Re: [ATM] Large Thin Mirror

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

A fringe is 1/2 wavelength.  Not quite as bad as a whole wave
but still quite bad.
The top piece of glass does have an airspace between it and the
bottom piece of glass but the bottom piece is only supported at 3
points which is where the basic problem exists.  There should be
a much better support for that bottom glass for best results.
In addition, one test was done with spacers between the two
pieces of glass so that they wouldn't tend to scratch if there
was some little piece of dirt between and that produced some real
errors in the shape as reported.
Bob May

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

----- Original Message -----
From: Ted Cohen <tcohen@blakeglobal.com>
To: <atm@atmlist.net>
Sent: Friday, May 09, 2008 5:00 PM
Subject: Re: [ATM] Large Thin Mirror


> Very Interesting. Those are huge errors if each fringe is a
wavelength. Note
> that by using a convex and a concave lens together means there
is minimal
> overall thickness gradient of the two pieces - gravitation is
acting equally
> at all radii - unlike the case of a single parabolic piece of
glass which is
> thinner in the middle and thicker around the edges. Also, I
assume the glass
> is laying flat so as not to be tilted. Thanks Bob for the
interesting data.
> I have no doubt this is a challenging issue.
>
> Ted.
>
>
> -----Original Message-----
> From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]
On Behalf Of
> Bob May
> Sent: Friday, May 09, 2008 4:36 PM
> To: atm@atmlist.net
> Subject: Re: [ATM] Large Thin Mirror
>
> 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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