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To make a good mirror

Started by kulginov, 12/15/2004 01:32PM
Posted 12/15/2004 01:32PM Opening Post
Perhaps a silly question...

What does it take for a newbie to make a 8" mirror no worse than 1/8 wave? Special abilities, price of materials, amount of spoiled blanks, any measuring equipment, how many hours?

Is it possible at all?

Dmitri
Posted 12/15/2004 01:50PM #1
Dmitri, here's an article on the subject I wrote for our club newsletter a little while back. It relates my experience. Hope you find it useful. (The title was supposed to be "Can You Do It?", but the webmaster who posted it to the site used the wrong text.)

Doug

http://www.umich.edu/~lowbrows/reflections/2003/dscobel.11.html
Posted 12/15/2004 02:45PM #2
My first mirror came out to be about 1/12th wave. It was an 8 inch F5.8. WOnderful little scope. Took about 20 hours to get it to polish, and another 40 to get it figured right. However, the last figuring session--when I finally "got it"--went from 1/2 wave to 1/12th in less than three sessions of five minutes each, and I predicted it would do exactly that.

Anybody can do it.

I should tell you that one of the many reasons I took up mirror making is because of my personality. I was a teacher and high school principal. All my life I worked with people and therefore success was always an approximation--never a dead on measurable goal. This extended to whatever project I was working on. Whenever I built anything, as long as it worked, I was satisfied.

But I could not do that with telescopes. Everything had to work. Precision mattered. So I had to change my personality.

I had to learn that when something was not working, I had to stop and regroup--not just bull my way through it.

When measurements did not come out right, I had to rework a piece, not just change the measurements.

I had to learn patience.

Anyway--I did it, and was really proud of the mirror and the changes in my personality.

In short--anybody with a modicum of intelligence can make a mirror.

I strongly agree with other advice you have in this thread--

If you want a telescope--buy an already finished mirror.

Get somebody to help you with the project.

Be patient. The first part comes remarkable easy. the last part (figuring and testing) torments.

You will never reach perfection. Let your next mirror be perfect. When this one gets to 1/8 th wave or so, put it in a scope. Then start on your next.

Alex
Posted 12/15/2004 03:37PM #3
Dmitri:

Hi! There are a number of clubs/places which offer help in mirror making.
Ed
Posted 12/19/2004 05:29PM | Edited 12/19/2004 05:32PM #4
Dmitri,
I'd recommend hooking up with a local club that runs a mirror-making class, or find someone in your area who makes mirrors. You'll learn faster from watching someone else--assuming they know what they're doing-- and a club may already have the necessary equipment.

A club in my area offers a dedicated mirror-making workshop each year, and gets Steve Swayze to come teach it. Starting with a pre-generated curve in a Newport glass blank, I was able to make a well-polished 10" f/6.5 of > 1/10th PV wavefront in 14 hours of work. I think the cost for the blank, materials, room & board was $375.

I assume you're doing this because you want to learn, not because it is the cheapest path to a decent mirror? Unless you can hook up with some workshop like the one I used (DelMarVa Stargazers) I'm not sure how much you'll save over buying a Discovery or similar mirror in the 8" range. Still, it won't match the thrill of looking at images through a primary you ground yourself.

Jim
Posted 12/23/2004 02:55PM #5
Thanks for all encouragement! Unfortunately I cannot follow the most frequent advise - to join a class. Here in Sweden there is one (perhaps two) mirror makers and a handful of people interested in making mirrors.

But it does sound like it's possible!

Dmitri
Posted 12/23/2004 05:36PM #6
Dmitri

You can make a difraction limited optic in the 6 to 8 inch range F5 or higher. I made two 6 inch full thickness F10 mirrors. They test 1/10 wave and give good star images.

I built my first 4.25 inch F10 telescope in Dec 1999 with supplied mirrors. Since then I built a 8 inch F6.25 motorized Dob using a mirror I bought used on this forum for $75.

Both of these telescopes have brought me many nights of joy and entertainment. The smaller scope gives me views up to 200X before starting to degrade but I never could resolve the cassini division. The large mirror gives me views to 350X before it degrades.

Now that I have completed my own mirrors I have been able to go back and test these commercial mirrors. The 4.25 inch tested 1/10 wave but had a significant amount of astigmatism which is probably why I could never resolve Cassini. The 8 inch mirror tested at 1/3.2 wave which is not
difraction limited but gives me stunning views of Jupiters GRS and festoons.

I guess what I am saying is if you do not already have a "great" mirror you can produce a mirror that will make you happy. Don't get hung up on the numbers. Follow the procedures that are documented everywhere and then when you think you are close or done star test the mirror and I think you will be pleased.


Good luck Dave

Posted 01/03/2005 03:45AM #7
I belong to the Fraser Valley Astronomers Society in Abbotsford, BC. We have lots of members who make mirrors and some who even have started businesses that aluminize mirrors.

We have one member who has lots of experience grinding mirrors by hand. He has put together a 12 page document that has helped a lot of people make great mirrors. Visit our website and look for the telescope making box on the left hand side (http://www.fvas.net), particularly Vic's mirror making link.

Good luck! I'm following Victor's instructions for my first 8" mirror.