Stupid question - Why cant you Mold a mirror?

"Timothy O'Connor" wrote in
:

This is a really stupid question (that someone asked me the other day,
and I didnt have any reasonable answer), but why is it not possible to
simply mold a mirror to shape, rather than grind it? I would have
thought that it would be possible get it done with a great deal of
acuracy these days. (obviously not, but Im not sure why)




This could be used to save a lot of grinding but you would still have to do
the final polishing and figuring the normal way. I suspect the reason is to
do with thermal effects - you poor molten glass into your mold - how well
is the mold going to retain its' shape? There is also the issue of
smoothness. It might work for very small mirrors and lenses though.

L.

(Wfoley2) wrote in
:

The
reason being no one has ever been able to program a random stroke only
humans are able to make randomenous work to there advandtage.

Actually, I think that the Chinese mirrors are ground and polished by
machines.
At least I read a post to that effect, I think by Roland.
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)




Some amateur mirrors are ground and polished by machines too. Ginding
machines are not hard to make. There are many ATM web sites detailing
how to make such machines.

L.

Llanzlan Klazmon The 15th wrote in
7.6:

(Wfoley2) wrote in
:

The
reason being no one has ever been able to program a random stroke only
humans are able to make randomenous work to there advandtage.

Actually, I think that the Chinese mirrors are ground and polished by
machines.
At least I read a post to that effect, I think by Roland.

Just one more point, If you are referring to Roland Christen of
AstroPhysics Inc, then am pretty sure that AP also use machines to grind
and polish their lens blanks (at least that's what it says on their web
site - it refers to OptiPro & Opticam grinders and polishers). Only the
final figuring is done by hand.

L.





Clear, Dark, Steady Skies!
(And considerate neighbors!!!)




Some amateur mirrors are ground and polished by machines too. Ginding
machines are not hard to make. There are many ATM web sites detailing
how to make such machines.

L.

You can. That's how the big 8 meter mirrors are made. You rotate the
glass on a turntable while it's annealing and it cools to a parabola
shape.

You still have to figure and polish afterward.

scotty

On Thu, 27 Nov 2003 12:08:30 GMT, "Timothy O'Connor"
wrote:

This is a really stupid question (that someone asked me the other day, and I
didnt have any reasonable answer), but why is it not possible to simply mold
a mirror to shape, rather than grind it? I would have thought that it would
be possible get it done with a great deal of acuracy these days. (obviously
not, but Im not sure why)

"Engineer Scott" wrote in message
news
You can. That's how the big 8 meter mirrors are made. You rotate the
glass on a turntable while it's annealing and it cools to a parabola
shape.

You still have to figure and polish afterward.
I would have thought that this initial molding would be a huge advantage for
ATM, in that you would not need to spend so much time with the initial
grinding phase.

I think its odd that someone doesnt sell this sort of stuff.

"Zane" wrote in message
...
On Sat, 29 Nov 2003 00:13:18 GMT, "Timothy O'Connor"
wrote:

"Engineer Scott" wrote in message
news
You can. That's how the big 8 meter mirrors are made. You rotate the
glass on a turntable while it's annealing and it cools to a parabola
shape.

You still have to figure and polish afterward.
I would have thought that this initial molding would be a huge advantage
for
ATM, in that you would not need to spend so much time with the initial
grinding phase.

I think its odd that someone doesnt sell this sort of stuff.

As Wfoley2 (I assume this is Bill Foley's new moniker ? ) pointed out
in an earlier post in this thread, supplying mirror blanks with a
basic spherical curve already molded (cast) in used to be fairly
common. Like you, I'd be surprised if you can't still get them.

Some 35 mm camera lenses had one or more molded (and not ground or
polished afterward) lens elements in them as of a few years ago. I'm
not up to speed on the latest in this area. Of course the accuracy
requirements for the surfaces involved are not nearly as stringent as
for telescopes.
The problem is that given people want lots of different focal lengths, you
end up needing lots of different curves on the blanks. Also when you grind a
mirror, your 'tool', is automatically ground to match the mirror, allowing
it to be used with finer grits, right up to switching to the polishing
phases.
Molded plastic lenses are common. Molded glass lenses are relatively rare.

Best Wishes

I think its odd that someone doesnt sell this sort of stuff.

As I stated in an earlier post, this HAS been done before. Now the question
would be, why did they STOP doing it???
I suspect that there was a drought of ATM's at that time, and the vendors just
went out of business or stopped making pre-figured mirrors.

Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

The problem is that given people want lots of different focal lengths

I think you hit the nail on the head - the prefigured mirrors had an F/8 curve
molded or rough-ground in, and F/8 sort of went out of style when big Dobs
became available. F/8 is OK for a 6" or 8", but sort of a problem if you are
going with a mirror over 12".
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)