Last touched 2001 December 6
Originally published in M-111, the Newsletter of the Richland (Ohio) Astronomical Society
Reflections in the Eyepiece
by Robert Bunge
They are a mystery to all too many amateur astronomers. For myself and a few others, they are words that incite a level of excitement, challenge and fun. Grit, pitch, slurry, foucault, compound, testers, readings - mirror making.
These days, few amateurs have tackled the challenge of making their own mirror. It's even more amazing just how many don't know the basics of how a astronomical mirror is made. Sometimes, I wonder if it is these amateurs whom I hear complaining the most about the high cost of optics. Hah... If you think optics cost a lot now, wait a year or two.
The commercial telescope making market has been in a slump. Throughout the late summer and into the fall of 1990, the two major U.S. telescope making firms, Meade and Celestron, were talking about merging. Why? Because of plummeting sales and a weak market. (In late 1990, approval of the merger was denied by the Federal Trade Commission. The denial was later upheld in Federal Court and has since been called off) Mind you, all this was happening long before Kuwait and the war in the mid-east. Last fall, at ASTROFEST - a long time gathering of amateur telescope makers (ATMs) - most people had decided the slump was tied to the poor quality of commercial SCTs, or Schmidt Cassigrain Telescopes. The theory was most amateurs had read reviews of SCTs in both major magazines and weren't impressed. Mind you, the reviews weren't bad, it's just that they weren't great and people read between the lines and decided not to buy a telescope.
Personally, I thought the reviews were far from nice. I about dropped my Sky & Telescope when one reviewer pronounced the curvy, twisted lines of one optical test to be OK. I was equally amazed to read two articles about visits to Meade and Celestron's factories in ASTRONOMY that clearly came across as public relation pieces meant to restore confidence in these companies to a disbelieving readership.
But, something else on amateur telescope maker's minds at ASTROFEST was that prices will rise in the coming months or years. If both companies believe that the best way to restore consumers confidence in their products is to produce better optics, then the prices HAVE to go up. You just can't produce diffraction limited optics for the price they are selling telescopes today.
So what does this have to do with grit, pitch, slurry, foucault, compound, testers, readings and mirror making? Simple. About ten or fifteen years ago, amateurs stopped making their own mirrors and started buying whole telescopes. Why? Some of the classic reasons include:
* Price - you can buy a mirror for less then what it costs to make one;
* Modern american lifestyle - the fast pace of life today doesn't leave enough time for long term commitments like making a mirror;
* "I don't have the skills" - A number of "skills" are needed to make a mirror;
* "I'm not good with math" - Reducing the test data of a mirror requires at least college algebra;
* "I don't have a good place to work" - Mirror making requires a certain amount of space and is best done in a low traffic area;
* "All the books say to start with a 6-inch, but I want a 12 or 14-incher" - The bigger the mirror, the harder it is.
One by one, these are starting to tumble - leading off with the price of optics. The cost of mirrors have gone up during past couple of years and will most likely continue to do so if amateurs continue to expect diffraction limited optics. In a letter to the editor in Telescope Making Magazine, Roland Christan of Astrophysics, once said a 1/20 wavefront 12.5-inch mirror costs at least $2,000 to make.
The life style excuse is only a matter of commitment. After a 12.5-inch blank had collected dust on a shelf for two years, I made a personal commitment to work four to five hours a week on it. A year later, the mirror is finished with a wavefront rating of 1/11 -- one of the best feelings of satisfaction I've had in years.
I've heard the one about skills over and over. Of course you don't have the skills! Any and all skills have to be learned and the only way to do that is to "do" it.
The one about math really isn't a bad excuse, but I find few people will admit to this. In my early days of mirror making, I didn't completely test my mirrors because I was not comfortable with the math involved. Today, computer programs not only allow me to run traditional tests faster with fewer errors, but also run series of tests that allow better judging of the quality of the mirror. They also allow me to analyze the data with different methods, one using lengthy calculations that a personal computer can do in seconds.
But, perhaps the best reason of all in the above list is a lack of working space. Nevertheless this can often be defeated by finding a location to work. In Columbus, mirror makers meet and work in the basement of a ATM who offers the space to those who don't a place of their own. For many years, Cleveland amateurs obtained space for classes at a local school.
The last one about not starting with big mirrors isn't bad either. However, I believe it isn't true. I have found bigger mirrors take longer, but they aren't always harder (short focal ratios have more to do with this than mirror diameter). In fact, I found bigger mirrors are easier to handle. John Dobson, famous for the mount he designed, should really be famous for the hundreds of mirrors he's made or help make. He once told me he thinks it is a waste to make anything smaller than 12 inches. In many cases, I agree.
In the end, there is a more important reason why more amateurs should make their own mirrors. That is so they know how their telescope works and just what has gone into it. Don't get me wrong, I'm not trying to make it a requirement that to be an amateur astronomer is that a mirror be homemade. It has been my experience that many advanced telescope makers who haven't made their own mirror sometime regret it. All four class members of a recent mirror making class in Columbus had all made at least two telescopes. Three had won awards at conventions and one has been published in several magazines. For each of them, it was their first mirror. Three of the four had decided making a mirror was a important part of telescope making and the class was their chance to learn from others. The fourth had a great reason. He wanted a small, fast mirror that isn't commercially available. But, in any case, these ATMs are now educated comsumers.
Even if they decide to purchase their next mirror, when they do so, they will have the skills and knowledge to test the commercial mirror and confirm the mirror is up to the manufacturer's claims. This will help insure they get a good product, which into allows the amateur community to better police the market. Recently, ASTRONOMY magazine has run articles on how to "test drive" your telescope even through they published similar articles only a few years ago. The publishing of these types of articles helps to insure truth in the commercial market.
So, I urge amateurs, even SCT owners, to learn as much as they can about optics and how a good telescope performs.
Back to Bob's Homepage.