Molding concave lenses.

Bughuggger wrote:

Just as a speculation, consider that the ordinary mirror, if it were cheap to
make, would not be eight or ten inches, but could be up to fifty inches. Then,
if one needed not necessarily a fine wooden tube but instead had merely a large
black tent. Then, considering the subject matter, would it then be possible to
view detail of the moon with a twenty inch mirror, even if the accuracy was not
very good. Thanks for the help.

I tell you bughugger, you don't have much experience yet, but you are
thinking right! What you don't realize is JUST how accurate a telecope
mirror has to be to give an image. The bigger the mirror the bigger
the problems! Just the weight of the mirror alone can bend it enough
to screw up the image! But one solution is the so-called "rubber mirror"
thing. You take an ordinary cheap but flexible mirror that is close
to the right shape. Then mount it on an array of piezo actuators that
can bend the mirror up and down. Using a laser and feedback into
a computer, you can actually send signals to the acutators that literally
BEND the mirror into the correct shape. This is an interesting
approach to huge mirrors where cost and weight are prohibitive.

Note that a parabolic oven mirror only has to be accurate enough
to focus to the area of the object to be heated. NOT good enough to
look at the moon! Another data point. I own a 36" Schmidt mirror
that came out of an old TV projector. A Schmidt mirror is spherical
rather than parabolic, but uses a corrector lens to give a sharper
focus. This system even WITH the corector plate produces a VERY
blurry moon image. Good enough for TV on a screen but sucks for moon
pictures. And THIS mirror is glass a couple inches thick and designed
for images!

But hey, the moon is BRIGHT! You don't need a 20" mirror... you
can make a decent telescope out of a large lens with a concave
side. Just silver that bowl side and build a telescope! Very
instructive and will work!

bjacoby

PS. If you want to see what we are talking about here, go find
one of those 6" or so cosmetic magnifying mirrors women use
to put their face on. Set it up as a telescope and you'll see
how ineffective an inacurate mirror can be. Also get a sheet of
black paper and cut maybe a 1" or so hole in it. Now put that
infront of your cheapo mirror and move it around. You'll find that
if you cut down the area of the mirror the image will improve
as the mirror is more accurate over small areas than the whole
thing.

--
Due to SPAM innundation above address is turned off!

bjakoby:
I tell you bughugger, you don't have much experience yet, but you are
thinking right! What you don't realize is JUST how accurate a telecope
mirror has to be to give an image. The bigger the mirror the bigger
the problems! Just the weight of the mirror alone can bend it enough
to screw up the image! But one solution is the so-called "rubber mirror"
thing. You take an ordinary cheap but flexible mirror that is close
to the right shape. Then mount it on an array of piezo actuators that
can bend the mirror up and down. Using a laser and feedback into
a computer, you can actually send signals to the acutators that literally
BEND the mirror into the correct shape. This is an interesting
approach to huge mirrors where cost and weight are prohibitive.

Note that a parabolic oven mirror only has to be accurate enough
to focus to the area of the object to be heated. NOT good enough to
look at the moon! Another data point. I own a 36" Schmidt mirror
that came out of an old TV projector. A Schmidt mirror is spherical
rather than parabolic, but uses a corrector lens to give a sharper
focus. This system even WITH the corector plate produces a VERY
blurry moon image. Good enough for TV on a screen but sucks for moon
pictures. And THIS mirror is glass a couple inches thick and designed
for images!

But hey, the moon is BRIGHT! You don't need a 20" mirror... you
can make a decent telescope out of a large lens with a concave
side. Just silver that bowl side and build a telescope! Very
instructive and will work!

bjacoby

PS. If you want to see what we are talking about here, go find
one of those 6" or so cosmetic magnifying mirrors women use
to put their face on. Set it up as a telescope and you'll see
how ineffective an inacurate mirror can be. Also get a sheet of
black paper and cut maybe a 1" or so hole in it. Now put that
infront of your cheapo mirror and move it around. You'll find that
if you cut down the area of the mirror the image will improve
as the mirror is more accurate over small areas than the whole
thing.

--
Due to SPAM innundation above address is turned off!

Thanks a lot. I like to get these general points right.

DW

Imagine you have a parabolic mirror and a large black tent. Would you then be
able to sit inside it, and watch the visions on a mirror in front of it like it
was a t.v. screen?

DW

"Bughuggger" wrote in message
...
Imagine you have a parabolic mirror and a large black tent. Would you then
be
able to sit inside it, and watch the visions on a mirror in front of it
like it
was a t.v. screen?
No.
You can watch the images, if you bring them to focus on a surface. This is a
'camera obscura', and was a popular device in Victorian times.

Best Wishes