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Grinding with thin telescope mirror kits:


Thin glass grinding differs slightly from "standard" mirror making practices. Keep these principals in mind when using your thin mirror kit and you'll be fine. Also, be certain to support your thin mirror properly when it's finally IN the telescope!



PRESSURE


When grinding thinner blanks, be conscious of downward pressure - if excessive, it may bend the mirror blank out of shape. While this 'bending glass' effect is small, it makes proper support and regular turning of the work important. At later stages (fine grinding and polishing) you should see that the stroke forces are distributed evenly across the mirror blank surface. To achieve uniform distribution over the entire surface area, make sure to grind your mirror blank using a slightly yielding backing substrate such as a thick section (1/2 inch or so) of wet newspaper or carpet, which will support the entire back of the mirror blank while at the same time having some "give" which allows the forces you apply to be spread evenly across the glass.


HAND AND FRICTION HEAT EXPANSION


When you place your hands on the back of your mirror blank, you change the shape of the glass - expanding it where it contacts the heat from your hands. Heat distortions can be "polished in" to your telescope mirror. Arrange a method of keeping the heat from your hands off your mirror blank (and tool if made of glass) while fine grinding and polishing - gloves for example, or other form of thermal barrier to keep the heat off your work. Ideally, your mirror blank, and the objects that contact your mirror blank (tool, water, grit or polishing compound, backing material, etc) should all have had time to equalize to room temperature. As for friction, if you've ever rubbed your hands together to warm them up on a cold day you can see how fast polishing will warm your mirror blank just as it warms your hands, though the effect on your mirror is not as desirable. The heat from polishing friction distorts thin and thick mirror blanks alike, and unlike the heat from your hands which starts at the back of your mirror blank, friction heat distortions emanate from the front of the blank. Heat from friction increases with fast strokes and heavy downward pressure, so slow polishing strokes and light pressures keep heat expansion in your telescope mirror blank to a minimum. Long polishing sessions using consistent pressure creates even temperature equalization between your mirror blank and tool. Short polishing sessions are used during parabolizing, so see that your mirror blank and tool start out at an equal temperature during these short sessions, and use light or no downward pressure to minimize friction heating.


THIN TELESCOPE MIRROR MAKING HELP ON THE WEB


There is a wealth of information on the net about the benefits of thin mirrors, and techniques for grinding thin mirror blanks. Here is one such website created by a guy with lots of experience in the realm of creating very fine homemade telescopes using thin (and ultra-thin) mirror blanks... his name is Mel Bartels. Highly recommended!

Mel Bartels ATM Telescope Website



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