making parabolic mirror with resin

(Jon Isaacs) wrote in message ...

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

Thanks for the suggestion. However, those mirrors would be too
shallow. The mirrors I saw in the experiment were fairly deep. The
bottom mirror which holds the onject is almost a small salad bowl
size.

jbc

(Jon Isaacs) wrote in message ...

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

Thanks for the suggestion. However, those mirrors would be too
shallow. The mirrors I saw in the experiment were fairly deep. The
bottom mirror which holds the onject is almost a small salad bowl
size.

jbc

Jonathan B-C wrote:

(Jon Isaacs) wrote in message ...

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


Thanks for the suggestion. However, those mirrors would be too
shallow. The mirrors I saw in the experiment were fairly deep. The
bottom mirror which holds the onject is almost a small salad bowl
size.

jbc

A bit pricey but take a look at
http://scientificsonline.com/product.asp?pn=3053491
(this is what has become of the old Edmunds Scientific)

I (Brian Tung) wrote:
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.

Oops--that's only if the mirrors meet rim to rim. If you have a spacer
between them, they can be of a slower (that is, higher) focal ratio.
The image might not be as bright, though.

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

Steve Taylor wrote:
Thanks for the suggestion. However, those mirrors would be too
shallow. The mirrors I saw in the experiment were fairly deep. The
bottom mirror which holds the onject is almost a small salad bowl
size.

Aren't they elliptical mirrors ?

You could do it with one ellipsoidal mirror, but it's probably easier
to do it with two paraboloidal mirrors--more people are familiar with
making (and/or obtaining) paraboloids. (Although they are deep!)

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

Jonathan B-C wrote:

Thanks for the suggestion. However, those mirrors would be too
shallow. The mirrors I saw in the experiment were fairly deep. The
bottom mirror which holds the onject is almost a small salad bowl
size.

jbc
Aren't they elliptical mirrors ?

Steve

John Oliver wrote in message ...

A bit pricey but take a look at
http://scientificsonline.com/product.asp?pn=3053491
(this is what has become of the old Edmunds Scientific)

Thanks for link. Looks different from what I've seen back in
college--definitely not a single mirror trick.

I was able to find other groups (not google) that had some posts on
resin or plastic mirrors (sometimes with spin casting as part of the
topic). But many of these date back to 1998 or earlier. Has there
been lost of interest because it's difficult/not cost effective? Of
course, the posts were related to telescope-quality mirrors (couldn't
find the topic anywhere else), but again, I'm wondering if such
quality is necessary for the illusion. I guess that would depend on
the quality/sharpness of the illusion...

Bye the way, thanks Brian for the suggestions...

In message , Jonathan
B-C writes
I was able to find other groups (not google) that had some posts on
resin or plastic mirrors (sometimes with spin casting as part of the
topic). But many of these date back to 1998 or earlier. Has there
been lost of interest because it's difficult/not cost effective? Of
course, the posts were related to telescope-quality mirrors (couldn't
find the topic anywhere else), but again, I'm wondering if such
quality is necessary for the illusion. I guess that would depend on
the quality/sharpness of the illusion...

I reckon you would probably be OK for this sort of trick. The spin cast
mirrors at that fast f ratio will be a bit ugly and heavy but it should
be possible. Try Scientific Americans amateur scientist archive for a
pretty decent write up on how to spin cast an epoxy mirror on a record
deck.

The problem with spin casting is you can't get anything like the
precision needed for an optical grade telescope mirror. But you can make
a really neat vicious burning glass or shaving mirror with some careful
polishing.

Your other option is to find a pair of reasonable grade parabolic
searchlight reflectors of suitable size for the illusion.

Regards,
--
Martin Brown

Jonathan B-C wrote:
John Oliver wrote in message ...

A bit pricey but take a look at
http://scientificsonline.com/product.asp?pn=3053491
(this is what has become of the old Edmunds Scientific)


Thanks for link. Looks different from what I've seen back in
college--definitely not a single mirror trick.


If we are talking cheap big "mirror" tech - Maurice Gavin once made
quite a success out of a membrane mirror made of mylar and vacuumed into
its form. Maurice used the thing purely as a flux collector for an
experiment of course and not an imaging telescope. I have a link
somewhere, I'll post it if I find it.

Contrary to legend, the shape of the membrane is not parabolic
unfortunately.

Steve