I'm sending my first telescope mirror off to Newport Glass to be coated.

[denbot]

I started working on a 10" f/5.6 almost two years ago. It did not take me long to realize just how difficult a project I started. My local community college offers one of the few telescope making classes in California. The instructor is adamant that you must make a 6" f/9 mirror before taking on a larger scope. I quickly ground and polished my pyrex blank, and then began somewhat tricky figuring process.

I am a natural at getting to sphere, but a paraboloid was very difficult for me to achieve without turning down my outer zone's edges. Tonight was the last lecture/lab, but the "shop" will be open for the next two Saturday's for us to finish building our OTA's or figuring our mirrors.

My Foucault Knife Edge test charts shows my mirror well within tolerances, but on the test stand it shows a micro-rippled surface. This won't affect the optical performance, but it does hurt my pride. I put in a lot of hours grinding and figuring to have to settle for very good viewing, but on a test stand, ugly optics. The fact that anyone "seeing" with the scope won't be able to detect the micro-ripples, but I won't be able to forget that they are there. Where it comes time to re-coat the mirror I'm going to re-figure it, but until then I'll have to present my Dad with a gift that is not what I wanted it to be.. sigh..

The upside is that I will use the mirror lap-tool to finish polishing, and figuring my long dormant original project. The whole point of the class was produce a handmade telescope for my pop's Father's Day Gift, and hopefully I will have learned enough to be able to complete my much more difficult 10" f/5.6 mirror project this summer.

[Manifestor_of_Light]

1. Very cool!! Excellent.

Groovy!!

[laconicsax]

2. Awesome!

Did you do the 6" mirror first, or just dive head-first into doing the 10"?

[denbot]

3. I already hi-dived in to the shallow end of the mirror figuring pool..

I took my instructors direction and made a 6" mirror. The larger 10" f/5.6 will be a couple of orders of magnitude more difficult to figure.

I am hopeful I'm up to the task.

Budget cuts are eliminating the telescope building class next semester, but I will take an Astronomy Independent Studies class in the fall to access test equipment and my Professor's expertise, if I need help finishing the larger mirror.

[laconicsax]

11. I just remembered something you may find useful.

If you can make good spherical mirror, you can flex it into a very good parabolic. Alan Alder wrote an article about it in the Nov. 2000 Sky&Telescope (http://media.skyandtelescope.com/documents/Adler-MirrorFlex.pdf">pdf).

[denbot]

12. You could use a very complicated mirror cell, with one more thing to collimate

Or you could properly figure a mirror. If you look at his graphs he does not compare it to a "control" or properly parabolized mirror. Not only to you have difficulties with compression of materials that flex the mirror, he does not mention the thermal properties of each component, and how the difference in "cool down" expansion/contraction of pyrex, and what ever materials in the mirror cell, from the carriage bolt, puller sponge, et al, would affect the "seeing", and the time it would take for the various components reach thermal stability.

A flex system is very complicated to that of a simple parabolized mirror cell.. Easy usually means popular. I have never seen a "flex mirror" set up at any star party. Twelve years after this article was published and in my small world not a single example in the field tells me that this is not the solution the author thinks it is. Don't get me wrong, flexing was briefly mentioned during a class lecture, and the article was interesting, but a flex cell way more complicated then it needs to be.

BTW: My mirror arrived today, without a single sleek.. :bounce:

[formercia]

4. I helped with one once

It's a lot of work. We used two glass blanks, face to face, with the abrasive between. I can appreciate the effort involved.

[5X]

5. I have always wanted to make my own scope, but

off the shelf dobsonians have gotten very inexpensive.
I ended up buying an 8" dob from Orion. Is a very nice
scope and I didn't have to try my patience, which I have
very little of.

Good luck with your 10".

[BlueJazz]

6. I built an eight inch scope while in college...turned out pretty good. I understand the...

...problem with micro ripples. I used (via my instructor) silver polish to smooth some of them out but don't know if this applies to you. I do remember the polish was extremely fine "grain" but forgot the name of it...only that it comes in a small 3 in container....Later...Oh...it's "Wright's silver creme". ...again I don't know if this applies to your project. :)

[denbot]

7. I used a "pitchlap" and 1 micron Cerium Oxide

I wanted to figure back to sphere for the 7th time this semester, eliminating the micro-ripple and re-figure to Paraboloid. My dilemma was that this was my last "official" class. I simply ran out of time to do the job I wanted to do.

[Lionel Mandrake]

8. Way to go!

I have had various telescopes but never made my own.

I used to go to the astronomy club at the same community college. I'm not sure if it still exists.

[denbot]

9. The South Bay Astronomical Society?

It still meets at the El Camino College Observatory.

Edit to ad club link: http://sbastro.org/

[Lionel Mandrake]

10. That's the one.

I'm glad to hear that the SBAS is still active.