University Lowbrow Astronomers

Can You Do It? Make your own telescope mirror, that is?

by Doug Scobel
Printed in Reflections: June, 2003.

Make your own Newtonian telescope mirror that is?  Most people think it is such a daunting task that they never even consider it.  But is it really that difficult?  When I started out in this hobby back in the late 60’s (I’m showing my age here), most everyone made their own mirrors - commercial mirrors were simply too expensive.  Nowadays, due to various reasons, the relative cost of commercially made mirrors is much less.  So if your main reason for making your own mirror is to save money, then don’t do it.  You’ll end up spending nearly as much money as what a finished mirror would cost, at least one in the smaller sizes.  But if you enjoy building things yourself, there is nothing like the personal satisfaction you get when you finally get a good mirror, accurate across the entire surface to within a couple millionths of an inch, that you made yourself, literally with your bare hands.  Plus, you will have learned a skill that few others have mastered.

But doesn’t it require sophisticated tools and measuring devices?  Well, yes, but they are easily made or obtained.  The primary (pun intended) tool you need is patience and the ability not to get frustrated.  Bad, or I should say unpredictable, things will happen, and you need to be able to objectively analyze what’s going on to figure out what to do when things don’t go according to plan.  This is where the help of someone else or a group with mirror making experience is valuable.  All the books and all the web sites simply can’t cover it all - there are too many variables.

But doesn’t the primary mirror have to be nearly perfect to be useful?  Not really.  There’s perfect then there’s perfect.  Even a mirror that produces an error at the wavefront of, say, 1/2 wave will produce a decent image at low to medium power.  And a 1/4 wave mirror will provide as good an image as atmospheric seeing will normally allow.  Besides, if you are working alone, then you have two chances of getting that nearly perfect mirror on your first try - slim and none, and slim just left!  Seriously, there is too much to be learned with just one mirror, unless you are willing to have the project go on for years.  My personal suggestion is that you try to make an OK first mirror, then go for a better one.  It will take much less time than trying to get a “perfect” mirror on your first try.  And don’t start out with anything larger than an 8 inch - it is generally accepted that the difficulty grows roughly with the cube of the aperture.  Stick to moderate focal ratios - say between f/6 and f/8.  Faster focal ratios require a very precise figure, while with slower ratios the paraboloid is so close to a sphere that it is next to impossible to figure.

What about grinding the curve - isn’t that difficult?  Actually, rough grinding the curve is probably the easiest part of the job.  Fine grinding without leaving larger pits behind is a little harder.  Polishing it out completely is a little harder yet.  Figuring is by far the most difficult part of the job, maybe by an order of magnitude.  The difficulty here is twofold - mastering the testing methods to analyze the surface, and then mastering the techniques you need to manipulate it.  This is where you need to keep a cool head, go slow, and not get frustrated.  Once you stop thinking objectively, or over-analyze things, things can easily get out of hand.  And by all means ask for help.  There are a few Lowbrows that have made mirrors, and there are also on-line forums that are helpful.

I made my first mirror when I was 14, a 6” f/9 (it was supposed to be an f/8, but I had trouble getting the curve deep enough), by myself using Thompson’s Making Your Own Telescope.  It probably came out no better than 1/2 wave - I don’t know - I really didn’t know how to test well yet.  After using it for a while, I re-ground it to f/4.5.  This ended up a little better, maybe 1/3 wave.  Next I made an 8” f/8, which ended up about 1/4 wave.  Then I refigured my crappy 1 (yes one!) wave Coulter 13.1” f/4.5 to about 1/4 wave.  Now I’m finally refiguring my 8” f/8 to be as good as I can get it - I won’t settle for less than 1/10 wave.  My point is that most people need the experience of easier projects before going on to more difficult ones.  There’s as much (if not more) art as science when it comes to testing and figuring, which simply takes time to learn.  But if you have modest expectations for your first try, then try again for that really good second mirror, your odds of success go way up.

So, can you do it yourself?  Only you can answer that question.  But if you put your mind to it, and keep at it, you just might surprise yourself.


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