27 replies to this topic

[largelenses]

Hi all,

  I'm looking to have a 4 inch glass disk silvered but it needs to have a very durable protective coating to survive multiple cleanings etc. It will be used for imaging mostly around 800 nm (with occasional visible spectrum) but without much magnification... probably less than 5x, so I should think that multiple protective coating layers would be OK too. Recommendations? 

Thanks

[rainycityastro]

Curious as to why you're not aluminizing it. 

[Oregon-raybender]

There is no coating available for sprayed on sliver, if that is what you are 

talking about. You can use a standard AL coating with a better overcoat 

to increase reflection at 800nm. AL has a dip at 880nm. You can 

go for a gold coating. 

 

Starry Nights 

[Bob4BVM]

If you mean a sprayed on sliver coating, Bob is (almost) right.

There is the Angel Guard anti-tarnish overcoat us spray-sliver guys use.

It gives a very mild level of protection to the silver,

But it is no way a hard coating like the oxides and other overcoats we see on standard vacuum coated mirrors.

 

There are vacuum coaters who can do silver with overcoats, but not a home job if that's what you are looking for.

 

CS

Bob

Edited by Bob4BVM, 24 June 2025 - 02:48 PM.

[hamishbarker]

Gold's excellent IR performance and I assume resistance to tarnish sound favourable, but from what I have read, it's much more difficult to successfully evaporation coat than aluminium. Maybe sputtering could be easier, but the sputtering target made of gold, even if thin, =$$$ !

I wonder if gold coating removal might also require use of aqua regia.

[SteveInNZ]

Could you electroplate gold onto a silvered mirror ?

 

Steve.

[luxo II]

nope.

 

The reflectivity of protected aluminium coatings in the NIR is well above 90% so I really don't see why you wouldn't use it, especially for a small mirror that is easily coated at modest cost.

 

Silvering is more relevant to those with big mirrors and no cost-effective way to aluminize them, short of replacing the mirror - as is the case in my country where 40 cm is the limit of the local coaters. Shipping a one-off mirror back-and-forth internationally costs more than replacing the mirror.

Edited by luxo II, 24 June 2025 - 08:08 PM.

[Oregon-raybender]

We used a standard AL coating for our equipment, 400 to 1200nm

with no issues, a slight dip at 800nm, but not a problem.

 

Starry Nights 

 

https://www.plxinc.c...irror-coatings#

 

https://wp.optics.ar...-Treatments.pdf

[triplemon]

Gold reflectivity just starts at 600nm,not supporting the "occasional visual range"use.
Its also a LOT softer than AL, making it rather unuseable for a particularly durable coating as requested.

More layers of overcoat as requested also does not translate into more durability.
If repeated cleaning is the problen, try CO2 snow or other particularly gentle cleaning methods.

Edited by triplemon, 25 June 2025 - 03:07 AM.

[calypsob]

Dichroic coatings are pretty tough

[Diego]

I'm guessing perhaps the OP may have the same issue I have (unless it's something else). Practically cero aluminizing services for telescope mirrors in the country. The only coating chambers are for lab or universities use, not open to the public. The other options are car headlight aluminizers that usually don't have the experience or knowledge on handling optics. For my 16" mirror I was quoted by the club that made it, a 30 day leadtime and they send it off to someone else. Considering how hard to get, and expensive a 16" mirror is here, I refuse to send it off to the unknown. I have considered silvering myself as an option if I can't find someone locally that I can trust enough....

Edited by Diego, 25 June 2025 - 07:19 AM.

[luxo II]

Diego much the same in my country - there is just one who can do upto 40cm, but for larger mirrors the international shipping costs more than the mirror is worth, which leaves two choices:

 

a) strip it and apply a silver coating yourself (and repeat as often as needed) or

b) replace the mirror with a new (overcoated) one, with the expectation that it will last 10+ years if well cared for.

 

A club mate of mine has an 18" in need of recoating and finally decided to replace with new from Hubble Optics. He wasn't confident about silvering.

Edited by luxo II, 25 June 2025 - 08:12 AM.

[Starman1]

Hi all,

  I'm looking to have a 4 inch glass disk silvered but it needs to have a very durable protective coating to survive multiple cleanings etc. It will be used for imaging mostly around 800 nm (with occasional visible spectrum) but without much magnification... probably less than 5x, so I should think that multiple protective coating layers would be OK too. Recommendations? 

Thanks

The thin film providers that do a lot of scientific equipment could easily provide an enhanced aluminum coating that would have a high reflectivity at 800nm and down to 425nm or so at the same time.

The layers of dielectric materials used in the overcoating enhance durability and increase reflectivity.

Coatings like these, if kept clean, will maintain high reflectivity for 10-15 years and be quite durable.

 

Gold is not usable visually though it has benefits for near IR imaging.

Silver is not as durable and will lose reflectivity faster.  Plus, a professional lab coating using IAD and dielectric overcoats will cost a lot more than aluminum.

 

You don't say where you are located.  Perhaps such enhanced aluminum coatings are not available where you are.

Have you done a web search for coaters in your country?

[triplemon]

He has a 4-inch mirror, like a medium size secondary. This can be shipped at reasonable cost anywhere across the world.

[Tom Stock]

If you mean a sprayed on sliver coating, Bob is (almost) right.

There is the Angel Guard anti-tarnish overcoat us spray-sliver guys use.

I believe "Angel Guard" is really just a repacked product called "Midas".

 

https://www.riogrand...037?code=335037

 

Not a durable coating by any stretch but I washed my 10" silvered mirror once about a year ago (very carefully) and it's still going strong. I'm about to complete year #2 since I silvered it ... Well worth the $15 it cost me to silver it... although the process is a little demanding and not for everyone.

Edited by Tom Stock, 25 June 2025 - 01:05 PM.

[IslandPink]

Silver can be coated and protected to a durable level ( multiple cleanings and years of service ) . There will likely be about 3 layers on top, and modern IAD significantly reduces porosity and hence durability.  My guess is you'll need a bigger commercial coater, and will need to put in some money and/or wait for the time they can fill a plant.

[DrPete4155]

If you can find a coating house I would just stick with al/sio.

 

Another option if you find someone who is willing, copper will reflect just as good as gold at your wavelength and the plus is it can be overcoated with sio just like aluminum.

 

Nothing special about it, just replace the aluminum with pure copper.

[Bob4BVM]

I believe "Angel Guard" is really just a repacked product called "Midas".

 

https://www.riogrand...037?code=335037

 

Not a durable coating by any stretch but I washed my 10" silvered mirror once about a year ago (very carefully) and it's still going strong. I'm about to complete year #2 since I silvered it ... Well worth the $15 it cost me to silver it... although the process is a little demanding and not for everyone.

Correct, same stuff as Midas

[Bob4BVM]

He has a 4-inch mirror, like a medium size secondary. This can be shipped at reasonable cost anywhere across the world.

Your'e right of course.

Funny how we tend to make mountains out of mole hills here

[Tangerman]

If you can find a coating house I would just stick with al/sio.

 

Another option if you find someone who is willing, copper will reflect just as good as gold at your wavelength and the plus is it can be overcoated with sio just like aluminum.

 

Nothing special about it, just replace the aluminum with pure copper.

Gold can also be protected, and copper is much more reactive. I don't really see the point of using copper.

[triplemon]

Guys, the OP is clearly stating that he occasionally wants to use the mirror for the visible spectrum, 400-600nm.

 

That very much completely rules out copper and even gold, as either have just a very few percent reflectivirty in blue and green. Most optical laymen also would inuitively not call gold to be a "silvered" thing.

In addition the improved IR reflectivity of gold only play out in the far IR range - at wavelengths of many micrometer. While the request here is for 880nm, in the very near IR, well under 1um.

 

As far as adding multi-layer coatings, these usually are more narrow band, the more layers are added. And most are optimized for the visual range, so reflectivity at 880nm might be somewhat reduced:


And yes, all these coatings could be redesigned and customized to more evenly cover both 880nm and the visual range, it all depends on how much effort (i.e. money) you want to spend.

Edited by triplemon, 26 June 2025 - 02:23 PM.

[Starman1]

Guys, the OP is clearly stating that he occasionally wants to use the mirror for the visible spectrum, 400-600nm.

 

That very much completely rules out copper and even gold, as either have just a very few percent reflectivirty in blue and green. Most optical laymen also would inuitively not call gold to be a "silvered" thing.

In addition the improved IR reflectivity of gold only play out in the far IR range - at wavelengths of many micrometer. While the request here is for 880nm, in the very near IR, well under 1um.

 

As far as adding multi-layer coatings, these usually are more narrow band, the more layers are added. And most are optimized for the visual range, so reflectivity at 880nm might be somewhat reduced:


And yes, all these coatings could be redesigned and customized to more evenly cover both 880nm and the visual range, it all depends on how much effort (i.e. money) you want to spend.

The EAL coating isn't bad, with around 85% reflectivity at 850-900nm, and >93% from 400-700nm.

Better in the visual range AND better from 800-900nm.  A winner.

[IslandPink]

Here's something I didn't know - M1 is being coated with Silver !  -

 

https://www.youtube....h?v=3-s_UZePP_Q

[triplemon]

Yeah - the dreaded chrome based adhesion layer on the glass, then pure silver and a silicon nitrate overcoat.

Compared to aluminum with silicon(di)oxide overcoat, silver and the nitrate are both chemically deposited, no vacuum needed. Plus, the nitrate is mechanically and chemically very resistant, several times more than Oxide. You often need to strip it with hydrogen fluoride. Which is one of the nastiest chemicals you can ever come into contact with ... the number one cause for limb amputations in the semiconductor industry. Ouch.

 

But overall the nitrate overcoat is widely used in semiconductors and also as AR layer in solar cells, so the hardware to deposit them is widely available.and the size of these mirror is nice to handle, too. The only issue is the nitrate creates significant mechanical stresses, so the layer have to be kept really thin. The 600nm they quote in the video sounds suprisingly thick to me - would not want it this thick on a semiconductor wafer.

I always wondered if that tech would be any good for our telescope mirrors, but it appears in our low altitude, humid and more importantly sulfur rich environments that overcoat would not prolong a silver mirror for long enough to put up with the significant chemical hurdles to re-coat them. BUT, given these materials are so widely used in mass produced electonic things, the odds are good that some of the bazillion of dollars poured into cost optimizing these things, some day there might come a new practical mirror coating out of that. The astronomers teflon pan moment  ....

Edited by triplemon, 01 July 2025 - 11:45 AM.

[hamishbarker]

are you sure they are chemically deposited for the ELT? I read some articles a while ago about the silver coating equipment for ELT and I think it was magnetron sputtering. I have no idea about the nickel chrome nitrate etc.