What's in it, to build a telescope!

Share practical advice and tips on making your own kit

What's in it, to build a telescope!

Postby Jacques Reusens » Thu Mar 06, 2008 11:03 am

Before I bought my scope, I read all I could about making my own telescope. As an old tinkerer, the prospect of creating something was becoming exciting. I have all the necessary 'infrastructure' like lathe, milling machine, cutoff saw, etc., so making up parts was no real obstacle. What I was not prepared for was the agonising process of producing a parabolic mirror!. I could see the students in a class who tried doing it, were physically suffering pain. In my view, anything taking more than an hour to produce a result is simply neither economic nor a good use of time, as you get older, you simply "don't have all the time in the world", probably the most stupid expression ever said by man. You can't beat the Chinese, they will produce something cheaper than you can buy the raw materials.

So, armed with such gloomy thoughts, I said, why on earth would someone spend weeks of manual labour pushing a glass disk over another to produce a mirror shape? surely an old washing machine motor and gearbox can reproduce the to and fro action required to rub the glass stocks over each other? When jem stones are polished, no one in his right mind would wind a tumble drum round and round for two weeks, unless of course there is some perverse satisfaction from pain and detention. A spark of enthusiasm. What if I made a 14' mirror, it would put the local guys to shame. You certainly can't buy it off the shelf. The cost?, the pain? the shorter life span?. Is it all worth it? Then what about the massive equatorial mount, the goto motors and alignment problems to mount such a beast. Such a scope cannot be moved, if at all, so an observatory structure is called for.

What happens then when you do a quick calculation and find it's going to cost the earth, just for the raw materials. To make it worse, the city camera shop wants to get rid of perfectly good and solid 10" scopes at half price!. Not only that, but there in front of you is a beatiful shiny and complete scope begging to be taken to a good home and show off it's wonders. Who can resist such temptation. I confess now I was hooked and bought it straight away.

Joe from the south!
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RE: What's in it, to build a telescope!

Postby ajohn » Thu Mar 20, 2008 1:56 pm

Just like you I have a lathe and milling machine etc too but have only completely made a telescope when I hadn't anything like that at all.

First scope was an 8ins F6.5 that should have been F6 mounted as a dobsonian. Next was a 10ins F6.5 mounted the same way. Both very portable and gave better views than an 8ins and then a 10ins schmit cassegrain that I bought later after the others had been sold. Why it's simple really the mirrors were both figured as good and as close I could get them. (Liar I bought the 10ins mirror but it was selected fro me and checked out fine.) What you have bought is very unlikely to be. It's a fact that a good parabola needs to me made to better than 1/8 wave peak to peak. Once upon a time manufacturers quoted surface accuracy and many where 1/4 wave rms. That's worse than twice as bad as 1/8 wave peak to peak. These days some quote 1/6 wave peak to valley and charge more for it. Mirrors must be smooth at that level too. I recently came across a test on a well known makers product. The errors exceeded 1 wave. You really would see the difference. So on this basis compare cost with like for like. Have to agree about the cost of abrasives etc but not much is sold. Try a quote from oldham optical for a 1/10 wave mirror. There are other suppliers about too. The costs of making one and the effort won't seem so bad then. It's a sad fact that kits with pre generated curves don't seem to be available in the UK any more but they can be ordered to any curve one wants from the states. That saves a lot of the work. Not as much as one might think though at reasonable F ratios. Those make sense in any case because they don't need such expensive eyepieces. If you have F4.3 expect to spend nearly as much on each eyepiece for it if you want to get the best out of it. The best is unlikely to be that good anyway for a number of reasons.

On portability I would say realistic maximum is about 14 or 15 ins on a fork mount driven with gears - if you want to make that too. They can be lapped. Worm drives just take up less space. You can even build you own stepper driven goto and get better results than available commercially unless one spends a fortune.

Much of the tube can be made of sonotube (thick cardboard) much better than it sounds especially if sprayed with paint etc. Some people have linked sections of tube with various pipe layouts - huge scopes that will go in the back of a car. I used a hexagonal ply tube cut with a fret saw set at an angle plus a straight edge and glued together at the edges. Biggest problem was getting the end square to the axis of the tube. I should have made that adjustable.

On books I would advice anybody who is making or buying a scope for that matter to read Texereau's How to make a telescope. If they are making a mirror dig out info on the Dall null test and the caustic wire test too. (Both in one of the ATM books 3, I think) I would also advise the last polishing stages to a sphere to be done with shortish strokes with the mirror on the bottom. Much easier to get a clean edge.

--
Regards
John
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RE: What's in it, to build a telescope!

Postby sftonkin » Thu Mar 20, 2008 5:00 pm

[quote]It's a sad fact that kits with pre generated curves don't seem to be available in the UK [/quote]

Hint: If you are doing a big mirror, use a sub-diameter tool for hogging out. Last mirror I made (over a decade ago) was intended to be a 12" f/6. Using a cast-iron 5lb dumbell weight TOT, I overshot in a few hours, and ended up with f/4.6.
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RE: What's in it, to build a telescope!

Postby ajohn » Thu Mar 20, 2008 9:43 pm

Another hint. There are some aweful tester designs about. The texereau book shows one that can be made mostly out of mdf and wardrobe rail or some other suitable round stuff. It does pay to have some 1ins by 1/8th brass though. Could saw up a brass hinge at a pinch. A web search will show how to add a web cam if you want and ronchi etc but I have my doubts about that one - too easy to fool yourself.

I also wonder about the 6 inch mirrors most books mention. Texereau sugests 8 and me thinks that's likely to be easier. 10ins might be easier still. F6 or F8 isn't too hard. F4.5 is another story but can be done. At 4.5 I would suggest finding out about the dall null test and or the caustic curve test. The usual tester can be used for both of those.

John
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RE: What's in it, to build a telescope!

Postby sallan » Tue Apr 15, 2008 10:43 pm

Hello everyone that reads this, just registered here and I will be looking for help here quite a bit hope you all like helping a struggling want to be amateur astronomer .
I am getting there has nearly cost me my missus though and just to make sure I am naglecting her enough decided to build said telescope in the magazine.
Being a carpenter not a problem making that part but now for the hard bit as I am looking into wasting several weeks grinding a piece of glass, have been doing my homework bought a book looked at several sites relating to this torture but still the testing of the mirror ,caustic test ,Ronchi and the foucalt test are not very clear to me if anyone can help me, any assistance would be greatly appreciated. As would any advice on where to buy the materials looking at galvoptics at the moment if anyone knows anywhere cheaper again would be very grateful.
Hopefully the articles in the magazine will be as expertly clear as the building of the scope.
Just cant wait to get this project finished and get on with the fun part.

[image]local://3068/32A666FAF0E34EE3B42D005F4AE5E511.jpg[/image]
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