A stupid question from a beginner

[kayakfisherman (Brian)]

Hi all

I am very keen on getting into astronomy, but have a very tight budget, and no interest in buying a toy telescope. I have thought a lot about building my own dob, but the main cost is the mirror, and it doesn't look like its much cheaper to grind your own. This got me thinking about something that has no doubt crossed the minds of many people before me, especially those on a budget, namely why does a mirror have to be made of glass - why not epoxy for example. Obviously if it could be done cheaper, it would have already been done, so there must be a reason, I just can't find one. So either my research skills aren't that crash hot, or my question is too stupid that anyone would even consider addressing the issue!!!

I can only guess that glass is the most stable substance available that can be polished to a sufficient degree of smoothness. If anyone can shed light on this it would be much appreciated.

Cheers
Brian

[madtuna (Steve)]

The only stupid question is the one you don't ask.
Welcome to IIS
I'm actually hoping someone pops in with the answer as I'd like to know too.

[Zuts]

Hi,

When I search for hardness, I find this article http://en.wikipedia.org/wiki/Mohs_sc...neral_hardness

Basically glass has a mohs' hardness of 6 to 7. Epoxy has a hardness of around 4. So I dont think it could be ground successfully. The material is too soft and when ground would heat up and the 'grit' would embed itself in the epoxy.

Just my 2 cents from googling

Normally people make scopes for the fun of it. I once ground my own 6 inch mirror and then had a go at a 12 inch one. These days you can get a second hand 12 inch dob for around 500 bucks. You would be hard pressed to make one for that price even if someone gave you a mirror. Mirror cell, tube, focusser, finderscope, base and other bits and pieces all add up.

Of course an 8 inch dob second hand can be found for a few hundred bucks if you are prepared to be patient.

If you want to make one though, then go ahead. Like me you will be stoked if you complete it and get to look at things through a mirror you made yourself.

Cheers
Paul

[leon]

I agree, it is the joy of seeing something that you made yourself, and it works, it has nothing to do with, if I make it myself it will be cheaper, those days were back in th 60's and 70's were it was cheaper to make your own.

By all means grind your own , I did in those days and it was a great achievement, and fun.

Leon

[kayakfisherman (Brian)]

Hi all

Thanks for all your replies and welcome.

You can get dobs very cheap, and a 12" dob of reasonable quality for $500 is extraordinary good value. Nevertheless, $500 is way out of my budget (if I was single it would be nothing!!!), and I still believe that putting aside the mirrors, the rest of the dob could be made for next to nothing out of scrap bits (except for the other optics - finderscope, focuser).

This is why I was thinking about alternatives to glass. Even the unground glass blanks are expensive, then there are the cutting compounds, etc.

Paul, thanks for the info re hardness. Epoxy is soft - but it can be cast/moulded with a high degree of smoothness. The problem with molding epoxy is that it shrinks. However, if the mould was done in a couple of stages, with say the last stage being applying a very thin layer of epoxy to the almost finished mirror blank - before re-inserting back it into the mould, it should eventually be possible after several castings to virtually eliminate shrinkage problems. Alternatively, the mould could be rotated, and let gravity and centrifical force build the mirror. I am not saying that it can be done with epoxy, for example, I have no idea whether epoxy holds its shape that well over time, and it would probably be impossible to aluminize. However, someone in this big planet of ours must have tried cheaper alternatives to glass, and hopefully documented their efforts.

Cheers
Brian

[Starkler (Geoff)]

The only viable alternatives i have heard of to glass are a lot more expensive, ie. exotic ceramics.

A mirror substrate is a just a support structure for a thin layer of aluminium but the stability and accuracy required means that few materials make the grade. Rest assured that if cheaper materials could be used to create the required optical surface, the Chinese would be using them in mass produced scopes

You could probably fashion a shaving mirror from many materials but for illustrative purposed consider these numbers.

A mirror with 1/4 wavelength surface accuracy is considered fairly poor for a telescope mirror. A 1/4 wavelength of green light is around 125nm. In laymans terms thats 0.000125mm deviation from a perfect parabola across the face of the mirror.

[Garyh]

I have found documentation about making solid aluminum mirrors (how to grind and polish) but I have no idea if they hold up to glass substrates..

[Geoff45 (Geoff)]

Originally Posted by Garyh

I have found documentation about making solid aluminum mirrors (how to grind and polish) but I have no idea if they hold up to glass substrates..

The problem with a solid aluminium mirror would be that once it eventually tarnished, you couldn't restore the polish without refiguring the mirror, whereas with glass you just remove the old film and put on a new one.

[bojan]

Guys...
Forget about alternatives.
Glass RULES..... Otherwise, plastic mirrors would have been here a long time ago.
The only so-so alternative is thin aluminised mylar and low pressure round chamber behind it, where if placed as cover it will form paraboloidal surface. BUT, the shape is subject to pressure differences and this works only for far IR, where departures from required shape are comparable or smaller than wavelength.
Another one may be speculum (alloy that people used for mirrors in the past, but it does not compare to glass.

Try www.surplusshed.com or similar places, there you can find cheap mirror.. the rest is , as you said, next to nothing.. except time and fiddling with plastic tubing and other parts and materials from Bunnings or places like that...
For finder (and eyepieces), use second hand binos, you can find them for $10 on Sunday markets...
And, besides, what is wrong with binos to start with?

[netwolf]

Brian,

There is a TAL1M for sale in the used equipment section for 150bucks with a good mount. These things are built well. Reviews i have read of this scope are quiet good but there is the odd bad review.

Regards
Fahim

[kayakfisherman (Brian)]

Bojan, that surplus store is fantastic. I just had a look at their catalogue, and I will definitely be following up with them after I get a bit more knowledge under my belt.

Fahim - thanks, that TAL1M looks like a great deal. I would like to start of with at least 10" though (wouldn't we all!!!).

Starkler - your statistics certainly put it into perspective; and I completely agree with you that if China could have done it cheaper, they would have already done so...

cheers
Brian

[netwolf]

Brian, I know what you mean my mates tried to convince me with electronic goto system with smaller apperture. But my first Telescope was a 10" Dob.
Surplus shed is a good way of getting some very good bargains.

Edit: Why not put up a wanted ADD for Mirror in the Classifieds section of the forums. Also contact Matt at Telescope and Astronomy, I am sure he has kits etc for DIY.

[Bassnut (Fred)]

If $500 is "way" out of your total budget, your in for a whole world of pain in Astronomy, and probably very off putting to boot in the process even starting this way.

[kayakfisherman (Brian)]

Hi Bassnut - I understand your concerns, but lack of dollars does not always have to be the insurmountable barrier it is perceived to be. For example, I love fishing, but not for tiddlers. Most people would believe the only way you can go for the big fish is to spend a small fortune on a boat, or pay a hundred dollars + to share a bench with a whole heap of strangers on a charter boat. Sometimes, to use the awful cliche, you just have to think outside the box a bit. So I bought a cheap secondhand kayak which I have set up to fish offshore. I even save money on bait and ice by buying it at the fish markets (for a third of the cost), and I make my own lures for next to nothing. Occasionally the odd boaty will go out of their way to be rude/obnoxious while I am offshore, but I put that down to them simply be jealous that I am fishing the same locations as they are in their $100,000 + boat. I am not a miser, its just that as a sole income earner for a family of five I cannot justify spending more than a few dollars a week on my own hobbies. It may well be the case that it is impossible to get into astronomy without having a lot of money to splash around, but I am not going to let the fact that others are spending a lot put me off investigating whether some corners can be cut on the costs.

Cheers
Brian

[Rod]

Hi Brian,

Have you thought about a plate glass blank? A 10 inch by 1 inch disk should be pretty cheap. A cermic tile tool costs next to nothing to make. You then just need to get the abrasives, pitch and cerium oxide which are available from some clubs or from GotGrit.com.

Hope that helps,

Rod.

[Starkler (Geoff)]

Originally Posted by kayakfisherman

Hi all

Thanks for all your replies and welcome.

You can get dobs very cheap, and a 12" dob of reasonable quality for $500 is extraordinary good value. Nevertheless, $500 is way out of my budget (if I was single it would be nothing!!!), and I still believe that putting aside the mirrors, the rest of the dob could be made for next to nothing out of scrap bits (except for the other optics - finderscope, focuser).

Seriously the dobs as mentioned are the cheapest way to get that kind of aperture, and as you have already found out you cant really build one any cheaper yourself.
Sometimes you see unwanted scopes like this on the second hand market and thats where I suggest you look. Its also not unknown here on IIS for some generous people to donate bits and pieces to keen would-be amateur star gazers.

I liked your story about the kayak fishing and I would love to see you in action

[toyos]

If money is the main issue, why not use that extra time you're going to spend to build the scope out of scrap to do some more paid work?

Making the mirror yourself, looking around for cheap parts and then building it properly sound pretty time consuming. It will cost way more than $500 worth of time, in my opinion. Wouldn't be worth it if it was all about saving money, would make more sense if it was for your own satisfaction.

[TrevorW]

Don't rush your decision look around sometimes a bargain can be obtained. Also you really don't have to start big some people are obsessed with size and size doesn't always matter.

Look at a 6" or 8" scope either DOB or Newt, either would be good for general viewing. Even a good set of high powered binoculars will do on an alt/azm mount.

[bindibadgi]

I would also suggest binoculars. They are better than they look, and will get you a good grounding in the night sky. If you're into DIY, you can make a parallelogram mount to put them on a tripod and keep them steady.

[Screwdriverone (Chris)]

The British BBC Magazine Sky at Night recently had a series on building your own dobsonian 8 inch scope out of wood and cardboard post tubes/loo roll tubes for the focuser which fit the 1.25" eyepieces.

I think it started in February or March issues of 2008 and is well worth a look to see how easy it is to build your own. The CD's that come with it have step by step DIY guides and pics & videos as well, with the only purchased bits being the mirror and the secondary spider & mirror.

Well worth a look if you can dig up a copy from their back issue department.

Also, Andrews Comms has some ex demo/used stock well worth a look for only $249 below:

GSO GS-500 6" reflector w/SV1 eq. mount/tripod - requires small parts $249.00
GSO GS-680 8" Limited Dobsonian - excellent condition $249.00

Cheers

Chris