TimC From The Workshop #2...(Draft)

On 1/29/2014 8:41 AM, Tim Crawford wrote:
From The Workshop #2...(Draft)
    Crafting a telescope mirror can be a rewarding endeavor. Once, in my earlier years I went downtown and bought a telescope mirror kit from Douglas Telescopics. I bought a 55gallon drum to use as a work top . I set it all up and opened Allyn Thompson's Making Your Own Telescope.... I was stunned ! It made no sense. I put everything away-for years! The reason for this short aside is to let you know it is not uncommon to be lost in the beginning when you try to do it a cappella. I strongly advise you to find someone to guide you, like you get in a workshop like ours. There are many in the group with hundreds of hours crafting mirrors. They have vast experience and can guide you through all aspects of crafting your own mirror.
    So you are handed a round blank and another tool of let's say the same diameter. As in the previous letter, you will use a grinding medium between the two to grind a telescope mirror. Before you begin though, there is one important step to do. You must bevel the outer edge of your blank. You may use a carborundum sharpening stone to do this. Soak it in water and using a downward 45 degree angle, grind slowly the edge of your mirror. For purposes of these essays, I will use Pyrex as the material we use as a mirror. This bevel will prevent chipping of the edge of your Pyrex blank while grinding. The bevel should be at least 1/8th inch wide. Choose a workplace that has somewhat constant temperatures and a work surface that has strong support.
    How does it all happen then? How do two flat surfaces become a perfect sphere and a perfect convex compliment of this sphere? The answer is an elegant one and a little obvious on hindsight. Let's not think so much about the abrasives here but instead, think of the two discs passing over one another. The disc on top just by its own weight has downward pressure on the lower disc. The upper disc is worn in the center and the lower disc is worn on the edges. But, let's think of it another way to further illustrate the process. If the upper disc is passed over the lower, using chordal strokes let's say, first a little to the right and then a little to the left, then the middle of the disc on top is getting worked full time, while the edges get a break when the disc is either right or left of center. The center has to wear more than the edges. Now, add one more element- controlled randomness. Sounds kind of contrarian but it is the most important. By walking around a worktop in one direction while turning the mirror the opposite direction while grinding, the discs become worked evenly, in fact perfectly. But, how deep do we make them? How do we stop from going deeper and deeper. Next edition we will cover this as well as terms you will likely use if you decide to grind your own optics.