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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
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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
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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
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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
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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
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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
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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
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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
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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
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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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