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pi1983
2006-Apr-25, 06:20 PM
hey all... I've been researching building my own telescope for a while. I'm trying to go out the box and build something great but for cheap. I looked into grinding my own mirror but I can't find any resources in my area. Any good websites? or know of any good place in (or around) Palm Beach Gardens, Fl.?
If I were to grind a mirror, what is the thinnest I can go? I know the ratio is 6 to 1 (6"d, 1"t or 12"d, 2"t) but can I go thinner? Like, for a 6 inch can I do 24 to 1? (6"d, .25"t)? And (gotta lot questions, sorry) is it possible to build an 18" segmented mirror using 6"d mirrors?
I want to get a couple 6" mirrors, (as you can prolly tell), and focus them at one secondary and then shoot a cassegrain beam back down the center... (but I what focal ratio does the secondary have to be?) I know segmented telescopes are tough because each mirror has to remain perfect but I think with some inginuity it can be done cheaply and effectivley. Am I wrong?!
Any help would be greatly appreciated!!
Thanks,
- pi1983

Kaptain K
2006-Apr-25, 09:55 PM
Let me get this straight. You've never built a telescope before and you plan to start with a multi-mirror cassegrain? That's like saying "I've never built an airplane before, I think I'll start with a supersonic jet! :doh: Start simple and work your way up.

f I were to grind a mirror, what is the thinnest I can go? I know the ratio is 6 to 1 (6"d, 1"t or 12"d, 2"t) but can I go thinner?
Maybe. But performance will suffer. The 6:1 ratio was established by a couple of centuries of trial and error. It is a compromise between thermal and mechanical stability.

...for a 6 inch can I do 24 to 1?
NO! Your going to grind the curve deeper that 1/4".

I know segmented telescopes are tough because each mirror has to remain perfect...
Not only that, but each segment is an off- axis paraboloid section - orders of magnitude more difficut than an on-axis parabaloid!

perfessor
2006-Apr-26, 01:04 AM
NO! Your going to grind the curve deeper that 1/4".

But he wants that hole in the middle for the cassegrain focus!

perfessor
2006-Apr-26, 01:27 AM
Hello and welcome pi. Let me add a few more substantive comments about building a scope. I have built several, including grinding my 8" mirror. Do it because you like to build things; you won't save a bundle of money ( a little, probably).

If you do everything right, you will end up with a good scope. If you make a few slight mistakes, you'll end up starting over. I'm speaking from experience on this point.

There are several books available with titles along the lines of "How to Build a Telescope". Get one, and read it three times at least.

Start small. I mail-ordered a 4.5" mirror with matching diagonal for my first scope. The experience you gain will be invaluable.

When I ground my mirror, I did it in a workshop run by an experienced telescope optician (thank you, Dan Joyce!). There is really no way to make all the right guesses, forsee all the looming problems, or evaluate your own technique. If you can't find such a person to help you, then plan on buying.

When I went through the workshop, a guy there was making a 3-mirror off-axis solar scope. Let me tell you, these guys knew what they were doing. Later, he brought in some amazing photos he had taken. Meanwhile, Dan was working on his 18" mirror. I'd never seen a hunk of glass that big. The whole experience was a lot of fun; but on my own, it would have been impossible.

Good luck!

glasspusher
2006-Apr-26, 05:19 AM
My first mirror was a 12.5" f/6. I had to go at it alone as I knew of no one who had made a mirror. It was a steep learning curve but not impossible. I just took my time and when things weren't going well, I stopped and would read the book some more. For your first mirror, a good size would be a 6" f/8 or f/7 to get the feel of grinding and polishing a mirror. Then start thinking about going bigger. And after making your first mirror then you can think about going as low as 10:1 on the thickness. Also, check out Wilmann Bell for books and stuff.