I know Iv'e seen the underwater version too
You should see how he cleans his mirrors, with his tongue! No Joke!
He recons that there is less scratches that way.
I've not built one, but i've seen a few in action. The mechanics are very simple, especially if you plan to use a tool the same size as the primary.
(and I'm sure you have googled the designs by now)
What they don't do, but my mirror making mentor David Sinden http://www.britastro.org/iandi/mariot04.htm did: Rather than clamp his mirrors in position he attached a threaded plate to the back with pitch. It was very neat, and the 'nut' screwed onto the lower spindle when on the bottom, and captured a peg coming down from the upper arms when on top. I'll draw a diagram!
Mirror making machines are diversion all of their own, not necessarily a labor saving device for the inexperienced.
Its probably the case that until anyone learns the subtleties of producing a superb mirror by hand, that they will never make a good mirror by machine . Part of the art of making mirrors on a machine is learning to introduce randomness to mimic the natural smoothing action of handwork . I still use an element of hand polishing in my mirrors and will always do so .
I've not built one, but i've seen a few in action. The mechanics are very simple, especially if you plan to use a tool the same size as the primary.
(and I'm sure you have googled the designs by now)
What they don't do, but my mirror making mentor David Sinden http://www.britastro.org/iandi/mariot04.htm did: Rather than clamp his mirrors in position he attached a threaded plate to the back with pitch. It was very neat, and the 'nut' screwed onto the lower spindle when on the bottom, and captured a peg coming down from the upper arms when on top. I'll draw a diagram!
Wouldn't the pitch move with the summer heat making the grinding uneven?
Wouldn't the pitch move with the summer heat making the grinding uneven?
You can (and will need to) change the hardness of the pitch to suit the temperature. (boil it to make it harder, add turps to make it softer!) The pitch is pretty firm stuff at normal room temp.
Secondly, it's a good idea to do your figuring at least in an environment which matches the temperature that plan to use the mirror. Not such a big deal with low expansion glass, but worth considering.
Finally, a little movement isn't actually a bad thing. It's the randomness of the hand movements that make such fine surfaces, and something machines find difficult to emulate. There should be no great forces involved either - just time and grit.
Secondly, it's a good idea to do your figuring at least in an environment which matches the temperature that plan to use the mirror. Not such a big deal with low expansion glass, but worth considering.
Hi Astrojunk
Could you elaborate there ? The relative mechanical dimensions of a glass blank are the same regardless of its temperature. Are you referring instead to the practise of undercorrecting a plate glass mirror to allow for possible overcorrection condition as the ambient temperature falls and the optics can't keep up, when in use under the night sky ?
Could you elaborate there ? The relative mechanical dimensions of a glass blank are the same regardless of its temperature. Are you referring instead to the practise of undercorrecting a plate glass mirror to allow for possible overcorrection condition as the ambient temperature falls and the optics can't keep up, when in use under the night sky ?
You're right, I suppose the critical factor is the even-ness of temperature and it was probably cheaper to keep the workshop evenly cold rather than evenly warm where I grew up!
As you have reminded me, it was always said to better to under correct mirrors - and back then we used to grind everything to f8 anyway.
Mark when using a machine would you still need to do the figuring by hand or are there some mirror manufacturers that don't bother?
I have never used a machine so I wouldn't know, but I can't see it doing the figuring.
A great deal of figuring can be done by machine. On smaller mirrors 10" to 12 " of modest asphericity it can actually be done with one polisher and a skilled operator all by machine .
In my experience a smoother zone free finish can be had with hand work and I'd guess I do 50% by machine and 50% by hand on my mirrors, as I like to take the figuring process from engineering science into art and a handcraft.
Large fast pro mirrors ( like the 8meter varieties ) are polished by computer controlled laps which actually warp to fit the shape of the mirror. They are not touched by the human hand.
As a contrast in technologies , the final corrective polishing runs on the 200 inch ( 5 metre ) Mt Palomar were done in 1947 by the late Don Hendrix by hand using 6" polishers. As the rib thickness had come out uneven in the casting, the mirror performed worse in its cell on the sky than it did hanging vertical on the test stand.
The final touches were made by taking `foucault-grams' on a star, where a knife edge null test was made on a real star and captured on a photograghic plate. The final rubbing down of the 1 wave high edge evident from the mirror `in-situ' in the Palomar observatory by Don Hendrix. Thats what I call `flying by the seat of your pants '
Wow - that mirror o matic was going fast wasn't it?!
The Draper machine was going at the rate that I remember David Sinden's machines went. He had one of those that he built for 18" plus mirrors, but his beam arm was much bigger - 2 or 3 meters long to minimise side to side movements.
. I still use an element of hand polishing in my mirrors and will always do so .
