My third telescope project pushes the boundaries of telescope technology, using kiln draping techniques to create a thin yet constant thickness meniscus-shaped mirror that will give bright views for cheap. This mirror will be 12" f/3.3, but only 0.5" thick. Only a few people in the world have tried making mirrors this thin and this fast.
Making mirrors this thin is a bit heretical for amateur telescope making. Telescope mirrors must hold the correct shape to within a fraction of a wavelength of light (500 nanometers). Glass may look rigid, but on small scales, glass is floppy and will happily bend under its own weight like a piece of paper. To resist bending, traditional mirrors are almost an inch thick (or often more).
However, large volumes of glass may take a long time - hours, even - to cool down when brought from warm indoors to cool outdoors. Because materials expand and contract with temperature, the difference in thickness between a larger edge and smaller center will change the mirror’s shape during those hours of cooling down. Traditional mirror owners must wait many hours for their mirror to cool down to ambient temperatures before the stars will look pinpoint. A meniscus mirror is thin - only 0.5" thick - and its curved shape gives it a constant thickness the entire way through, reducing the problems of cooling.
My 0.5" thick meniscus mirror with 0.25" sagitta, bending-wise, should like an 0.75" thick traditional mirror with a thickness of 0.5" at its center. Less glass means it is easier to carry and cools down faster in the cold night air.
Also, buying thick glass cylinders costs hundreds of dollars. I bought a glass countertop from an used furniture store for $20 and cut a hole out of it.
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Five or so 30 minute long sessions of long parabolizing W strokes have finally turned my center hill into a center hole. The dip at around 80% to 90% diameter remains, while the 90% to 100% diameter edge zone is turned up.
I made two changes to get better pitch contact: I am filling a plastic container with hot water and pressing the pitch to the glass underwater, and I am only polishing for 30 minutes at a time instead of going for a full hour.
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I thought I was close to a sphere, but some 1/3 center over center strokes look like they just made my oblate mirror shape less spherical. I realized I have an oblate spheroid, with a center slightly higher than it should be.
My theory is: my mirror has a slight central hill. When I press the pitch lap on top of it, the pitch forms a ring shape, with not much contact in the center.
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More wide parabolizing strokes are bringing my 12" mirror closer and closer to a sphere…
I have a slight worn down edge, overall a tiny bit oblate, as well as a tiny central hill, but it’s all very small.
Here’s a Ronchi test picture of the mirror today compared to two weeks ago. As a reminder, straight up and down is spherical.
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I’m around 10 hours into polishing, and it looks like I’m very close to polished out! I can see barely any tiny pits under a microscope. I think I’m done.
Next up, figuring. I can test a mirror’s shape easily. I have been doing that already despite not quite being done with polishing.
The problem with a thin mirror is that glass (and anything else) will happily bend a few hundred nanometers under its own weight.
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Polishing continues. I’ve noticed that my short center over center strokes seem to be creating a hill in the center of this mirror, so I’ve switched to longer 1/2-diameter strokes and 1/3 side to side motion in order to wear down the center of the mirror. It seems to be working, according to these ronchi test images before and after. I’m jumping the gun a little by figuring while polishing but there’s a long way to go so I should be fine.
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My mirror is now reflective enough to see the entire surface and get my first Ronchi test image! But I’m not done with polishing - it may look reflective by eye, but the same region under a microscope reveals tiny pits left over from 5 micron grit are still there. I’m not polished out until all those are gone.
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My pitch lap polishing tool wasn’t making good contact with the center of my mirror. I can tell because I’m just beginning polishing and so I can see that the mirror’s outside is more reflective than the inside. Also, the tool looks unevenly worn.
I tried heating up the tool using a tub of hot water so the pitch would flow and then pressing the tool against the mirror some more (pictured) but it seems to have only helped slightly.
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Another hour and a half of polishing and my piece of glass is looking much more reflective!
I was stroking with tool on top, but I noticed the center of the pitch lap wasn’t making good contact. I suspect the plaster backing tool that the mirror rests on isn’t evenly supporting the mirror; it picked up a slightly wavy surface I was hoping wouldn’t cause issues. When I poured the tool I used wax paper to ensure it wouldn’t stick to the mirror, and the wax paper crumpled slightly from being flat on a non-flat mirror surface.
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I finally started polishing this 12" mirror! It has taken so long and many failures to get here. I took the pitch lap tool that I made and carved some bonus channels into it, squirted on cerium oxide mixed with water, and got polishing. An hour and a half later, the outside of my mirror is polished and now is smooth enough to reflect light! It looks like my pitch lap wasn’t pressed for long rnough to make good contact with the mirror, though, because I can see that only a ring around the outside is being worn down, not the center.
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