Why are mirrors always made of glass?

[DavidOfBanff]

Is it because it is the only material that can be ground smooth enough?

Just curious.

[Starbabe]

Quartz would be nice

Also a reflective coating that won't RUST would also be nice, or at least seal it in so it cannot oxidise

2 things that put me off buying a reflector, oxidisation and fragility.

I would pay more if those two problems where reduced greatly.

Rust is a problem here.

Quartz is tougher and can be ground to more precision, glass cannot. That's what they say about quartz diagonals.

[andrew s]

David - Glass is a very generic term. Reflector mirrors can be made of plate glass, low expansion glass (e.g. Suprax) and low expansion crystalline materials like quartz and Zerodur. The main difference is the coefficient of thermal expansion. With plate glass it is very small but high enough for the mirrors figure to change as the temperature changes during the night. With low expansion glass this is not an issue for normal astronomical use and for low expansion materials like quartz and Zerodur none at all.

All can have a figure put on them that would meet most needs but plate glass is not recommended due o the expansion issue (which also complicates the initial figuring).

The Aluminum coating on most astronomical mirrors is over coated with either SiO2 or a high reflectivity coating.

Other materials don't have the combination of being able to be figured accurately and have a very low thermal coefficient of expansion.

Andrew

[barkis]

In the early years of telescope making, telescope mirrors were made of Speculum metal. Of course their reflective qualities were quite poor, and had to be burnished fairly frequently. They had Silver deposited on the surface which deteriorated fairly quickly exposed to the atmoshphere.

Modern astro mirrors, can be made from Plate Glass, Pyrex Zerodur, Cervit, Fused Quartz. All rather expensive, but Ideally suited, mostly for their very low coefficiency of expansion.

Many of the large Observatory telescope mirrors are made up from many smaller mirrors, and honeycombed to reduce the weight.

Ron.

[johninderby]

Yes you can get better mirror coatings. Orion Optics UKs Hilux high reflectivity coatings give a 25 year + lifespan to their mirror coatings. The Hilux is also a tougher coating that's less prone to scratches.

John

[assasincz]

I guess that in the end, it is all down to price of researching and developing the technology and appropriate procedure, price of manufacturing process in question, price and accessibility of materials used, the amount of time and number of people (and their qualification) necessary to make the damn thing - all that in balance to maximum usability, durability, functionality and quality within given restrictions.

If it weren't, I guess we would not pay the equivalent of +-£300 for an 8" OTA that your kinds could inherit.

[DavidOfBanff]

Thankyou for those very interesting answers.

So now I'm wondering if the mirror were of solid aluminium and the optical tube of the same aluminium, if the changes in size of the two parts with varying temperatures would keep the focus unchanged?

But I dont doubt that minds infinitely greater than mine have already been there and found why it wouldnt work.

[barkis]

For observation, and even moreso for imaging purposes, a telescope should always be at ambient temperature.

All the elements in the scope itself must be equal in temperature, otherwise unwanted tube currents resulting from the ongoing cooling process, will disturb the incoming light, and distort the image. The eye may compensate for slight image shifts, but a camera certainly wouldn't.

Ron.

[ollypenrice]

It is a strange thought, though, that all your big chunk of glass really does is hold a few microns of aluminium in the right place!

Olly

[andrew s]

Ron - You can't figure solid Al as well as glass you always get a rough surface (at the wave lenght of light level) . The layer of Al deposited on the glass takes up the figure on the glass as long as it is not too thick.

Andrew

[andrew s]

Olly - not just the glass but the whole mount etc just for the microns Al and some dopped gms of Si and a few electrons !!!!!

[Merlin]

In the classic American AMATEUR TELESCOPE-MAKING ( 3 vols ), there's a photo of a Newtonian mirror made of polished granite.

[skybadger]

Anything that will hold a polish against differential expansion will do. I have seen concrete mirrors before (never mind concrete springs and boats) and once dreamed of casting one in a spinning oil drum at f/2.

FGranite is interesting in that it has the potential to be microporous and flawed - I guess we are looking for a homogenous material without a tendency to defects

Apatite, calcite, sodalite, serpentine, garnet, diamond, carborundum seem to fit the bill.

Mike

[studdedsole]

Who's going to be first to try this as a garage project for those cloudy nights - a liquid mirror telescope

HowStuffWorks "How Lunar Liquid Mirror Telescopes Work"

[laser_jock99]

One of the nice physical properties of glass and and it's variants is it's hardness. To an optical worker this means it's good for polishing and an nice polish is what you need on a mirror. In lasers we use some exotic materials for optics- some like ruby, saphire & YAG are really hard (nearly like diamond) and can only be worked using various grades of diamond ginding/polishing compound. They are difficult to get to shape- but they polish up nicely on final figure.

Other optical materials like Magnesium Fluoride, Zinc Selendide, KTP & DKDP etc. can be more diffucult to work due the softness of the material. They require totaly different polishing techniques and achieving very flat (<10th wave) surfaces can be a nightmare.

Fortunately glass falls somewhere in-between making it relatively easy to both generate a curve and achieve a good polish.

[fwm891]

In the classic American AMATEUR TELESCOPE-MAKING ( 3 vols ), there's a photo of a Newtonian mirror made of polished granite.

I think was 'obsidian' (excuse the spelling) I sold my copies of ATM1,2 & 3

[Peter Drew]

The other factor, so far overlooked, is "stiffness", glass, and its derivatives, is very rigid physically when of the required thickness.

[skybadger]

Who wants.to fund my attempts with diamond?

Explains why my zinc selenide Windows were so expensive. I did also get some in rock salt, wonder how hard they were to polish?

Mike

[FraserClarke]

So now I'm wondering if the mirror were of solid aluminium and the optical tube of the same aluminium, if the changes in size of the two parts with varying temperatures would keep the focus unchanged?

Metal mirrors, especially thin ones, are actually a very sensible choice. If you have a single material construction, all the thermal stress problems go away (focus is a bit different though, because it's a combination of a mechanical expansion of the tube and the optical power/focal length of the mirror). Most people assume that the thermal expansion of metal mirrors makes them useless (it's about 10x worse than plate glass), but they also have such a high thermal conductivity (about 100x better than plate glass), that they can react to temperature changes very quickly. This means that a metal mirror will follow ambient much better than a glass one. I think metal mirrors (overcoated with aluminium in the same way as glass mirrors) would actually be a pretty good choice for amateur telescopes. I'm not sure how easy it is to get a good polish on the surface though?

People are working on carbon fibre mirrors now, which should be very interesting if the finer points of polishing can be sorted out. I know some have been made successfully, but the process is still quite involved.

As others have said, a lot of it comes down to how good a polish the material can take. We know how to do it relatively easily for glass-like materials. We can do it for metal mirrors (though true optical polish quality is still hard and expensive to achieve I believe). Not 100% convinced that people can do it reliably for carbon fibre yet.

[Peter Drew]

The majority of the posts pretty much answer the original question, other materials than glass are harder to produce and more expensive.

[laser_jock99]

I wonder if you could make a metal mirror out very low thermal expansion alloy like Invar 36

Carpenter Invar 36® Alloy

I searched the web but can't find anyone that's tried. Though most proffesional telscopes have copious amounts of Invar 36 in the structure. We use a couple of tonnes a year at work.

[Ericgraham]

A guy in Canada used to make ceramic blanks called BVC. Has anyone here had any experience with them?

[yesyes]

One could also try using an old satellite dish. Nice short focal length... ;-)

[fwm891]

A guy in Canada used to make ceramic blanks called BVC. Has anyone here had any experience with them?

Made a 22" f4.5, great material lower expansion than plate glass, grinds and polishes very quickly.

After finishing fine grinding with 5 micron grits it took a good (not full) polish in 7 hours using polishing pads and cerium oxide. I've taken longer than that on 12-inch LE and Pyrex...

A.J.'s Telescope Workshop -- BVC Telescope Glass and Mirror Manufacturer

[themos]

I think Silicon Carbide is used for your more exotic optics.