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Stupid question - Why cant you Mold a mirror?



 
 
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  #1  
Old November 27th 03, 01:08 PM
Timothy O'Connor
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Default Stupid question - Why cant you Mold a mirror?

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)



  #2  
Old November 27th 03, 02:16 PM
Matthew Ota
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Default Stupid question - Why cant you Mold a mirror?

Well for one thing, the mold would have to have the same accuracy on the
curve as the finished product. So you would spend the same amount of
time figuring a convex mold to make your mirror.

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)




--
--------------------------------------------------------------
Matthew B. Ota
Orange County Astronomers Astroimagers SIG
http://www.ocastronomers.org/
Telescopes In Education (TIE)
http://tie.jpl.nasa.gov/tie/index.html
Jet Propulsion Laboratory Saturn Observation Campaign
http://saturn.jpl.nasa.gov/news/soc/
--------------------------------------------------------------

  #3  
Old November 27th 03, 02:18 PM
Timothy O'Connor
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Default Stupid question - Why cant you Mold a mirror?


"Matthew Ota" wrote in message
...
Well for one thing, the mold would have to have the same accuracy on the
curve as the finished product. So you would spend the same amount of
time figuring a convex mold to make your mirror.

If your making a living out of it, then you could just bust your butt
getting that one mold right, and then just crank the mirrors out...


  #4  
Old November 27th 03, 02:48 PM
DBogan3220
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Default Stupid question - Why cant you Mold a mirror?



It probably has something to do with the accuracy of the mold making
process. I mean where are you going to get a mold that is within a fraction of
the wavelength of light. Do you understand what 1/4 wave or 1/8 wave means? We
are talking millionth's of an inch here folks!

The only way to get this done correctly is grind and polish and test
that is how it is done. In the end everything has to hand retouched for the
final figuring. Machine made optics don't cut it for Astronomical purposes. 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. The reason you want
random strokes is this is how you avoid zones!

May I suggest some reading material. Amatuer Telescope Making Books I, II,
and III these guy's have all the info on what it takes to make an optic
including testing

Clear Skies
Dwight L Bogan
  #5  
Old November 27th 03, 03:08 PM
Roger Hamlett
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Default Stupid question - Why cant you Mold a mirror?


"Timothy O'Connor" wrote in message
...
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)

It is just down to accuracy. The first problem, is that with most molding
processes, you need some form of 'release agent' to allow the mold and
molding to seperate. This degrades slightly the accuracy. Then you have the
problem that most (all?) molding materials change slightly in size as they
set (whether by chemical hardening, or by cooling). Again this distorts the
shape slightly (this is why designing molds to produce even reasonably
accurate surfaces is a complex process). Then you have the question of
whether the material itself is rigid enough to hold the shape when complete.
If you think in terms of other industrial processes, using casting or
molding, high accuracy surfaces (even though, in terms of a mirror, these
are relatively poor in accuracy), are usually still 'finished' by some form
of machining process after molding. The levels of accuracy required for a
good mirror, are several orders of magnitude beyond those achieved by all
but very complex molding processes. These processes are themselves
expensive, and then have tooling costs, that make normal mirror grinding
look very cheap indeed. You are talking an entire surface, which is accurate
to better than about 1/6000th mm...
There are some molding processes that approach this accuracy, but normally
only over quite small areas (these are the processes used for some lens
manufacture - these generally still only produce relatively 'budget'
lenses). The question at the end, is then still left, as to the suitability
of the final material for anything bigger.

Best Wishes



  #6  
Old November 27th 03, 04:02 PM
Wfoley2
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Default Stupid question - Why cant you Mold a mirror?

Actually, for a while there were some companies selling molded blanks, but this
was only to save time grinding and polishing. So far as I know, there just
aren't any molding processes yet which can give better than 1/4 wave accuracy
and smoothness. There were also some blanks with the shape roughly ground in
to save the ATM time. I do not know if these are still available or not. I
would think that, in this day and time, our technology would allow extremely
precise molding, but do not know of anyone actually doing it...

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


  #7  
Old November 27th 03, 04:04 PM
Wfoley2
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Default Stupid question - Why cant you Mold a mirror?

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!!!)


  #8  
Old November 27th 03, 04:28 PM
Chuck Simmons
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Default Stupid question - Why cant you Mold a mirror?

Wfoley2 wrote:

Actually, for a while there were some companies selling molded blanks, but this
was only to save time grinding and polishing. So far as I know, there just
aren't any molding processes yet which can give better than 1/4 wave accuracy
and smoothness. There were also some blanks with the shape roughly ground in
to save the ATM time. I do not know if these are still available or not. I
would think that, in this day and time, our technology would allow extremely
precise molding, but do not know of anyone actually doing it...


It is done for very small glass optics. The process is unworkable for
glass optics of any size. The examples I know of that are molded glass
are aspheric objective lenses for DVD players. These are diffraction
limited.

Chuck
--
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons
  #9  
Old November 27th 03, 05:42 PM
William Hamblen
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Default Stupid question - Why cant you Mold a mirror?

On 2003-11-27, 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)


Some mirrors for very large telescopes are molded to reduce the amount
of work needed to grind and polish. It isn't possible to mold a mirror
accurately enough to use as it comes from the mold.

--
When the fog came in on little cat feet last night, it left these little
muddy paw prints on the hood of my car.

  #10  
Old November 27th 03, 11:13 PM
Paul Hyndman
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Default You Can (was: Why cant you Mold a mirror?)

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)


Well... may not you or I, but it has been tried with varying degrees of
success. Most notably, the Goddard Space Center team has used the carbon
replication technique to make multiple copies of ultra-lightweight
high-accuracy mirrors. Dr Peter Chen gave a great presentation on this at
the 2001 Stellafane convention:

http://www.astro-nut.com/stchen.html

Cheers,

Paul

--- http://www.astro-nut.com ---


 




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