FAQ's on Telescope Making

(with asnwers!)


Can anyone share the basics of making a mirror?

I'd suggest getting a kit or look for a local astronomy club if possible. A 6" diameter mirror is the minimum size I would make, and it was my first many years ago. Smaller mirrors are actually harder and show so much less of the night sky. I would recommend an 8" today, as it lets you see much more than a 6 inch, having nearly twice the area. A 10" is the maximum I would suggest for a first mirror. A 10 inch or larger requires a lot of perseverence- you must not be one to get discouraged easily or it may never get done or it amy take years. Neither is what you want, of course.

Making an 8" mirror won't take much more time than a 6", and I'd say about 50 hours effort is required- that's working time, real time might be 80. That's an average. After experience, you could make the next in about 25% less time.

If you make a mirror, you need to be able to test it. While you could just star test, this requires good seeing and isn't very convienent. So a Foucault tester is really a near must-have. Why not then make it first before even starting the mirror? I have directions on making one on my web page, which I think compliments what Berry has written. It's also a little simpler and maybe less intimidating, but functions quite well. I also recommend using a Ronchi screen to start- many find it easier, in my experience at Chabot and Boston ATM Workshops.

Holding a Telescope: the Mount

As far as the mount goes, I think It's hard to beat the simple Dob (Dobsonian). And this too I recommend starting while you make the mirror. You may sometimes want a break from the 'grind' of mirror making- wordworking fits the bill nicely. You just want to decide on the focal length ahead of time and leave extra length to the tube in case you miss it by an inch or two (not a big problem). Having the tube and mount almost complete also allows star testing as soon as possible, if desired.

The secondary spider, focuser, etc can all be made in advance, once the mirror diameter is chosen. Since they affect balance and that affects the rocker part of the mount, it's better to wait on the side boards until you have a good idea of the tube balance with everything installed.

What Focal Length is Best?

Now I can't tell you what focal ratio is best- it depends on use. Planetary viewing has very different requirements from Deep Space objects (like Nebula, star clusters, galaxies), and unfortunately, they are somewhat mutually exclusive. I can say f/5 is about as fast as you might want to try for a first scope (to go faster - lower #) is much more difficult to figure. Fast scopes are good for deep space views, becuase of wide field of view and their light cone. A f/10 is good for planetary use, but starts to get difficult to read the shadows in testing. A good compromise is f/7. But f/6 or f/8s are common and no more work.

What if I Wreck the Mirror?

If you mess up your mirror, you can always re-make it- it just takes more time. Only breaking the mirror (or getting it very thin- not likely) can stop a determined mirror maker.

Pitch? Laps? Help!

Lap making from pitch is a bit messy (chips stick to things), but easier than some books suggest. I never found melting it difficult or even remotely dangerous, but as a kid the books had me scared for a long time. Just don't walk off while it cooking and don't cook on high so that it boils or over an open flame. Use electric and a coffee can and relax.

How do I do the Silvering?

Aluminizing is the standard way to coat glass mirrors now. Very few attempt to silver, since it tarnishes quickly and easily. So mirrors are sent to good folks who do that kind of vacuum coating.


More to come! Eyepieces, finders, you name it!