Cleaning Mirrors

[Carbon Brush]

Mods, please move to a different section if thought more appropriate.

Not a 'Do I need to clean this?' question. More seeking thoughts on How do I clean this?

I used to be told that a mirror was essentially aluminium deposited over glass. It tarnished over time (2 to 5 years) then needed recoating.
This was discussed when I was in an astro shop in 2003 looking to buy a reflector and I was factoring in the cost per year for a mirror recoat fund.
I also remember during the 2000s an astro club talking about having their big scope mirror recoated in this sort of time frame.
It might be said that with such a short life, mirrors don't get chance to get mucky🤔
Any coating over the ali was quite soft, needing the 'very gentle chemicals no touching' cleaning regimes advocated by many.

Then I heard of additional harder coatings. Maybe Hilux from Orion Optics? Sorry if I'm wrong.
This claimed a longer life and was (I think) a bit more tolerant of cleaning.

Now in 2022 I have several old reflector scopes with mirrors that still look quite good.
I have cleaned long neglected mirrors (dead flies, bits of leaves, general dust, damp) and they have come up quite well.
The coating has obviously been very restant to chemical exposure.
I read threads now about mirror cleaning using fingers - which was an absolute NO in the past.
Clearly (no pun intented) things have changed.

Does anyone have information about modern mirror coatings to back up more aggresive cleaning regimes?
Do any scope manufacturers publish information cleaning their mirrors?

Anything that anyone knows with information traceable back to a mirror or scope company would be good to hear.
Rather than 'I got away with it on the last 5 mirrors I cleaned.'

Another one on recoated mirrors. This was told to me by someone who had a recently recoated mirror, and been advised by the optics company involved.
If you leave the mirror in a sealed container (circular biscuit tin comes to mind) for a couple of months after coating, it will last longer.
I asked the reasoning behind this but he didn't know.

Looking forward to hearing comments and contributions.

David.

[Uplooker]

Hi David,

mirrors can become very dirty before there is any impact on image clarity. When you do clean them you can certainly use your finger tips - as long as they are free from dirt/ particles. Submerge the mirror in water with the tiniest bit of washing up liquid added. Let it soak for a bit, then very gently rub over stubborn areas. Rinse off under tap water, then a final rinse when the mirror is at an angle with Reverse osmosis (RO) water. The RI is available for peanuts from any aquatic fish shop. The RO has very little dissolved in it and therefore will not leave residue/ witness marks on the mirror. Mirrors and their coating are significantly more robust than th majority of people give them credit for.

I suspect that the majority of manufacturers would recommend people send their mirrors back to them for cleaning due the likely liability they would be exposed to if people screwed it up.

[CraigT82]

Modern mirrors have an overcoat of silicon dioxide (i.e quartz - very hard) on top of the aluminium. This top coat is protects the reflective surface for many years. 

Not sure about modern mirrors being kept sealed after coating but I think older mirrors without a protective overcoat were advised to be kept sealed up for a couple months after coating to allow the aluminium to develop and oxide layer. 
 

OOUKs hilux coatings is a silicon dioxide overcoat which has some trickery in the application process which increases the reflectivity (and also the longevity apparently). 

[Carbon Brush]

@CraigT82Thanks for the comments.

The quartz coat makes perfect sense.
It seals the aluminium against water and oxygen. As well as providing a hard coat that will withstand moderate abrasion.

The uncoated mirror in the biscuit tin also makes sense. I'm not sure if there was silical gel in the 'story' I heard.
If so the ali would be oxidised, which does form a protctive coat.
By keeping water out, you avoid the formation of aluminium hydroxide which I think doesn't have the same stable surface properties as oxide.
Having said this. My chemistry was a long time ago.

 

[Pixies]

Does this mean that all the advice about the careful and gentle cleaning of Newtonian mirrors really applies to the old uncoated ones, and more modern mirrors can take a little rougher handling?

(Not that I'm going to risk it, mind!)

Edited July 30, 2022 by Pixies

[CraigT82]

2 minutes ago, Pixies said:

Does this mean that all the advice about the careful and gentle cleaning of Newtonian mirrors really applies to the old uncoated ones, and more modern mirrors can take a little rougher handling?

(Not that I'm going to risk it, mind!)

I wouldn’t say that TBH, the protective coating is still incredibly thin and even a tiny of grit pulled across the surface will scratch through it. It’s probably best to consider it a protection from atmospheric elements rather than mechanical attack. 
 

Eventually tiny holes form in the overcoat which allows the aluminium beneath to be exposed and that eventually leads to holes right through the coating…. That looks like this:

[ONIKKINEN]

Just lightly cleaned my OOUK mirror and the coating looks pretty much exactly like it did 2 years ago with a few (user error) scuffs here and there. That 2 to 5 years per recoat sounds very aggressive to me, doubt that's actually necessary.

I dont overthink the cleaning process, not that i need to since it wasn't really all that dirty. Puffed away all the surface dust and particulates with a blower, brushed the not-so-mobile particulates off with a camel hair brush that came with a camera cleaning kit, then few drops of lens cleaning fluid which i swirled around without applying any pressure with a sealed lens cleaning paper (single use). Then finally sprayed the mirror with a small amount of water and removed whatever was left of all that drying up with a fresh microfiber cloth. Didn't even take the mirror off the cell, overall took maybe 5 minutes but again this was not a mirror that "needed" cleaning, i just happened to have the mirror off so might aswell clean it.

24 minutes ago, CraigT82 said:

I wouldn’t say that TBH, the protective coating is still incredibly thin and even a tiny of grit pulled across the surface will scratch through it. It’s probably best to consider it a protection from atmospheric elements rather than mechanical attack. 
 

Eventually tiny holes form in the overcoat which allows the aluminium beneath to be exposed and that eventually leads to holes right through the coating…. That looks like this:

This looks nasty, how old is the coating here? I assume this is some significant magnification under a microscope, but still looks ruined.

[CraigT82]

26 minutes ago, ONIKKINEN said:

This looks nasty, how old is the coating here? I assume this is some significant magnification under a microscope, but still looks ruined

It was a primary from a 12” dob which had been kept outside in a shed for 20-25 years, so it’d had a good innings. Pic was taken using one of those little clip on macro lenses on a phone. This pic gives a bit more perspective (believe it or not the views with this mirror were actually pretty good)

 

[Astrobits]

Hi David,

A couple of points for interest sake.

As far as I am aware, all mirrors are overcoated with Silicon Dioxide ( silica) ( normal ) or multi-layer ( enhanced reflectivity ) except those professional telescopes used for spectroscopy and unless the owner has specifically asked for a plain Al surface. The silica can interfere with spectra, adding it's own fingerprint, so not using it is preferable/mandatory in the professional world.  Certainly the mirrors in amateur telescopes that I am aware of over the last 50+ years have had an overcoating of one sort or another.

Although it is usual to quote that mirrors have a silica overcoat, it is not possible to evaporate silica directly in most coating chambers. Therefore Silicon Monoxide is evaporated onto the mirror and this will slowly oxidise to Silicon Dioxide over the following days on exposure to air. The coaters that I used back in the 90's always waited a day before packaging up mirrors to allow the surface to harden a bit.

Overcoated mirrors will last many years provided they have a tight fitting lid when not in use. My own 16" has the original coating from the time that I made it about 22 years ago, and it is still perfectly useable.

Nigel

[Astrobits]

I have just opened up my 16" which has been in storage for the past few years ( due to observatory damage, I really must get round to re-building it ) and taken a pic of the mirror.

22+ years and counting:

Nigel

[Carbon Brush]

I guess most of us are using coated mirrors now. But whether they have a molecule thickness of coating, or a good dollop is unknown.
That will probably determine life before dimming.
A scratch is though still a scratch.

While we still need to be cautious, a 'modern' mirror is more likely tolerant of abuse.

Thanks all for your contributions.

[Astrobits]

Many years ago I was shown a small mirror produced by an overseas company. Reflections in that mirror were all surrounded by a nice coloured halo. No, it wasn't back silvered, it had an exceptionally thick overcoat!!!!!!!!!!!

Nigel