aluminium mirror

[TimmySand]

Do you guys think it would be possible to make a fair quality aluminum reflector mirror?

This guy made a small one:
http://www.optics.arizona.edu/SSD/concave.html

Glass is very expensive and I don't have the patience to walk around a barrel hundreds of times. Aluminium is cheap and much softer than glass. I am not sure but I think polished aluminium is almost as reflective as regular mirrors.

By the way why doesn't any make cheap reflector mirrors? There is a concave mirror in my bathroom, why doesn't anybody just put a reflective surface on the outer surface of one of those? It probably wouldn't be perfect, buy hey for the price I dont think you could go wrong.

[aurora]

What would you consider "cheap" for a finished mirror?

Here's a 4.25 inch mirror for $80.

http://www.buytelescopes.com/product.asp?t=70&pid=3069&m=

Or try links from the Atronomy Mall

http://astronomy-mall.com/

[Veeger]

Aluminum mirrors have risen and fallen in popularity over the years. They are fairly inexpensive and fairly easy to grind and shape. They can oxidize depending on how they are coated, but the biggest draw-back I ever heard of was the temperature sensitivity. Aluminum may distort as it expands and contracts with temperature. Glass does as well, but the coefficient of expansion is much smaller.

Good luck if you try it and let us know how it comes out.

Best,
Veeger

[hhEb09'1]

By the way why doesn't any make cheap reflector mirrors? There is a concave mirror in my bathroom, why doesn't anybody just put a reflective surface on the outer surface of one of those? I'm not sure, but the outer surface may be flat. Besides, I think I've seen some examples somewhere where people have used bathroom mirros in telescopes. Like you say, not perfect. :)

Here's an article from ten years ago, about frugal telescope making (http://www.upstateastro.org/stars/frugalscope.html). While looking around I also found this image of an MMT at Whipple. Awesome picture, once you figure out what you're looking at. I'm going to submit it to APOD. :)

[redshifter]

Do you guys think it would be possible to make a fair quality aluminum reflector mirror?

This guy made a small one:
http://www.optics.arizona.edu/SSD/concave.html

Glass is very expensive and I don't have the patience to walk around a barrel hundreds of times. Aluminium is cheap and much softer than glass. I am not sure but I think polished aluminium is almost as reflective as regular mirrors.

By the way why doesn't any make cheap reflector mirrors? There is a concave mirror in my bathroom, why doesn't anybody just put a reflective surface on the outer surface of one of those? It probably wouldn't be perfect, buy hey for the price I dont think you could go wrong.

It would depend on how you define 'fair quality', but unless you have a lot of experience grinding and shaping mirrors, you're probably getting into more than you bargained for.

A parabolic mirror is a fairly involved process which can be pretty time consuming, esp. with the more accurate high end parablolic mirror, and they don't come cheap. A cheap spherical shaped mirror might be available, though I've not seen any spherical mirrors in anything larger than about 4.5". A cheap concave mirror (is it even spherical?) with a reflective coating thrown on would probably make an extremely poor primary mirror for a reflecting telescope. Not worth the trouble and expense of sending a mirror like that out to be coated IMO.

[Veeger]

As for bathroom mirrors, if you want to use one why bother to have it coated on the front surface since image quality is obviously not a factor. Bathroom mirrors are much too thin and full of distortions. As a kid, I actually tried one because I was desperate for aperature. I rigged a simple mount with off-axis focuser and actually got a highly distorted, poorly focused image of a street light, full of chromatic abberation. At the end I decided the poor quality was not worth the effort. (My first hand ground mirror (from Edmund's Scientific), actually fared just slightly better, but I gave up, as I never had the money to have it coated. All them hours spent basically for nothing. :cry: )

-Veeger

[JustAFriend]

Here's a 4.5" mirror for $36:

http://www.scopetronics.com/atmmirrors.htm

They have all the way up to 12" mirrors. Heck you can get a refractor lens for less than $50 from surplusshed.com

Aluminum is a bad material due to the mentioned thermal properties and bathroom mirrors are just a bad idea....

[TimmySand]

Here's a 4.5" mirror for $36...

...Heck you can get a refractor lens for less than $50 from surplusshed.com...

...Aluminum is a bad material due to the mentioned thermal properties

That $36 mirror is quite a deal but I already have a 4.5" reflector, I was hoping to make the aluminium one around 8 inches.

Also I already made an 80mm refractor telescope from surplusshed.com (the whole telescope cost me less than $40)

How bad would the thermal properties be for aluminium? Once the telescope cools down shouldn't it be fine?

[Veeger]

Yep. If you grind it and correct it at the same temperature.

In my opinion, it depends on how important a good image is to you. The imperfections may be barely noticible if you are not a seasoned observer. Mirrors of that size are usually parabolic, not spherical, due to aberration and so the surface shape is more critical. What can you live with?

If you do manage to go through with it, give us an update. I'd be interesting in hearing about the results.

Best,
Veeger

[Hornblower]

The greater expansion coefficient should not distort the optical surface, provided the aluminum is perfectly homogeneous and properly annealed. The high thermal conductivity results in uniform expansion or contraction. For long-exposure photographic work, the focus shift could in principle be corrected with some sort of temperature-compensating tube assembly. For critical astrometric work the slight change in image scale could be a problem. For visual use none of this should be a problem.

Now the bad news. Here is an interesting publication on the topic of aluminum mirrors.
http://tco.gsfc.nasa.gov/ft-tech-super-alum-polishing.html

As they pointed out, grinding with abrasives does not yield a good optical surface on a piece of aluminum. They machine it to the desired curve with a diamond cutter on a special milling machine, and then do the polishing and fine figuring with a special polishing compound.

These mirrors are made for special purpose professional instruments. My educated guess is that the necessary tooling would be prohibitively expensive for an amateur telescope maker. You will be better off with glass, even with the expense of having the mirror coated.