by Stephen Lieber
In the process of making a telescope of my own design I have learned much about this unusual craft. It is my hope to be able to share this knowledge with others. As different issues come to mind I hope to write them down and make them available to others. These articles are not intended to be a complete course on telescope making. There are some fine texts available for that. Rather I have found that some of the popular texts are dated or have some gaps. These articles are designed to fill in a few of the gaps. Also every person who gets involved with telescope making will bring a new perspective or discover new observations. My own observations may be of some help to others.
The starting point for any discussion of telescope making should be the wisdom that has already been handed to us from others. The central textbook is How To Make A Telescope by Jean Texereau. This is published by Willmann-Bell, Inc. P.O. Box 3125, Richmond, VA 23235, telephone (804) 320-7016. This book covers just about every aspect of telescope making, and his advise on polishing and figuring is second to none. Willmann-Bell also publishes other books that are useful. They also sell some high quality grinding and polishing agents.
Another excellent publisher is Sky Publishing Corp., 49 Bay State Road, Cambridge, MA 02138-1200, telephone (617) 864-7360. Their magazine, Sky and Telescope, also has a column written by amateur telescope makers. Some of them are extremely helpful. Their website has programs that can be downloaded free of charge. Some of these programs are related to telescope making. It is worth a visit.
Today’s large thin mirrors present their own problems with grinding and polishing. Bob Kestner wrote some excellent articles on this topic that were published in Telescope Making Magazine. They are in Volume 12, p. 30-35, volume 13 P. 36-41, and volume 16 p. 32-43. This magazine is no longer published, but it is worth the work to find a copy. Many amateur telescope makers hold on to the old copies.
The project of making a 17-1/2 inch telescope was made possible only with the help of many other people, most of whom are part of the Amateur Astronomers Association. Dr. Phil Pinches, Stew Rorer and Barry Levin each provided years of experience without which this project could never have been completed. My thanks to them and to many others who contributed to this project.
One final note. Be patient. This project was a much larger undertaking than I ever expected. Through constant effort and the help of others it could be done. If you are thinking of making a telescope do not be deterred by the complexity of the job. As you work, others will help. You will be surprised.
Grinding a Large Thin Mirror
Years ago amateur telescopes were made with mirrors of a relatively modest size. 6 and 8 inch telescopes were considered large. A 10 inch telescope might be the envy of a club or observing group. In the recent past people have learned how to make large mirrors. This started with the improvements in telescope design and mirror making that were popularized by John Dobson. It is easy to understand this as a type of engineering. It is not possible to simply take a good design and then scale it up. Such things do not work, not for buildings, not for bridges, and not for mirrors. Making things larger will introduce new complications that could be ignored with smaller designs.
For mirrors the deciding factors are weight and flexure. During the grinding and polishing process very great pressures are generated. The glass cannot be allowed to flex, even a very small amount. If it does the glass will relax when the pressures stop. This means that the mirror will have an unplanned and irregular shape when the work is finished. For smaller mirrors the answer has been to make them thick enough in order to be stiff. We would use 8 inch mirrors that were 1-1/7 inch thick and 10 inch mirrors that were at least 2 inches thick. These are referred to as being “full thickness mirrors”. Full thickness means a thickness to diameter ratio of 6 to 1. For larger mirrors a full thickness would quickly result in a mirror that would be too heavy to pick up.
Thin mirrors become very important for larger instruments. These mirrors can have a thickness ratio of 10 to one or even greater. They are not only lighter, they also cool down to the outside temperature much faster. They also cost much less. The difficulty is that they are much more flexible. Once people found out how to manage this problem, then a whole new world of telescope making opened up. Today a great many people have telescopes in the 12 to 18 inch range. More ambitious amateurs have been making mirrors in excess of 36 inches.
Choice of Materials
Pyrex is the material of choice. It is relatively cheap and available. The coefficient of expansion is small enough that it will not change shape too much during the course of the evening. The issue here is that as the temperature of the outside air changes the glass also expands and contracts. Ordinary glass expands much more. The best Pyrex is that which has been slowly annealed. It has a lower coefficient of expansion than does glass which has been annealed quickly. Also it is better to get glass which has been cut from a sheet. Glass that has been molded into shape will have much more stress in it. The stress will be circular in nature. This will result in subtle changes in the curvature of the glass as the temperature changes. A number of years ago many people were purchasing molded glass. Today this is rather rare.
This is not to say that ordinary glass will not work. It simply will be much more problematic. The 60 and 100 inch telescopes on Mount Wilson were made of ordinary glass, and they perform perfectly well. John Dobson started making telescopes from the portholes of ships. These were available at salvage prices at the time. Still the low expansion rate of Pyrex is far superior.
More exotic materials can perform even better, but they can be unrealistically expensive. A mirror blank of fused quartz would cost several thousand dollars. Personally I have other things to do with my money than spend it on such an expensive mirror.
Two full sized tools are needed for grinding. One will be used for the front and the other for the back. Many people have used plaster in order to conserve weight. Some people say that dental plaster is the best. Getting a 50 pound sack of dental plaster might be a problem. I have used cement. It is quite strong, and for the 17-1/2 inch mirror that I made, it was not a problem to handle.
Making a tool means making a mold for the wet concrete. A 1-1/2 inch strip of cardboard is what I used. A layer of plastic tape was put on the inside, so that the cardboard stayed dry. The strip was the size of the circumference of the glass. This needs to be reinforced. At first I simply put wire inside. No good! A good reinforcement is needed so that the concrete does not crack and flex. Exploded steel is the material of choice, and is available at lumber stores.
The grinding is done against unglazed ceramic tiles. These are often sold in hardware or tile stores. It is important to use tiles from a single batch as subtle differences in hardness can greatly affect the results on the mirror. For my mirror I cut these into 2 inch squares and attached these to the tools with epoxy. The tool needs to be coated in order to block any grains from falling off and scratching the glass. What I did was to coat the entire tool with epoxy. Other people have used polyurethane or other materials. The tool needs to be fully coated on all sides.
The reason for using two tools is that it is necessary to grind the back perfectly flat. If the back is not flat, then the glass will deform around a high spot. This in turn will produce astigmatism. It is possible to measure the flatness by making a straight edge. There is no need to purchase one. Simply take some fine wire and pull it straight. It can be held straight by tying it to a piece of bent wood the same way that a bow holds its string straight. No great force is needed.
Once the back is straight the tool that was used on it is still important. The tool and the back of the mirror are equal in shape. The tool can be put on the barrel. Wet a few layers of newspaper and place them on top. I usually spray water on the paper after I lay them down. After this comes a layer of pile carpet, trimmed to size and moistened. Then the mirror is placed on top. The idea of the water is to hold everything in place.
The mirror is now being supported by the tool that was used to grind the back flat. In this way the mirror has all of the strength of a full sized tool. Yet when the mirror is finished it will weigh much less. Be careful! Even well supported glass will still flex. The smallest imperfection will still cause astigmatism. The answer is to stop every few minutes and rotate the mirror a few degrees. This will randomize the error and not concentrate it on one spot of the glass.
The central concept is that errors cannot be completely eliminated. Mirror makers have learned to randomize errors instead. The same thing goes for grinding and especially polishing strokes. The errors are unavoidable. Just randomize the strokes. Lengthen them a bit then shorten them. Vary the pressure a bit. This is one of the beauties of making mirrors by hand. A machine can easily cause astigmatism and other problems simply because they do not randomize the errors.
An Unsual Problem with the Foucault Test
The Foucault test is an extremely accurate way to test concave surfaces. Like any accurate test it is also extremely sensitive. Things like heat currents and the like are famous for causing problems. One day Prof. Phil Pinches and I encountered a completely unexpected problem.
Most people try to test mirrors by setting up to test equipment in their basements. As I live on the beach a basement is not practical. The water table is only a foot or two below ground level. My Foucault tester is in my living room. One windy day the shadow on the image was shifting back and forth constantly. This had a regular period between two and three seconds. The amplitude suggested the mirror and stand were moving closer and further by several hundredths of an inch.
We knew that this was happening, but why? The wind was gusting outside, but the gusts were random. This movement we were seeing was regular. Perhaps we were observing the regular movement of the waves on the beach. There was a severe storm offshore at the time, so the surf was high.
A call to an engineer solved the problem. The house was experiencing what is called “vortex shedding.” This is turbulence that occurs as the wind whips past the house. Airplanes often try to control this phenomena by putting little tips on their wings. On my house this was causing the house to vibrate with a 2 to 3 second interval. The gusts of wind were like a guitarist striking the string with a pick. No matter how often the string is struck the frequency does not change. The striking only adds to the amplitude. In the same manner the gusts of wind were adding energy to the house and increasing the amplitude. The actual frequency of the house did not change. Thus I did not see the gusts themselves in the tester, only the frequency of the house itself.
How to control this? Isolate the vibration. Normally this would mean putting the equipment on a cement floor in the basement. But I do not have a basement. If I did I would have to swim. Instead the engineer recommended building two piers of equal height. One to support the mirror stand and one to support the tester. They could even be made of books. Lay a layer of bubble wrap on top of them, and place a board or plank on top of them. Then place the mirror stand and tester on either end of this. In lieu of a board a ladder could be used. In this way the vibration is isolated.
There are many sources of vibration that have to be considered in a test area. My house is not the only one where this problem has occurred. As it is these heavy gusts to not occur every day. Other people may have more consistent vibrations where they do their work. If so this type of solution may be needed.