Grinding thin telescope mirror blanks:


Thin glass mirror making techniques differ only slightly from "standard" mirror making practices. Keep these principals in mind when grinding and polishing your thin mirror blank, and you'll be fine. Also, be sure to support your thin mirror properly in the telescope:



FORCE


When grinding thin mirror blanks, be conscious of downward pressure, which if excessive, may bend the mirror blank momentarily out of shape. While this is not a problem during rough grinding (as the amount of flex is exceedingly small) the effect becomes important as you reach the later stages of making your telescope mirror. At later stages you should see that the stroke forces applied to your thin mirror blank aren't excessive, and that they are distributed evenly across the mirror blank surface. To achieve uniform distribution over the entire surface area, 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.


HEAT


When you place your hands on the back of your mirror blank, you change the shape of the glass by expanding it where it directly contacts the heat from your hands. If sources of heat are not kept from the glass, distortions can be "ground 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 laid across the back of the glass to keep the heat off your mirror blank. Ideally, your mirror blank AND the objects that will be contacting your mirror blank (tool, water, grit or polishing compound, backing material, etc) should all have had time to equalize to ambient room temperature.


FRICTION


Ever rubbed your hands together to warm them up on a cold day? Friction from fast polishing will warm your mirror blank just as it warms your hands, though the effect is not necessarily 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 creates heat distortions emanating from the front surface of the blank. Since heat from friction increases with fast strokes and heavy downward pressure, slow polishing strokes and light pressures will keep heat expansion in your telescope mirror blank to a minimum. Long polishing sessions using consistent pressure creates good temperature equalization between your mirror blank and tool. Short polishing sessions are used during parabolizing - 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 Mirrors and Mirror Making


There is a wealth of great information on the internet regarding the benefits of thin mirrors, and techniques for grinding thin mirror blanks. Here is one such website created by someone with much experience the realm of creating fine homemade telescopes using thin (and ultra-thin!) mirror blanks... Mel Bartels. Highly recommended!

Mel Bartels ATM Telescope Website