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.

Step 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.
<p>On the question of why not rotate the mold to get a bit of a central dip, Remember that we would be working with materials at 2000 degrees (F or C, I assume C) and that temperature would melt most spindles and bearings. I did see a photo where they were rotating the whole kiln to make a mirror more than 3 or 4 feet in diameter for a telescope in Arizona. I think it was Roger Angel's project.</p>
<p>Just a thought... why not make your mold out of a plastic that will withstand the temperature of molten wax. I think it was the Egyptians that invented the 'lost wax' process, but it is better known as 'investment casting'. You make a positive out of wax by casting it in a mold that you can reuse. Once the positive is made, make a mold of 'refractory ceramic material' around the wax, heat the mold and remove the wax, then cast your mirror. I know; easy for me to say.</p>
<p>So... Question, if I wanted to make a telescope with enough power and resolution to read news print on the moon, how large would my mirror need to be, and what would the final product weigh? How accurate would the finished product need to be? Could I operate it at low altitude, or would I need to find a mountain top somewhere?</p>
<p>&gt; how large would my mirror need to be</p><p>Super big. </p><p>According to <a href="http://boards.straightdope.com/sdmb/showthread.php?threadid=61205" rel="nofollow">http://boards.straightdope.com/sdmb/showthread.php...</a> to resolve a flag on the moon you'd need a 10,000 inch telescope in orbit to see the flag as a single pixel in your sensor.</p>
<p>Buggery! There has GOT to be a better way to get live high resolution pics of the moon's surface!</p>
Wow! Now THAT is cool! Question though,: Why aren't large 'scopes made with precision lightweight materials like, say, magnesium alloy, then silvered to create a lightweight first surface mirror? Assumming there not...
<p>The reason that light weight metals are not used is due to their thermal expansion. Magnesium alloys expand 7 to 8 times more than plate glass, thus distorting your surface.</p>
<p>To back up colin.bravi's point - using any lightweigh (aluminum, magnesium, titanium, etc). metallic alloy would give you a larger coefficient. There does exist an alloy, Invar, which has a coefficient of thermal expansion that is half of borosilicate glass. However, it's density is ~8 g/cm^3 as opposed to ~2.5 g/cm^3 for borosilicate.</p>
<p><a href="https://ssl.instructables.com/member/xarlock667/" rel="nofollow">xarlock667</a>, No offense, but you have no idea what you're asking--you're vastly overestimating the resolution power of telescopes. The biggest telescopes on Earth, not amateur but the professional ones, can resolve objects down to the size of a football stadium on the Moon. Literally, that's not an exaggeration. Your mirror would have to be, I don't know, maybe the size of Los Angeles. Possibly Arizona. </p>
<p>As I was saying... creative and resourceful. It would appear to be easy to cut the bold head off a shanked bolt, a couple of fender washers. a lock washer and a couple of nuts, and there you have it!</p>
<p>As a buddy of mine says, &quot;It ain't braggin' if it's true!&quot;. What an awesome achievement! I admire your resourcefulness and creativity. Add to that the fact that your instructions for us aspiring to do the same are great! I'd love to buy one of these in a 14&quot; diameter... if I could afford it.</p>
<p>I have a good big kiln, I might try to make a mirror. Thank You for the good instructable sir.</p>
hi i from Medellin Colombia beatifull mirror you giftme one 14&quot; for a social telescope thanks you fredyalexp@yahoo.com<br><br>happy childrens
<p>This may be the best Instructable I have yet looked at. Your approach to the problem of casting an astronomical capable mirror has been thoughtful, and extremely analytical. It has also been made very clear for those of us who have studied this instructable. Not only have your methods used sound engineering and good technique, but there is also a very useful note of pragmatism in your sourcing of materials and creating the actual honeycomb mold structure. Very impressive and extraordinarily well done. Congratulations.</p>
<p>I assume that the honeycomb is to add stiffness to the mirror, right? I would suggest &quot;next time&quot; to consider isogrid(triangular shapes) or orthogrid(square shapes) cause these patterns add also flexural rigidity while honeycomb is rigid mainly in compression in the direction normal to surface(but needs support on the other side).</p><p>With isogrid maybe you can also avoid the back plate of the sandwitch and save even more weight (isogrid is used to produce light and stiff panels for rockets and spacecrafts </p><p>http://hypersizer.com/industry/projects/Pretest-prediction-composite-isogrid.php </p><p>)</p>
<p>Thanks for posting this. I've got an older 8&quot; Meade, but I've promised myself that I wouldn't go larger diameter unless I built it myself (why? cost.)</p><p>This inspires me. Thanks!</p>
<p>How do you silver it?</p>
I've heard that mirrors can be put in a rotating mold that makes a convex surface. The rotation of the mold full of molten glass makes a perfect convex shape.
This is my favorite instructable I've ever seen. I've been tossing around the idea of building a telescope, and had no idea you could cast your own blanks at a reasonable cost. <br> <br>You rock! I'll definitely be checking out your site!
A pressure washer is supposed to be very effective for removing the plaster too. <br> <br>GREAT project.
Awesome instructable! I'm curious, have you tried vibrating the kiln to get out the bubbles? I haven't worked with one so I don't know how well it'd work, but I know that's how they get carving ice so clear.
<p>This instructable is awesome. The finished mirror (on your website) looks truly professional. Congrats.<br> <br> <br>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.<br> <br> <br>My recipe was inspired by the flux recipes from the &quot;Post-Consumer Container Glass Remelting Process Assessment&quot; via the Clean Washington Center (CWC).<br> Link:<br> <br> <br><a href="http://www.cwc.org/glass/gl965rpt.pdf" rel="nofollow"> http://www.cwc.org/glass/gl965rpt.pdf</a><br> <br> <br>Lots more useful (technical) glass info here:<br> <br> <br><a href="http://www.cwc.org/glass.htm" rel="nofollow">http://www.cwc.org/glass.htm</a><br> <br> <br>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).<br> <br> <br>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.<br> <br> <br>Hope it helps.</p>
Very nice and thorough Instructable. I have just acquired an old Quickfire 6 kiln and your instuctions on building your mold encourage me to try casting. <br>Thanks
Beautiful. Thanks to you, stars twinkle brighter and unseen nebulae emerge to amaze us. Great work!
Any thought to spinning the mirror as you cool it to get it a rough shape? Could save you from having to remove one pound of glass in rough grinding! <br> <br>Best Wishes
About the plaster: <br>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? <br> <br>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. <br>
This is a work of art. Thank you for sharing the process.
Excellent work!!!!
Wow! I'm almost speechless after reading your instrucable. Excellent work.
Wow! Now THAT is cool! Question though,: Why aren't large 'scopes made with precision lightweight materials like, say, magnesium alloy, then silvered to create a lightweight first surface mirror? Assumming there not...
Awesome work!
fantastic project! very inspiring.

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