Step 12: Bonus Material: Rough and Fine Grinding the Mirror

Photo #1 shows the rough grinding process. The mirror blank is ground against a ceramic tile tool with #60 carbide grit in between. Using the correct grinding stroke results in the mirror becoming concave as the tool becomes convex. This process is continued until the mirror reaches the desired curve. The process was all done by hand, and took several weeks of Saturdays, (and raised a few blisters).

Photo #2 shows the mirror at the end of rough grinding. I was aiming for an f/4.5 mirror, so the required depth of the curve is 0.175 inch. Once the required depth is reached, the rough grinding is complete. Amazingly, the rough grinding has removed over a pound of glass. The mirror now weighs in at only only 8 pounds 15 ounces. Now the fine grinding can begin to clean up the rough surface left by the rough grinding.

Photo #3 shows the sequence of abrasive grits that must be used to get from rough grinding, through fine grinding, to polishing.  Successively finer grits are used to clean up the pits left by the previous stage. Eventually the mirror reaches the stage where polishing with Cerium Oxide can begin.

Photo #4 shows the mirror at the end of fine grinding. The surface is not quite transparent. It still has a slight frosted look to it. Rough and fine grinding are done outside to prevent contaminating the polishing room with large grits that could cause scratches on a mirror. Once it is time to polish, the mirror gets thoroughly cleaned and I go into the polishing room. See the next step for polishing.
<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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