Why glass ?

[beamish]

Interesting thread but as an aside might I ask why mirrors are coated in aluminium these days ? I was under the impression that silver coatings were far more reflective ( in XS of 99%) but don't see it offered as the norm-please don't quote price as silver aint expensive ! ????????

[skye at night]

I have wondered if a laminated (thin layers of glass or ceramic) mirror would work... maybe a final figuring by traditional means or ion/laser...

Steve

[skye at night]

Interesting thread but as an aside might I ask why mirrors are coated in aluminium these days ? I was under the impression that silver coatings were far more reflective ( in XS of 99%) but don't see it offered as the norm-please don't quote price as silver aint expensive ! ????????

Perhaps the cost IS an issue as the silver would have to be near pure to avoid easily oxidised elements like copper etc in the 'mix'.

Yes, Sterling is cheap, even fine/britannia silver, but then 9K gold is a way different species from pure gold...

Maybe just a matter of 'what works' as the alu coatings these days are usually quartz (or similar) overcoated...

Not sure about reflectivity of silver, but I do know it is a better electrical conductor than copper, but we do not have our houses wired with twin and earth silver core cable...

I'd quite like to know the answer myself.

Steve

[Rui]

Well, if anything else was better or cheaper, it would be already commercially available.

As I see it, until now nothing better came up. At least considering the ratio cost/efficiency.

[beamish]

Perhaps the cost IS an issue as the silver would have to be near pure to avoid easily oxidised elements like copper etc in the 'mix'.

Yes, Sterling is cheap, even fine/britannia silver, but then 9K gold is a way different species from pure gold...

Maybe just a matter of 'what works' as the alu coatings these days are usually quartz (or similar) overcoated...

Not sure about reflectivity of silver, but I do know it is a better electrical conductor than copper, but we do not have our houses wired with twin and earth silver core cable...

I'd quite like to know the answer myself.

Steve

Umm, p'raps ? but surely silver coatings would be electrostatically deposited either from a pure Ag source or from solution either way impurities wouldn't be an issue I don't think.

Perhaps it IS available as an upgrade to stnd coatings ?

[Tycho Brahe]

I was thinking of recoating my mirror with silver. I mentioned it to my boss and he said not to cause it was Rubbish, and to stick with aluminium... He didnt expand greatly but my guess is its way too soft, so will scratch easily and doesnt take coatings well so cant really be protected... He also said to us aluminium oxide instead of a protective quartz coat...

[FraserClarke]

Glass is cheap, robust (there is a telescope mirror in Texas with 6 bullet holes in it from when the telescope operator went mental -- apart from 6 small holes, the mirror was fine), stable and takes good polish. If you include cost as a factor, there isn't a better substrate available. There are others, which you can use in specialist situations, like Silicon Carbide (light, stiff, expensive), Metal (OK in the IR beyond ~1 micron) and Carbon Fibre (light, stiff, possibly not stable enough?).

Metal you can turn/machine (you can machine glass too), but you still need to post polish if you want to use it in the optical, and that's what costs time/money. The surface quality you get from machining is orders of magnitude worse than you need for a usable optical surface (some mentioned making a diffraction grating )

Silver is used as a mirror coating, but it's harder to use and needs a protective coating to stop it tarnishing. It's also less reflective than Al in the blue (<370nm)

[FraserClarke]

Slightly off topic, but why does a "Glass" mirror have to be ground out, why cant you just pour moulton glass into a mould to achieve the perfect shape?

Quite right, you can

Steward Observatory Mirror Lab - TECHNIQUES

This place is insanely cool. I was lucky enough to have a look around it a few months ago -- very very impressive The pictures don't really do justice to the scale of the place (or the mirrors).

(most small mirror blanks are cut from a larger flat sheet, hence why they start off flat...)

[ponytale]

I think molten glass/pyrex is around 1500 degrees celsius ( red hot) the mold would draw out heat rapidly and change the structure of the glass/quartz crystals and silicates , then it would shrink away from the mold as it cooled , then it would need to be anneald and would probably change shape again and nead regrinding ? ponytale:icon_eek:

[ollypenrice]

What's wrong with glass? Is it really fragile? I don't think it is. I wouldn't want to drop our 20 inch on my toe. It is very hard, which is the main thing, so if Warthog pelts the reflective surface with screwdrivers it will hold up better than most other materials!! The reflective coating is always going to be fragile so care will always be needed.

The transition, historically, from research refractors to research reflectors was entirely dependent on the arrival of glass as a support for the aluminium surface.

No, I think glass is a good choice. However, in May I hope to be able to review a rather different mirror for you, still glass but watercooled... Aha!

Olly

[Macavity]

Sine it's traditional to mention refractors on a reflector thread (LOL) - I was wondering recently if they could make lenses based on Fresnel technology. Hmmm... maybe not!

But came across: Canon EF 70-300mm lens - Wikipedia, the free encyclopedia

Perhaps the future may produce different (revolutionary) technologies.

[simon84]

A little bit off topic here but I'm a glazier and have been involved in lifting and fitting some very big bits of glass all over the country.

One of our customers asked my boss just how tough the glass was we were installing which was two sheets of 10.8mm toughened glass laminated together.

To answer the customer my boss threw a 5lb club hammer at the sheet and the hammer just bounced off.

Was quite funny watching the customer duck for cover.

[ollypenrice]

A little bit off topic here but I'm a glazier and have been involved in lifting and fitting some very big bits of glass all over the country.

One of our customers asked my boss just how tough the glass was we were installing which was two sheets of 10.8mm toughened glass laminated together.

To answer the customer my boss threw a 5lb club hammer at the sheet and the hammer just bounced off.

Was quite funny watching the customer duck for cover.

Exactly! Tough stuff and as hard as... no, harder than nails!

Olly

[LukeSkywatcher]

I don't know, they use plastic for spectacle lenses now and apply different coatings to it.

Yeah but have you ever owned a pair of glasses with those non-reflective coating on?

I have and they are the biggest pain in the backside ever. I swear the coating actually attracts dust and grease.

I can only imagine that telescope mirrors made in the same fashion would suffer the same problem.

As Darkknight said................i think it is the reflective properties of glass that are the main reason why glass is still the material of choice in making mirrors.

[Tycho Brahe]

The antireflective coating is usually TiO2, so if pretreated right should work like the pilkingtons self cleaning glass, you should just be able to pour on water and sluice all the Rubbish away!!!!!

[simon84]

The antireflective coating is usually TiO2, so if pretreated right should work like the pilkingtons self cleaning glass, you should just be able to pour on water and sluice all the Rubbish away!!!!!

I've installed lots of that "self cleaning glass", it dosnt work.

[EA2007]

couldn't you just have a parabolic primary with metallic liquid on one side then have the other side as a large magnet, this would hold the metallic liquid in place and it would probably be quite smooth?