How to Grind a Telescope Reflector Mirror
Telescope mirrors are rounded pieces of glass used to focus the weak light entering from the night sky onto a secondary mirror that bounces it to the eyepiece. They must be very finely honed to achieve the exact focal length needed for a given telescope. This is hard work, but there are many reasons to do it. Grinding your own mirror may be cheaper than buying one, or it may be the only option if you want a mirror of an unusual focal length or size. There is also an element of pride in hand-honing such a precise piece.
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Instructions
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Initial Grinding
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Buy a circular glass or pyrex blank. These are available as part of mirror grinding kits or from your local glass shop. An old ship's porthole would work as well.
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Calculate how deep your mirror's "saggita" needs to be. This is the section of a sphere that must be dug out of a flat disk to create a lens. The radius of curvature is twice your desired focal length. The depth to be dug depends on the width of the mirror; consult a depth table to find out how much material you must dig out. For example, with a 1000 millimetre focal length and a 150 millimetre lens width, the curve must be 1.4 millimetres deep at its center point.
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Bevel the edges of the disk by rubbing it with a wet sharpening stone at a 45 degree angle. This will prevent the edge from chipping during the grinding process.
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Make a grinding tile tool to dig out the lens. To do this, glue pieces of shattered ceramic tile to the surface of another, matching glass disk or circular piece of ceramic, and coat the whole thing in epoxy or boat varnish.
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Support the mirror blank on a piece of carpet and sprinkle #80 grit on it. Spray it down with water to keep the dust from entering the air, as this could cause lung damage.
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Dig out (or "hog out") the center of the disk by placing the grinder tool on the work surface and pressing the disk on top of it, turning it in your hands to make circular strokes. Your strokes should be 1/3 of the mirror diameter long. Spin the grinding tool around randomly to keep the grinding even, and add grit and water whenever you stop hearing the loud grinding noise from the disk.
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Switch to "tool on top" grinding by setting the mirror down with the growing depression facing upwards. Continue to add grit and grind with random strokes, moving the tool until the bowl reaches the edge of the glass blank.
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Measure the depth of your bowl. If you need to deepen it, switch briefly back to having the tool on the bottom.
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Clean the disk, the entire work area, your clothes, and your hands very thoroughly to remove all grains of #80 grit. If any remain, they will cause scratches during fine grinding.
Fine Grinding
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Check the consistency of your grinding by drawing a grid on the disk with a marker and then starting to grind again with a fine grit. If the marker wears away evenly, you're grinding evenly. Unworn patches show uneven spots that must be ground away.
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Continue grinding with the same method, moving to finer and finer grits of abrasive. A sequence of #120 grit, then #220, #400, #800, and #1000 is appropriate. To know when to switch grits, look at the mirror with a magnifying glass. If all the pits in the surface match the current grit, you can switch to a smaller one. This process should take many hours.
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Mix the finer grits with water. Above #400, you will need to add 1 part grit to 6-8 parts water in a bottle and shake before applying it to the mirror surface. At this point, you will not be removing much depth, just smoothing the lens. When the lens has been ground with the finest grit of abrasive and shows a completely even surface when grid marks are drawn on, grinding is complete. The glass can now be aluminized and polished to turn it into a mirror.
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References
- Photo Credit telescope image by Warren Millar from Fotolia.com
