I saw a posting from my astronomy email group that indicated the DelMarVa Astronomy Club would be hosting the 9th Annual Mid-Atlantic Mirror Making Seminar. Well, I wasn't interested in a seminar, I go to too many already in my day job. But, it turns out this is a 3-day weekend (Fri-Sun) of total immersion mirror making. They promise that you come Friday morning and you will leave with a finished mirror by Sunday afternoon. Bottom line: TRUE.
So, I contacted Don Surles, who runs the workshop, told him I wanted to make an 8" f/6, and signed up. It cost $300 for the pyrex mirror blank, already hogged out to a rough sphere by a machine, the glass tool, and all the food you could eat and a clean bunk to sleep in when you get tired. I arrived in Smyrna, Delaware at the Mallard Lodge early on a Friday morning, checked in and was given my tools and instructed what to do. I had made a 6" mirror a decade before, so this was not totally new to me. Hogging out a mirror blank means using a very coarse grade of the abrasive, carborundum and grinding two flat pieces of glass (the mirror blank and the tool) against one another till the top piece becomes convex and the bottom piece becomes concave. It is hard work but the result is truely magic. There were about 10 to 15 other mirror makers there that weekend. Because the mirror blank that you get at DelMarVa is already hogged out by a machine, you start shaping the convex hole into a portion of a sphere using less coarse carborundum. The expertise of the several helpers allows you to get to a sphere reaasonably quickly. Then you spend a lot of time making the glass smooth using finer and finer grain carborundum and other abrasives. Once the glass is smooth and polished, as judged by the 'filament test' as seen in the rightmost picture below, you work on figuring. Figuring is the process by which you alter your stroke, using a special tool that they help you to make, to change the shape of the convex glass from a sphere to a parabola. The latter figure is what is necessary to focus parallel rays of light back to a point at the focal point of the mirror. All this work was supervised extensively and the testing of the figure was done by an imported expert, Steve Swazye. Sometime on Sunday morning, after two 14-hour days of grinding, polishing, and figuring, Steve tested my mirror for what seemed like the 100th time and said "I don't see anything wrong with this mirror". Ok, then!!! Steve used a Barlowed Ronchi test to determine if the figure of the mirror was good and, if the lines are all parallel and straight from edge to edge, it is indicative of a good mirror. On my mirror (finally) they were. So it seemed as if it came out well. Star testing and high power observation of Saturn confirmed these bench tests. The completed telescope works very well. I highly recommend this approach to mirror making. It was so much fun I am thinking about going back to do a 10".
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