The first photo below shows a 12.5 inch diameter, 1.5 inch thick, telescope mirror blank, made from recycled glass, that has been lightened by casting the mirror in a mold that made lots of hexagon-shaped pockets in the back of the mirror. This is the story of how I did it.
Why light-weight mirror blanks? Well, telescope mirrors are made from thick glass. Thick glass is heavy, no, really heavy. Most people don't realize how heavy thick glass actually is because they usually only deal with small pieces of thin plate glass. Handing a large telescope mirror to someone who has never seen one before, is a revelation to them about just how heavy glass really is. I like to tell them that it is as heavy as granite. So imagine the piece of granite that was cut out of a granite countertop to make a sink opening. That piece of granite is about the same size and weight as a large telescope mirror. It's heavy stuff.
In a large telescope, the primary mirror can constitute 1/3 or more of the weight of the entire instrument. Supporting the massive weight of the mirror requires that the rest of the telescope be sturdily built. The result is that large telescopes can be shockingly heavy. I know this first hand because I have thrown out my back several times moving my relatively "easily portable" 17.5 inch Dobsonian Telescope. So I decided to try making some-light-weight mirror blanks, just to see if I could do it.
Let's start this story at the beginning. About two years ago I bought a small kiln at a garage sale and began experimenting with making my own telescope mirror blanks by melting together pieces of thin scrap glass in my kiln to make the thick glass needed for telescope mirrors. The process works great. I started out small, producing 6, 8 and 10 inch diameter blanks up to 1.5 inches thick in the small kiln (Photo #2). Then I saw a much larger kiln for sale cheap on Craig's List. I bought it (Photo #3) and scaled up my mirror making process. The kiln was in rough shape. Well, to be honest, it was near the end of its useful lifetime and about ready for the landfill. But I fixed it up and gave it new life. So not only am I using recycled glass, I am also using a recycled kiln. Fixing up the kiln though is a story for another Instructable. I have now produced wonderful solid mirror blanks in the bigger kiln, up to 14.5 inches in diameter and 1 1/2 inches thick (Photo #4). Once I had the process of making solid blanks pretty much perfected, I wanted to try making light-weight mirror blanks.
Two years of trial and much error went into developing this process. This Instructable just presents the finished product. If you would like to see the whole history of my glass casting (mis)adventures, please visit my web site at http://www.mdpub.com.
Why light-weight mirror blanks? Well, telescope mirrors are made from thick glass. Thick glass is heavy, no, really heavy. Most people don't realize how heavy thick glass actually is because they usually only deal with small pieces of thin plate glass. Handing a large telescope mirror to someone who has never seen one before, is a revelation to them about just how heavy glass really is. I like to tell them that it is as heavy as granite. So imagine the piece of granite that was cut out of a granite countertop to make a sink opening. That piece of granite is about the same size and weight as a large telescope mirror. It's heavy stuff.
In a large telescope, the primary mirror can constitute 1/3 or more of the weight of the entire instrument. Supporting the massive weight of the mirror requires that the rest of the telescope be sturdily built. The result is that large telescopes can be shockingly heavy. I know this first hand because I have thrown out my back several times moving my relatively "easily portable" 17.5 inch Dobsonian Telescope. So I decided to try making some-light-weight mirror blanks, just to see if I could do it.
Let's start this story at the beginning. About two years ago I bought a small kiln at a garage sale and began experimenting with making my own telescope mirror blanks by melting together pieces of thin scrap glass in my kiln to make the thick glass needed for telescope mirrors. The process works great. I started out small, producing 6, 8 and 10 inch diameter blanks up to 1.5 inches thick in the small kiln (Photo #2). Then I saw a much larger kiln for sale cheap on Craig's List. I bought it (Photo #3) and scaled up my mirror making process. The kiln was in rough shape. Well, to be honest, it was near the end of its useful lifetime and about ready for the landfill. But I fixed it up and gave it new life. So not only am I using recycled glass, I am also using a recycled kiln. Fixing up the kiln though is a story for another Instructable. I have now produced wonderful solid mirror blanks in the bigger kiln, up to 14.5 inches in diameter and 1 1/2 inches thick (Photo #4). Once I had the process of making solid blanks pretty much perfected, I wanted to try making light-weight mirror blanks.
Two years of trial and much error went into developing this process. This Instructable just presents the finished product. If you would like to see the whole history of my glass casting (mis)adventures, please visit my web site at http://www.mdpub.com.
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Signing UpStep 1: Designing the Mirror Blank
I played around with some layout ideas in Google Sketchup. I wanted to make a mold that is the negative shape of the finished mirror. I had settled on a mirror 12.5 inches in diameter and 1.5 inches thick for my initial experiments. So I tried arranging hexagonal prism shapes inside a circular wall or dam 12.5 inches inside diameter that will retain the glass. I had the idea of assembling the mold using one of my kiln shelves as a base. So I built up all the parts and played with sizes and spacing of the hexagons. The hexagonal prisms would form pockets in the back of the glass mirror that would lighten it. I settled on hexagons 1 7/8 inches in diameter (as measured across the flats) and 1 inch tall. That would leave 1/2 inch of glass on the top face of the mirror for grinding in the curve. The spacing between the hexagons would be 3/8 inch.
All the parts of the mold would need to be built of materials that could withstand glass fusing temperatures. The hexagons would be made from plaster and silica mixture. The outer wall would be 2.5 inches tall and cut from soft firebrick with a band saw and the pieces glued together with furnace cement. The kiln shelf base will easily withstand the temperatures involved.
With an initial design in hand, I set out to make it happen. I wish I could just CNC mill the mold out of a big block of refractory material. Or even use a 3D printer to print a plastic negative of the mold that I could pour castable refractory into. Oh well, maybe someday. Till then, I have to piece it all together the hard way.
All the parts of the mold would need to be built of materials that could withstand glass fusing temperatures. The hexagons would be made from plaster and silica mixture. The outer wall would be 2.5 inches tall and cut from soft firebrick with a band saw and the pieces glued together with furnace cement. The kiln shelf base will easily withstand the temperatures involved.
With an initial design in hand, I set out to make it happen. I wish I could just CNC mill the mold out of a big block of refractory material. Or even use a 3D printer to print a plastic negative of the mold that I could pour castable refractory into. Oh well, maybe someday. Till then, I have to piece it all together the hard way.
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You rock! I'll definitely be checking out your site!
GREAT project.
This instructable is awesome. The finished mirror (on your website) looks truly professional. Congrats.
In case you didn't know, it is possible to add flux to float glass to lower its melting point. I successfully did this with bottle glass after looking at the prices of commercially available glass billets.
My recipe was inspired by the flux recipes from the "Post-Consumer Container Glass Remelting Process Assessment" via the Clean Washington Center (CWC).
Link:
http://www.cwc.org/glass/gl965rpt.pdf
Lots more useful (technical) glass info here:
http://www.cwc.org/glass.htm
My flux recipe was a combination of sodium carbonate, sodium tetraborate, and potassium nitrate. I didn't add the more toxic fluxes (lithium, lead, barium) nor the de-colorants (antimony, manganese).
I've also read about people using an angle grinder fitted with a diamond sanding pad mounted on an adjustable arm to sand/grind glass as it turns on a potter's wheel / lazy susan. I'd post the link, but can't remember where I read it.
Hope it helps.
Thanks
Best Wishes
Does the heat actually reverse the chemistry that's set in motion by the water? If the removed plaster dust is mixed with water, will it set like new plaster?
A way to lighten the mirror further, is to make the hexagons at the rim taller, even slope their tops (down toward the center) slightly, so after grinding/figuring, the top surface is of uniform thickness.
There is a technique used in casting called "shell mould casting."
The pattern is usually wax, and it's dipped in a refractory slurry / ies, to build up a sufficiently strong and thin shell. The shell is dewaxed or flash heated, to rapidly melt the surface layer of the wax inside, and after dewaxing, the shell if often sunk in sand as a backing support and the metal is poured into it.
I was thinking that it's possible to cast up a single once piece shell of refractory, to melt the glass onto.
Kind of like slip casting.
And you can use an acid wash to remove the plaster - much easier on a thin shell.
And by removing the hex shapes, one could use a router to make a simple linear webbing that runs radially under the mirror.
"Why aren't large 'scopes made with precision lightweight materials like, say, magnesium alloy, then silvered.. etc."
"Why are 'scopes not precisely made with light weight materials like magnesium allow etc.."
That is a good question.
I think it's because everything is a compromise.
Glass offers HARDNESS, STABILITY and it's INERT and can be resilvered many times over a long working life span.
And without too much hassle, the mirrors can be made and ground by anyone who has the persistance and determination to do so.
Tho to be fair, a uniformly pressed or cast magnesium mirror base and an extruded magnesium frame - does have a great deal of merit expecially for thermal equlibrium - warm day / cold night, and identical expansion co-efficients.
I guess tho, that glass is a traditional material for optics and given the fact that really GOOD mirrors gan be made by anyone, it's stable and holds it's shape for ever, and it can be resilvered almost indefinitely, that I think would be the reason why all and especially larger and fixed location telescopes use glass mirrors.