Parabolic mirror

[kristin1125]

I'm building my first telescope, a Newtonian reflecting, and I'm going to attempt to create my own mirror by purchasing a custom bend piece of glass then having a metal shop treat the back of the glass with sliver. If this won't work, let me know because I'm very new to all of this.

Main question: do the dimensions of a parabolic mirror have a limit.. Meaning, is there a certain equation needed for it to be used in a telescope. I just don't know how deep it should go. I want it around 20 inches. Any insight would be GREATLY appreciated.

[barkis]

I think you may have a wrong interpretation of what a telescope mirror is.

I'm a bit bemused by the silvering of the back of the glass,

The optical surface is the face of the disc, and has to have a very very precise surface indeed.

20 inches is a huge project too, and many difficulties lie ahead.

Have you read any books on mirror making?. If not, I would advise you to try your local library, and ask about mirror making books.

There are two popular ones, one by the French phycisist Jean Texereau, and another by the American mirror maker. N.E. Howard.

Ron.

[ronin]

The depth depends on the diameter and the focal length.

You do not silver the rear of the glass, it is a front silvered mirror.

Really best advice is to work out a focal length you want and buy a ready made parabolic mirror. Someone must make them try Zumbuto or visit Hubble Optical and buy one of theirs. If you get a Hubble one pay for the quick delivery, the not quick is about 8-10 weeks.

20 inches of glass is heavy, you will be looking at 2" think (5cms) so that is close to 10,000cc and at a specific gravity of 2.6 that means 26Kg or 55lb for the mirror alone. Throw in all the other construction required and the scope will be 70-80 lb easily, quite likely close to 100lb in total weight.

Also consider a 20 inch scope at f/4 will put the eyepiece at something like 80 inches from the ground when looking up - how high is your eye from your feet? To have your eye at 80 inches you would need to be about 7' 6"" tall.

[JamesF]

Yes, I think you need to do a fair bit more research before you go any further, into mirror-making and telescope building. For the latter I'd recommend Kriege and Berry's "The Dobsonian Telescope".

As you've not done it before I think the recommendation would be to start with something considerably smaller -- say about 6" or 8".

James

[endure]

Why do mirrors need to be made of glass? If what's needed is a bowl with a specific shape that reflects light why can't the bowl be made out of something light (carbon fibre? shaped thin steel?) and coated with a reflective surface?

[foundaplanet]

Just to help folk out a little here what experience do you already have with astronomy. Also you will need a big garage if it is going to take 20".

[JamesF]

Why do mirrors need to be made of glass? If what's needed is a bowl with a specific shape that reflects light why can't the bowl be made out of something light (carbon fibre? shaped thin steel?) and coated with a reflective surface?

They don't absolutely need to be made from glass and some aren't. However, for the amateur glass is relatively cheap and easier to form to a high level of accuracy than one might expect. It's not at all uncommon for practiced amateur mirror-makers to achieve a mirror accurate to within fractions of the wavelength of light. To obtain the same results with other materials usually involves exceptionally expensive tooling way beyond what an amateur could afford.

James

[LeeB]

While the glass of a telescope mirror is really only a support for the reflective surface, it needs to fulfil a number of key criteria, it needs to be stiff, thermally stable and be able to be worked to a very high polish and figure and allow a reflective surface to stick to it.. Early mirrors were made of a special alloy called speculum but they tarnished very quickly and would not take aluminising or silvering well so glass replaced them, More modern glass is also more thermally stable (pyrex , borosilicate etc) and the best modern mirrors use it . As to the other points raised I believe a carbon mirror would be impossible to work to within 1/4 of a wavelength of light, and a thin metal surface would distort horribly. Good question though.

[umadog]

I think you're underestimating the scale of the task, the size of the resulting instrument, and what will be required. To be quite honest, unless you're already tooled up for all this stuff, it will be cheaper and quicker to buy a ready-made telescope. If you're set on building yourself then first do a lot of reading. Buy the Kriege book and read it all. Do some Googling on what makes a telescope mirror a telescope mirror. It's a first surface mirror not a rear-silvered mirror. Making a good large mirror is a highly skilled (years of experience) job.

[umadog]

Also consider a 20 inch scope at f/4 will put the eyepiece at something like 80 inches from the ground when looking up - how high is your eye from your feet? To have your eye at 80 inches you would need to be about 7' 6"" tall.

To be fair, this is what ladders are for

[Moonshane]

not necessarily so high - take off maybe 14" for the distance from secondary to focus point and then maybe add back on 6"-8" for the distance from ground to mirror face depending on design. so just under 7 feet. this means just a small step potentially, and only then when at the zenith not ladders and nothing when not close to zenith.

I do agree though that glass is the only major alternative for the DIYer and also that given the comments in the OP some reading is definitely advised before the project starts. anything is possible though with a bit of skill, determination and imagination so don't let us put you off! a smaller project might be a good idea first, perhaps a 10-12" scope. this could then be retained as your grab and go with your eventual 20" - you'll find it good to have both I'll guarantee that.

do be aware though that the smaller the scope, the less likely you are to save money on a bought one.

[Moonshane]

ps, if thinking of imaging then (assuming you were referring to a dobsonian design - I was) this is not the optimum for imaging at all.

[brantuk]

This site is worth a look around - it'll give you a lot of info about how big mirrors are made what sort of tollerances you need to work to and how to measure them. It won't give you all the detail but it's a good springboard for further research. There's also plenty of pics showing a mirror workshop and the kind of gear you may need to make your own big mirrors. Hth

http://www.obsession...ptics/index.php

[ronin]

To be fair, this is what ladders are for

Entirely agree, just then the object drifts out of view especiually as the mag will be pretty high, and you cannot follow it when up a set of steps so need to climb down, relocate the object or find a new one, back up the steps and get in a maybe 15 seconds of viewing before starting the whole climb down/locate/climb up process again.

If the scope were on a motorised base then the eyepiece moves away leaving you up a ladder with no eyepiece to look into.

Intention was to indicate the problems associated with what is a large scope, at the aperture mentioned you are getting into the area of big aperture but close to impractical to use. Which when thinking about it, practicallity has rarely been a major factor in astronomy. :laugh2:

[brantuk]

"If the scope were on a motorised base then the eyepiece moves away leaving you up a ladder with no eyepiece to look into."

Interesting point - if you're up a ladder with no tracking then you still have to nudge the scope to track the object as it moves across the sky - so eventually you have to push the scope out of reach and reseat the ladder anyway. The fact it's being moved electromechanically in a tracking dob shouldn't really make a whole lot of difference to the process. If anything tracking gives you a longer view with the object centered before having to move the ladder.

[swamp thing]

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Entirely agree, just then the object drifts out of view especiually as the mag will be pretty high, and you cannot follow it when up a set of steps so need to climb down, relocate the object or find a new one, back up the steps and get in a maybe 15 seconds of viewing before starting the whole climb down/locate/climb up process again.

If the scope were on a motorised base then the eyepiece moves away leaving you up a ladder with no eyepiece to look into.

Intention was to indicate the problems associated with what is a large scope, at the aperture mentioned you are getting into the area of big aperture but close to impractical to use. Which when thinking about it, practicallity has rarely been a major factor in astronomy.

If I assume (correct me if I'm wrong) we are still talking about a 20" f/4? Then there needn't be steps, a small hop up will suffice. I will be using a small ally light weight folding work platform with mine. This will allow for observing for pretty much as long as one wants without repositioning.

The scope will move very easily by steering it from the upper cage, both finders will be up there too, so not much need to keep getting up and down. In fact how often you move your observing chair will be how often I need to move my hop up. As Shane said, only when up near the zenith will it even be needed. The rest of the time standing (or sitting) on the ground like everyone else. Only really giant scopes meed big ladders. A 20" f/4 really isn't that big.

[umadog]

Entirely agree, just then the object drifts out of view especiually as the mag will be pretty high, and you cannot follow it when up a set of steps so need to climb down, relocate the object or find a new one, back up the steps and get in a maybe 15 seconds of viewing before starting the whole climb down/locate/climb up process again.

Using a ladder (or s step stool in the case of a 20") is not as bad as you imagine. You can easily track an object for fairly long periods (minutes) whilst up the ladder. It's true you'll probably have to re-position the ladder if you move to different object--that's a bit of extra work, but it's not so terrible.

[kristin1125]

Thanks for all the great input! i have a a lot read about.

[ollypenrice]

Entirely agree, just then the object drifts out of view especiually as the mag will be pretty high, and you cannot follow it when up a set of steps so need to climb down, relocate the object or find a new one, back up the steps and get in a maybe 15 seconds of viewing before starting the whole climb down/locate/climb up process again.

If the scope were on a motorised base then the eyepiece moves away leaving you up a ladder with no eyepiece to look into.

Intention was to indicate the problems associated with what is a large scope, at the aperture mentioned you are getting into the area of big aperture but close to impractical to use. Which when thinking about it, practicallity has rarely been a major factor in astronomy. :laugh2:

No, this isn't so. I have a 20 inch F4 and you can easily track by hand while up the ladder. 15 seconds is way off as an estimate for deep sky observig at low powers, which is what big Dobs are good at. At high powers I still thik 15 seconds is a bit short!

However, several things on the thread make me think that the OP should do more homework. To the best of my knowledge the rear of the glass is only silvered on the Riccardi Honders design, or is it just the Honders?

Olly