, thermal expansion, etc.) throughout the disk and b)
that
> it is evenly supported so that it will deform by similar
amounts
> during strokes across different diameters.  This maintains a
figure of
> revolution.
>
> Mike Lockwood
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>

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