Become a better telescope mirror maker

Well sure, apprentice with an expert, study the literature paying particular attention to the writings of the first opticians who faced the initial barrage of obstacles to overcome, and check-in with fellow mirror makers.

There is more than that though. Here’s my story or how I learned to be super cautious with optics making.

I'm the type to read all the books, study all the literature, especially going back in time to the early practitioners who faced the initial blast of problems.

My first mirrors, well, to be blunt, were pretty hit and miss. Some were awesome and some were terrible. How could that be? I dutifully followed the instructions in the telescope making books. Was it going to be necessary to star test each mirror, then redo it if it turned out badly?

About that time I took physics lab in college. It wasn't like in high school where we could 'fudge' the experiment with a wink and a nod. What I learned above all else is that experiments are hard for very real and for cognitive reasons.

I learned to over compensate for my over-confidence by surrounding myself with every conceivable test. I used Foucault and Caustic and Poor-Man's Caustic and Ronchi and Star Test; much later I experimented with Hartman. But you know what I learned from all that is that it is stupid to blindly over-test. It lowers the odds of a bad mirror escaping for sure, but I didn't really learn more than testing is hard.

What I really needed was an understanding of what I wanted to accomplish with the mirror and how to test for those seminal requirements. So I studied various ways that the masters broke down mirror making. I picked an optimal path of fewer steps with sharpened goals: generating the curve by rough grinding, fine grinding to repair the damage caused by rough grinding, polishing to repair the damage caused by fine grinding. And most of all, I studied the various approaches to parabolizing such as small and large tools, deepening the center versus deepening the edge. I selected tests to verify each stage. For instance, the polishing stage doesn’t need an accurate wave rating because the mirror is nowhere near parabolized, but the polishing stage does need a test for large and small scale smoothness in order to answer the question, “Am I done with polishing? Can I commence a parabolizing run?”

And it turned out that that wasn't enough.

What I discovered over the years was that I needed a self-adapting process that I could learn from, that would alter and improve my technique for every unique mirror situation.

So that's why I am so eager to grab people by the shoulders and get them to see that it's not just about the stages and the tests, but how they are used and for what purpose and at what stage in the mirror making process.

That's why I do not think it very enlightened to use a single test all the way through. But that's the common approach, and so people end up arguing test 'x' is better than test 'y', and nonsense like, 'I'll show you a bad mirror made using test y, therefore test y is bad'.

Keeping in mind that there are 20 ways to make a mirror for every 10 opticians then surely we can do better. With the OODA loop (Observe, Orient, Decide, Act) I have been able to push hard on the boundaries of what is commonly accepted as the limits to amateur mirror making.

One of the good uses of OODA is the idea of validated learning (I hate the concept of 'failing quick' by the way, it's perverse). So I don't as much make mirrors as attempt to learn from them. For example when I made my 13.2 inch [34cm] f3.0 meniscus constant thickness mirror, I systematically attempted to parabolize the mirror a dozen different ways. From that emerged a standard approach to smoothly parabolizing a very fast mirror using of all things a slightly oversized pitch lap.

It’s not sufficient to learn a test well and to stick to the same approach for every mirror. In my mind these are but ingredients to be used in making a tasty meal. Shall I say, “Food for thought?”

Mel Bartels