Gold mirror coating?

[JJDOBBER79]

Hi all, I have a question, there are aluminium, silver and gold. I understand that silver reflects better but doesn't last. What's the advantage/disadvantage of gold coatings as compared to aluminium. I can't find a lot of info on this.

[Shiraz]

gold has relatively low average visible band reflectivity and introduces a minus blue colour cast. Great for IR though.
https://assets.newport.com/web600w-EN/images/5694.gif

[Steffen]

I think (and the graph shows it) that gold has a very high reflectivity, it just cuts out the blue, giving the image a yellow tint. In some paper on scatter of vapour deposited coatings (wish I had bookmarked it) I remember reading that it has low scatter, too.

Gold also doesn't tarnish, so it doesn't need to be overcoated and will stay shiny forever. This benefit is somewhat spoiled by the fact that non-overcoated vapour deposits are mechanically fragile. Chemical cleaning is easy, though, you can wash it in almost anything you like :)

Cheers
Steffen.

[MrB]

It would be an ok coating for an astrograph used purely for shooting Ha and SII tho :)
OIII? Oh well!

[Stefan Buda]

I haven't tried evaporating gold but it should not be too difficult, so if anyone is interested in converting their astrograph (up to 23cm Diameter) I'd be happy to do it.

[Pinwheel]

If Gold is too yellow then what about using Platinum?

[mental4astro]

Platinum has a melting point of 1773°C, 700°higher than Gold. Vaporizing platinum is very difficult. It can be done, but the expense of the process and gearing up makes it a poor choice compared to silvering & overcoating. Just too hard.

[Wavytone]

Jas,

If you look at the reflectivity of gold as a function of the wavelength of light it isn't as good as silver or aluminium in the visible range (400-700 nm). Worse, it isn't constant, either - it's quite poor in the blue and increases in the red, hence the "gold" colour which will affect the images you see in it.

The best is fresh silver, if you're prepared to recoat it every few weeks. There are kits to do it at home using a water bath and a variety if chemicals - it isn't hard, however be aware that silver nitrate is a very unstable explosive and quite dangerous if it forms during the reaction.

The next best IMHO is fresh aluminium with a quartz SiO overcoat, this stops the aluminium oxidising (preserving its high reflectivity) and protecting the soft metal from scratches when you need to remove dust and grit.

Third is BerAl, an alloy of beryllium and aluminium. Hard, durable, and reflectance
around 85% long term.

In the infrared (> 1,000 nm), gold is an excellent reflector and combined with the fact that it won't oxidise or react with other airborne pollutants, it certainly is the coating of choice for infrared laser mirrors.

http://en.m.wikipedia.org/wiki/File:Image-Metal-reflectance.png

The most durable highly reflective coating for mirrors in the visible range is a multilayer di-electric coating with a zinc cryolite outer surface. Zinc cryolite is also used for the anti-reflection coating on plastic spectacle lenses that are typically lake green or yellow; it is incredibly hard and scratch-resistant (harder than glass), won't react with most chemicals, and the thickness of the layers in the coating governs whether it acts as a mirror, or as an antireflection coating.

While it is cheap in mass-produced optics (spectacles) as a one-off for mirrors it costs somewhat more than quartz coated aluminium.

The downside however with zinc cryolite is that it cannot be chemically stripped to recoat damaged mirror - it has to be ground off. So if you do go down the path of a multilayer dielectric coating, it is effectively for life - it's cheaper to buy another mirror than regrind, re polish and recoat.

There was a discussion previously about this where I pointed out that the costs of common mirrors for newtonians are now so low that re coating is now so low that mirrors are effectively disposable - the cost of replacement is less than the cost of re-coating.

The only reason to recoat now is because you have a truly exceptional mirror that cannot be replaced by a cheap one from china. From the perspective of an accountant, it is also cheaper to buy and replace a cheap Chinese mirror every 2-3 years than to fork out the $$$ for a dielectric coating.

The only exception to this is because you have weird optics that can't be easily replaced by cheap Chinese mirrors. Like a maksutov, or a Ricardi-Honda astrograph.

[Don Pensack]

Multi-layer dielectric coatings can be made more reflective than over-coated aluminum but only at one angle. Reflectivity changes both in percentage and in wavelength according to the angle of reflectance.
Accordingly, multi-layer dielectric coating is OK for a star diagonal or a secondary mirror in a newtonian, where the angle of reflection is a particular angle with a small variation from side to side.
On a primary mirror, though, especially one with a fast focal ratio, a ray reflects from the mirror at widely differing angles depending on which section of the mirror is being reflected from.
I suppose it would be possible to vary the coating materials in circular segments but the process would require masking many times and it would be very complicated.
So the best coatings for a primary, reflectivity-wise, remain aluminum with 3-5 layer overcoating (call "enhanced" coatings). Or overcoated silver, if recoated at more frequent intervals than aluminum.

[JJDOBBER79]

Thank you all. As always, a lot of info to digest.,