carborundum) for this task. There are many ways of doing this. We have large, heavy work-benches instead. These shards will cut your fingers, and they will also scratch your mirror. ), If the sagitta is .125 (1/8"), then the focal ratio is half the mirror diameter. With no units. ). Here’s a diagram that I hope will help:  It is supposed to show a mirror (blue) with an insanely short focal length of 7.03 inches and a diameter of 10 inches. One way: do about 8 circular strokes. Rough Grinding of a Telescope Mirror. So if you plan on making an 8” diameter mirror with a 40-inch focal length, 40” divided by 8” gives you an f-ratio of 5. (This needs to be demonstrated!) On the opposite, a shallow mirror (e.g. For the sagitta (depth of curve) required, it’s easiest to use the formula for a parabola facing upwards with its vertex at the origin, namely,  . We don’t. In fact, I recommend overshooting it by perhaps 5 thousandths of an inch, since the hogging stroke does not create a strictly spherical shape. And that’s what we see in the diagram, too, if you look carefully. ... we use smaller and smaller abrasives to remove all the remaining pits from rough grinding and prepare the glass disc for polishing and figuring. Contrary to the 12" example in the previous video, very little CeO and low turntable speed is being used. However you do it, it doesn’t take too long, and it will prevent lots of problems. Note that a spherometer will typically NOT measure the entire diameter of your mirror, so you need to account for that smaller diameter. Rough grinding a telescope mirror. You should calculate and record what your numerical goal is for your chosen method of measurement. We use both hands to make the mirror travel in a circular path in either direction for about 6 turns, grinding the center of the mirror mostly against the edge of the tool. Using the formula   , and plugging in 5 for x and 7.03 for F, we get that y (the sagitta) would be about 0.889 inches, a bit more than 7/8” of an inch of glass to remove. It worked great. Place protective layer of plastic on mirror face that stretches up to top of cardboard dam, Place 1" square unglazed ceramic tiles (Daltiles) 6mm thick, webbed side down, on plastic that covers mirror face, Follow Hydrostone directions and add it to water (for these 30" tools, I used 25 pounds of Hydrostone mixed with 1/3 by volume of water), Then mix it with a heavy duty drill with mixing attachment for a couple of minutes, Add generous amounts of vermiculite (~1:1 by volume), Power mix as above, adding a touch of water when it becomes too thick to mix nicely, Pour over tiles that are laid on plastic that is covering the glass surrounded with a cardboard rim, Use trowel and shaking/tipping to make top flat, If the sagitta is .0625" (1/16"), then the focal ratio is the same as the mirror diameter. I like to rotate them both in the same direction, like the hands of a clock, with the top piece of glass going a little faster than the bottom one. If you wanted a 10” diameter mirror with the exact same focal length, we would call that f/4. telescope making, mirror grinding, homemade tripod, eyepieces, astronomy . I made my polishing tool for my 12" mirror out of three layers of 3/4" particle board glued together. Posted by gfbrandenburg in History, Telescope Making, (Part two of my description of the essential steps in making a telescope). Change ), You are commenting using your Facebook account. In the process of Rough Grinding one side gets the the desired sperical curvature using an abrading medium (silicon carbide) and a very simple grinding tool.