making parabolic mirror with resin

I'm thinking of making a parabolic mirror with a resin by spinning in
a dish. Has anyone tried this method? Also, is there a method of
silvering a resin?

Thanks,

jbc

Jonathan B-C wrote:
I'm thinking of making a parabolic mirror with a resin by spinning in
a dish. Has anyone tried this method? Also, is there a method of
silvering a resin?

Thanks,

jbc

How do you propose to determine your f ratio?
How will you polish and figure a resin blank?

Looks like you have your work cut out for you.

Regards,
Etok


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On 13 Oct 2003 18:13:35 -0700, (Jonathan B-C) wrote:

I'm thinking of making a parabolic mirror with a resin by spinning in
a dish. Has anyone tried this method? Also, is there a method of
silvering a resin?

The resin won't have the right shape after spinning, just a close approximation.
That means you still need to figure it, and shaping an polishing any polymer is
going to be somewhere between very difficult and impossible.

That said, if you make a resin mirror this way you can aluminize it the same way
glass mirrors are aluminized. You should also be able to chemically silver it
the way people did with mirrors years ago (the useful life of silver coatings is
quite short, however).

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Johnathan

Is then an excersize to see if you can do this? If not you would be
much better off purchasing a pyrex blank or even a plate glass blank
and grinding your own..

Rich


On 13 Oct 2003 18:13:35 -0700, (Jonathan B-C) wrote:

I'm thinking of making a parabolic mirror with a resin by spinning in
a dish. Has anyone tried this method? Also, is there a method of
silvering a resin?

Thanks,

jbc

I'm thinking of making a parabolic mirror with a resin by spinning in
a dish. Has anyone tried this method? Also, is there a method of
silvering a resin?

Have you made any calculations as to the deflections of the mirror under
gravitational loading? Glass mirrors need to have diameter to thickness ratios
of better than 12:1 to avoid deformation by gravity as the mirror is moved.

Since most polymers are some what less dense than glass but are about 1/10th as
stiff, this will be but one of your problems.

in my view, the biggest problem will be to get a surface that is accurate to a
fraction of a wavelength of light, something on the order of maybe 1/10,000 of
a mm or so. Given that a human hair is about 1/10 of a mm, this is a big job.

Jon

Chris L Peterson wrote in message

The resin won't have the right shape after spinning

Do you mean because of shrinkage when it cures (certainly a
consideration) or because the fundamental shape of the surface of a
spinning liquid is wrong?

If the latter, then those spinning mercury mirrors must have incorrect
shape too. They clearly work fine (albeit only at zenith). Since
there's no way to figure the surface of a liquid - how is their shape
corrected for?

A special "correcting" secondary perhaps?

Cheers
Beats

On 14 Oct 2003 08:30:26 -0700, (justbeats) wrote:

Do you mean because of shrinkage when it cures (certainly a
consideration) or because the fundamental shape of the surface of a
spinning liquid is wrong?

The theoretical shape of a spinning liquid is paraboloidal. The problem is that
the resin isn't a well behaved material like analytical grade mercury. The final
shape will be wrong because the resin won't cure with absolute uniformity. There
will be local variations in density which will end up as high or low points,
damaging the figure. There will also be variations that couple in from bearing
vibration, air currents, etc.

Consider the fact that large glass mirrors are routinely made by spinning the
melt, and the resulting blank still needs to be figured and polished (and glass
is a much more stable material than polymers).

BTW, the best spinning mercury mirrors have RMS wavefront errors around 1/3 wave
and P-P errors around 2 wave. Not bad, but not as good as you can get with a
figured glass mirror.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Thanks all for the replies. Sorry I didn't respond until now...I'm
not looking to make a telescope-quality mirror. It sounds dumb, but
I'm trying to replicate an experiment in optical illusion I saw back
in freshman physics. The illusion is a 3-D image formed by a pair of
parabolic mirrors, one on top of other, facing each other. An object
would be placed in this mirrored cavity formed by the parabolics, and
an image of the object would appear to float just on top of the top
mirror. I'm sure many of you know this illusion. I just wanted to
see if I can make a pair of mirrors for myself quickly and do the same
trick. Do you think that I need to make high quality mirrors for this
purpose? As for silvering, I remember seeing a professor (way back
when I was in junior high) would had chemically silverd a microscope
slides--I'm not sure if he chemically deposited silver or aluminum,
though. Can you chemically deposit aluminum onto a glass or resin?

Thanks,

jbc

The illusion is a 3-D image formed by a pair of
parabolic mirrors, one on top of other, facing each other. An object
would be placed in this mirrored cavity formed by the parabolics, and
an image of the object would appear to float just on top of the top
mirror.

Not familiar with this illusion but I would think that two magnifying makeup
mirrors such as sold in drug stores might work nicely. They are something
under $20 each and have a nice folding stand. The ones I have seen are
definitely spherical in that they magnify significantly, seem to have a
diameter of about 6 inches.

This would be the cheap way to go and if they didn't work you could return them
and get your money back.

jon isaacs

Jonathan B-C wrote:
Thanks all for the replies. Sorry I didn't respond until now...I'm
not looking to make a telescope-quality mirror. It sounds dumb, but
I'm trying to replicate an experiment in optical illusion I saw back
in freshman physics. The illusion is a 3-D image formed by a pair of
parabolic mirrors, one on top of other, facing each other. An object
would be placed in this mirrored cavity formed by the parabolics, and
an image of the object would appear to float just on top of the top
mirror. I'm sure many of you know this illusion.

Yes, Edmund Scientific used to sell this stuff. You need to make sure
that when you clamshell the two mirrors together, the focal point of
the top mirror is the point lying right at the center of the bottom
mirror (and generally, vice versa, too). This means a mirror in the
vicinity of f/0.7, I think.

That way, when you lay a penny on the bottom mirror, the top mirror
reflects light from it into parallel beams; the bottom mirror then
focuses that light into a real image located right at the hole in the
top mirror, and the penny appears to float in mid-air.

Err on the long side--that way, if you're off, you can use a shim to
put the penny on.

Brian Tung
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt