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Does any1 still grind their own mirrors?????

weareborg

By weareborg
in DIY Astronomer

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weareborg

weareborg

Posted
Posted

OK as the title suggests, I was wondering if any1 out there still grinds their own telescope mirrors???

Im seriously thinking about having ago at this, as its something that Ive always wanted to try. Obviously Id start small, say 8" and if all goes well, go with a 12-16"

Wot you guys and gals recon?????

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barkis

barkis

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I used to make my own Mirrors, I do have enough materials including a 6" Pyrex mirror blank with a plate glass grinding tool. Grits, Alux. fine grinding grits, and Optical Pitch. Rough cut plate glass blanks about 15" diameter too.

I have a partly finished 16" f4.5 Mirror. but I still have a yen to finish that. It was intended to build myself a Dobsonian.

I had a grinding machine also, but I broke it down and scrapped it.

A stupid thing to do really, but it took space up in the garage, and the local astro. society didn't want it so. C'est La Vie.

Ron.

There always here if you need stuff too.

http://www.galvoptics.fsnet.co.uk/telescope.htm

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barkis

barkis

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I am in the planning stages of a 12" Dob, will grind a mirror out of plate glass.. I have four guitars on the waiting list to build, then I'll really concentrate on this scope.

Can't wait.........

I hope you've been reading the N.E. Howard book in your spare time Sam.:icon_eek:

Ron.

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weareborg

weareborg

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Ive actually got a copy of this book. Bought it many yrs ago from the library book sale. Think I paid about 50p for it. Its been awhile since I read this, so Im re-reading the chapters on grinding oyur own mirror. I agree with you Sam, his style of writing is very easy to follow. Nothing too technical for the novice to follow. A true classic. Would love a copy of "How to make a Telescope" by Jean Texeroux, just so that I can say that I have read it. Don't suppose anyone knows where i can get a copy from do they????

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barkis

barkis

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Friar Dobson still grinds his own...

He 'says' he has a stock of old ships' porthole glass that he uses..

Steve

Are you referring to John Dobson Steve?

He ain't a Monk is he? I know he has the heart of one.

Ron.:icon_eek:

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skye at night

skye at night

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Are you referring to John Dobson Steve?

He ain't a Monk is he? I know he has the heart of one.

Ron.:icon_eek:

Not sure if he was 'officially' a Monk Ron, but he did spend over 20 years in a monastery (surely that makes you a saint!) before astronomy gripped him.

Steve

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barkis

barkis

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Not sure if he was 'officially' a Monk Ron, but he did spend over 20 years in a monastery (surely that makes you a saint!) before astronomy gripped him.

Steve

Probably where he got his Infinite patience from.

I don't think he would have attempted hogging out a mirrors curve in a Monestary. Imagine that awful grinding din echoing through the place?. No carpets or curtains to dampen the noise.

The guy's earned his place in Folklore though. He did take visual Astronomy on to the Pavements of America. :icon_eek:

Ron.

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Beulah

Beulah

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Go for it!!! Making your own mirror is a hugley satisfying experience, unfortunately amateurs in the UK seem to have lost the will to do it.

John

I have read articles and articles on this subject, and once or twice I read "a ready made mirror will cost the same as a home ground one" and "CNC Machining produces a perfect mirror as good as a hand ground one"....

Doesn't put me off, though!! :icon_eek:

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barkis

barkis

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Believe me, the satisfaction you will get from producing an optical marvel, will stay with you forever. The beauty of a perfect paraboloid under the knifedge of the Foucault test is something to behold. I'm sure it is rarely seen these days. Too many other tests which don't show it.. What a pity they didn't test the Hubble Mirror with this £5 device before sending it into space. It could have saved them Millions of Dollars.

Ron.

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callump

callump

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Not sure if he was 'officially' a Monk Ron, but he did spend over 20 years in a monastery (surely that makes you a saint!) before astronomy gripped him.

Steve

I thought it was a Buddhist monastery, but apparently it was Vedanta which appears to be an ancient indian religion/philosophy.

A top guy that I have been lucky to spend a few hours with on his occassional visits to the UK.

/callump

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callump

callump

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To go back to the question of the OP.

No, not many people grind their own mirrors these days, but there are a few about.

Its not particularly difficult to grind a small mirror, but you do need to spend a lot of time and patience. Larger mirrors need a bit of experience to get a good figure, but again patience and time and persistence will win through.

So, not really suited to those looking for instant gratification.

Of course if you make your own mirror and telescope, you can get something out of the ordinary or run of the mill - like a long focal length planetary newtonian.

So, yes, go for it!

/callump

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Sfarndell

Sfarndell

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I'm a bit late on the response, but I am in the process of making an 18" mirror from 1" thick plate glass. It's my first one and I am almost finished figuring with no real issues along the way. It's an ambitious task, and if I had to do it again, I'd start smaller with a 12"-14" mirror.

I joined the Camden Telescope making society which was beneficial, but not essential. There is a lot of information on the web which should provide enough guidance. Texereaux is excellent, but I only read it after I had polished my mirror out. Most of my knowledge came from the web (stellafane is particularly useful). Initial grinding and polishing is relatively easy, but time consuming.

Figuring is where the real skill comes in. If you join the Zambutomirrorgroup on Yahoo, there is a lot of useful information and tips on figuring.

After good hygiene to prevent scratches, the most important thing is to rotate, rotate, rotate....

Good luck!

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Beulah

Beulah

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Sfarndell,

That is really encouraging for me to read that your first mirror is an 18"! :D

I have a 12" and 14" mirrors waiting to be made - and although I have craftsmanship skills, have been a little overawed when reading the process - that theoretically you can make a great quality mirror by hand.

I'll look into that Yahoo group you recommend, thanks. :)

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Glasspusher

Glasspusher

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It is good to know that people are still making mirrors here in the UK, here are a few thoughts that I hope will prove helpful to those considering making their own mirror.

It is possible to make a good mirror from ordinary plate glass, unfortunately this material is only available in thickness up to 25mm (1 inch). Borosilicate (low expansion) glass is available in thicknesses up to about 45mm but is more expensive than plate glass. Thinner plate glass (25mm) is cheaper but tends to flex during working increasing the chances of astigmatism being introduced into the mirror, therefore it is always preferable to work with thicker material. Plate glass has a poorer coefficient of expansion that borosilicate glass and as a result longer cooling off periods are required when testing. On the plus side plate glass grinds and polishes quicker than borosilicate glass.

Choice of focal length is important as with thin mirrors it can significantly affect the centre thickness which reduces the stiffness of the glass, again increasing the chances of flexure during working. Longer focal lengths are more desirable in maintaining centre thickness, they are also easier to figure, but at the expense of increased telescope length. My advice would be to go for a longer focal length, F5 or F6 if you can cope with the length of the tube.

Careful thought should be given to a choosing a suitable diameter, the temptation is to go as big as possible. A limiting factor may be the size of the lap you can physically push if you are working by hand. During polishing it is advisable to use a lap about 75 to 80% the diameter of the mirror being worked. I personably would not want to push a lap much more that 16 inches in diameter, which translates to a mirror of about 20 inches in diameter.

How are you going to test your mirror? This is a question well worth giving a lot of thought to before starting grinding. The success of the project will ultimately depend on how well you are able to test the mirror. I am going to suggest the simplest test possible in terms of additional equipment required, this method was originally suggested by Mel Bartels. The test is really two tests, the first is the matching Ronchi test and the second is the star test. In the matching ronchi test a computer program is used to generate the appearance of a perfect mirror as seen with a Ronchi ruling at various distances inside and outside of the radius of curvature. Details may be found here:

http://www.bbastrodesigns.com/ronchi.html

The mirror is figured until the actual appearance under test is the same as the computer generated images. This qualitative test is not used to finish the mirror; this final stage is done by star testing the uncoated mirror in the assembled telescope. The mirror is figured until the star test results show that it is good enough to get coated. Information about star testing can be found here:

http://www.astunit.com/tutorials/startest.htm

http://aberrator.astronomy.net/html/body_star-testing.html

So the above enables the testing of a mirror without expensive additional equipment and does not require years of experience to perform, it enables relatively inexperienced mirror makers to accurately test their mirrors.

John

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  • 2 months later...
Merlin66

Merlin66

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Posted

Great fun and very satisfying!

I've ground a few mirrors up to 12" f5 and had tremendous fun with them.

We had a "granny" grind a 6" f7 on her lap while watching TV....

Better than knitting she said and the results were very very good; within three months she had built her own 6" Dobbie and had a smile which would illuminate Xmas!!!!

Go for it!

Ken

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acey

acey

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this final stage is done by star testing the uncoated mirror in the assembled telescope. The mirror is figured until the star test results show that it is good enough to get coated.

That's very interesting - I was aware that Herschel tested his scopes by observation but thought the modern way was Foucault testing etc. This approach sounds very appealing - even if it means having to wait for a clear night. I've never ground a mirror but definitely intend to give it a go some time, it sounds fascinating.

I got grannie to make a scope shroud so who knows what next!

Andrew

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