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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I’m slowly dealing with the crack’s aftermath. Slowly, I ground through the big hole I made when burnishing the crack. As I ground it down over the week, the crack wasn’t visible under the loupe unless I put a flashlight sideways and used a finger to cover the top of the mirror, illuminating from the side. After watching the hole get smaller and smaller, I took a look… and the crack was still barely visible.
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Tragedy!
I dropped my tool onto the glass from an inch up. I thought I was fine, but close inspection revealed a crack a millimeter long. I was advised to try to burnish it out because the crack could travel very deep and spread.
I ended up putting a screw in a drill and using the head as a bootleg buffing wheel with my 30 micron water mix to try to dig out the crack and its subsurface damage.
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18 minutes later, I ran out of grit and the unground region is less than 0.5mm wide. I’m calling it: good enough! I’m done grinding the back!
Total back grinding time: 5 hours 57 minutes
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Grinding the back has gone smoothly. At first I didn’t get good contact between tool and mirror; one orientation would slide freely but after turning the mirror 90 degrees any attempts to slide would lock up. Eventually the tool wore down and I got good contact.
In these pictures, my goal is to spread the frosted area (where the mirror has been sanded down to a sphere) everywhere. Any smooth areas are where the grit hasn’t made good contact (lower areas).
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When grinding thin mirrors, there is something called the “twyman effect”: grinding one side adds stress that can slightly curl the mirror’s other side. It can lead to slight astigmatism in thin mirrors that disappears once you grind the back side to relieve that stress. (See https://quinsightspectre.com/16-25-f-3-1-meniscus-mirror/ !)
To avoid any problems, I decided to follow the wisdom and grind my meniscus mirror’s convex back through #220 grit. Plus, it would get rid of any saddle shape, which I could see existed when beginning to grind the front.
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Looks like after 3 hours of grinding at #220 grit, the divot that has been in my glass since the start has been ground out! Yay! I’m glad I went back to coarser grit; this could have easily been six hours had I stuck with #320.
A mirror with a short focal length will make objects look brighter because they’re zoomed out, but is much harder to parabolize at the very end of the mirror making process.
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I’ve gone to 200 grit from the finer 320 grit. I’m glad I did; an hour of grinding later and my last tiny divot still has’t ground away. Had I stuck with 320 grit, I’d need to spend twice as long to get to where I am now. Hopefully another hour will get it out.
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A problem! After 6 hours of total grinding, I measured my mirror’s focal length and found it was 37". That’s weird because before it was 39" - maybe my hands were applying pressure in the center of the tool? Lower focal length means more zoomed out (and brighter) telescope views, but makes the final stage of parabolizing harder. If focal length / mirror diameter is less than 4, you also get “coma” that distorts the stars towards the edges of the view.
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Today’s hour of mirror grinding has completely gotten rid of the low zone! Now I just need to grind down through that one tiny divot.
I noticed it seemed like my tool was pushing the water I sprayed out off the mirror. I used a blade to try to carve notches in between my hexagonal tiles, and afterwards I could feel the tool sliding more smoothly, allowing water to flow between the tool and the mirror.
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The cloudy part is where the grit has sanded the glass. Clearly part of the glass is higher and making less contact. I bet it’s because the furnace cement mold was slightly curved because the wooden mold was slightly curved thanks to the grain of the wood