Re: [ATM] Basic mirror support question...

[Mauritz Andersson <mauritz.andersson@gmail.com>]

Dale,
Regarding mirror supported on sponge, I'll have to mildly disagree
(unless I misinterpret your statement) on the required flatness
accuracy of the supporting surface. The sponge acts to even out
irregularities. Consider a mirror supported on, say 9, soft pads at
same position as 9 point whiffletree. Let's say the pads are soft
enough to be compressed by 1 mm. Then if the surface under on of the
pads are 10 microns higher (some 20 lambda) that pad gives a force
which only deviates by 1 % from the others which is probably about the
accuracy one can expect from a well fabricated whiffletree. With most
reasonable mirror thicknesses that does not give deformations more
than fractions of a wavelength.

I think the mayor advantage of a whiffletree is that it is stiff
_at_the_same_time_ as giving uniform force i.e. the mirror does not
move down when pointing telescope upwards and in addition can give
better ventilation on the back.

See also the work on mirror flexing by Bill Kelley, Alan Adler which
essentially supports mirrors on sponges (in addtition to
parabolizing). (www.flexmirror.com)

Best regards,
Mauritz Andersson

On 08/02/06, Dale Eason <atmpob@yahoo.com> wrote:
> Some people wonder then if it would be alright to use
> a stiff flat surface and a spongy pad to assure that
> the mirror is supported at all the necessary points.
> Yes it would if the supporting flat plate and mirror
> back are flat to 1/8 wave and the spongy material has
> a uniform spring constant to 1/8 wave all across its
> surface. None of that is likely.  So we use wiffle
> trees or astatic supports to provide proper support
> were necessary.
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