Article From ATMoB March 1999 StarFields news letter
Last year I have finished re-figuring a 4.5 inch plate glass mirror from an old Tasco scope I bought back when I was in high school. This scope has been a joy and frustration to me. When I paid $150 for this scope, it was the biggest investment I had ever made. At the time I was paid $1 a day for helping my father deliver milk for our family business. Since I was only working once a week during the school year plus vacations, it took me a significant length of time to earn this amount of money. Not having ever looked through a scope before, I had a nothing to compare the image to. It was OK during the day and looking at the moon, but for smaller objects like the planets, it was less than enjoyable. Saturn and its rings were just one blur.
Over the years, my interest in astronomy beyond knowing the constellations and backyard viewing with binoculars had been book orientated. This scope, as bad as it was, was my symbol that I was an amateur astronomer.
When I joined the ATMoB, I started an 8 inch mirror. It was not long into my grinding when my interest in my old scope was awakened. I found it packed away in the back of the barn and cleaned it up. When I collimated it, I found that my memory of the poor image was not exaggerated. I took it up to the club house and Bob Collara set up the mirror in the test tunnel. It was clear why the image was so poor. The mirror showed a number of concentric zones with a peak in the middle and a very turned up edge. It looked like the mirror had been figured by a lap attached to a drill press.
I thought the mirror was junk and should be thrown away but Bob convinced me that refiguring it would make a good project. There were a number of advantages to this. My 8 inch mirror would take some time to complete and considering my perceived mechanical ability, I had doubts it would ever get to be part of a usable system. If I could refigure this mirror, I would have a nice portable 4.5 inch that I could use in the mean time since I already had a tube and mount for it. Bob said that the thin primary plate glass mirror would cool to ambient very quickly and hence maintain its figure quite admirably. He also noted that a thin plate glass mirror can out perform a thick Pyrex mirror up to about 10" in diameter because even though the Pyrex has a lower coefficient of expansion than plate glass, a thick Pyrex disk will never stabilize in a 3 degree F drop per hour observing session. Also since plate glass is softer than Pyrex it would be easier to work, but because it was so thin, about 0.375", it would deform easily with the pressure of figuring. To overcome this we made a jig from a piece of 2 by 10 plank in which we routed a 4.75 inch diameter indentation to a depth of 3/8 inch to hold the mirror. Bob obtained some uniform closed cell foam used in optical shops to uniformly support optical glass on the spindle during fabrication and we cut a piece of it to fit the indentation in the jig. This would keep the mirror completely flat during figuring.
All figuring except parabolizing was done with the mirror on the bottom. There was no way to parabolize with the mirror on the bottom. It took me a long time to remove the concentric zones. This being my first try at figuring, I was less than aggressive in my technique. Now that I have this experience, I do not think my next try will take as long.
When I had finished parabolizing, we tested the mirror in autocollimation via foucault test built by Fred Miller. With a Ronchi grating, we got absolutely straight fringes and in a knife edge test the surface blinked gray all at once. Testing a parabola in autocollimation requires no mathematical data reduction. It will blink just like a perfect sphere when using a knife edge tester. No wavefront measurements where made, which I may want to do some time in the future, but I was more than pleased with the results of the testing we did.
I had the mirror coated in Seabrook in October, thirty five years after I bought it. Bob tested the secondary. It turned out to be more than 1.25 waves convex so I ordered a replacement from Orion. In the mean time, I have replaced the .965 inch focuser with a 1.25 inch focuser so I can use standard eyepieces. I am now thinking about replacing the mount with something more stable. We have also talked about modifying the mounting cell.
I have found this project to be very rewarding. I feel much more knowledgeable about mirror making. I have turned all the frustrations with this scope into positive experiences. I now have a good optical instrument to boot. I can't wait until people start making fun of my using a Tasco scope so I can show them what has been done with it. -Bill Toomey-