Anti-Scratch Coating

[Captain Scarlet]

It's my understanding that most mirrors and lenses these days are, in addition to anti-reflection coatings, coated with a scratch-resistant layer on at least the surfaces that are exposed to the elements and likely to get contaminated and therefore cleaned and touched. I've tried to search the literature for what those coatings are actually made from, and it's surprisingly difficult to get any concrete information.

The search results are mostly of opticians trying to sell you scratch-resistant coatings for your new specs. Even the remaining 5% of search returns say "we employ CaptainMagenta-Lux,  the latest proprietary anti-scratch technology" or somesuch without giving any useful details. Schott's site says something like "give us a call and we'll give you more info via personal conversation". It all seems very hush-hush. Commercial secrets, I guess.

I believe that most such coatings are SiO2, i.e. Quartz, which is pretty hard stuff and should resist a reasonable amount of abuse. My only qualm came when further looking for information on household dust, to find that a reasonable proportion of household dust comprises, yes indeed, small particles of quartz! Which makes me reluctant to use the "soft fingertip" method of cleaning lenses and mirrors.

Another far harder material that seems to be used for some, especially IR,  optics and watch-glasses is Sapphire or Carborundum, which is much harder than Quartz. So my question: is Sapphire ever used for scratch-resistant coatings? If so, I'd be much happier to use a fingertip cleaning-method on it. Indeed if I were confident my scope had a Sapphire coating, I could happlily scrub away with wire wool! Well perhaps not, but you get the point.

Comments or descriptions of actual industry experience welcome...

Cheers, Magnus

Edited February 15, 2021 by Captain Magenta

[LukeSkywatcher]

Fingertip cleaning method?. I'm imaging that's as bad as it sounds. You should use a rocket blower to remove any stubborn dust/crud.

I have heard that primary mirrors do have a very thin coating on top of the silvering. I doubt its sapphire. Wouldnt that make everything a false colour. 

[Captain Scarlet]

I find that a rocket blower gets rid of perhaps half the dust, the rest stays firmly in place.

I've read of 3 extremely respected people in the industry recommending and using the fingertip method on their optics, and I do, or rather did, buy the arguments about it, before I read that there's significant quartz in house-dust.

The story is that if a surface is quartz-coated, quartz being really quite hard, and you gently, I mean gently, rub your soft fingertips over the mirror or lens surface in the presence of water + detergent, you'll end up with a good result. My own direct conversation was with a senior person at Leica who recommended the technique for their spotting scopes and binoculars. He even went on to say to do it under a slowly running warm-water tap, but their optics are nitrogen-purged waterproof! This is actually how I clean my bins and spotting scopes.

Sapphire in the jewellery world is indeed coloured, but synthetic "pure" sapphire is clear, and incredibly hard. Almost no commonly-occuring substances are harder, only diamond dust really. Sapphire's colours arise from other contaminants in the Carborundum crystal structure. In fact there's an in-joke in the gemstone world that says there's only one colour in which Sapphire gemstones don't occur: Red. That's because they have a different name: Ruby! Ruby is in fact red sapphire.

M

[Peter Drew]

I think most amateur available mirrors are overcoated with silicon dioxide as a protection for the softer aluminium.  Mirrors, like star diagonals, can be dielectrically overcoated for greater protection but the process is very expensive.    🙂

Quote

 

 

[niallk]

I run my mirror under a shower head to clean it.  Any loose dust is sloughed off.  I do then barely rub the surface with washing up liquid covered fingers... under constantly running tepid warm water.  Seems ok; lots of folk do it?  Yup SiO2 overcoat on mine too.

[Alma]

No industry experience I’m afraid, however I’ve seen Swarovski’s excellent Swaroclean coating described as “hydrophobic”, I.e. water repellent.   There’s a Wiki article here which gives details on hydrophobic technology :-

https://en.wikipedia.org/wiki/Superhydrophobic_coating

 

[PEMS]

I would think mirrors are SiO2 coated to prevent oxygen getting at them, aluminium is actually very reactive and forms an AlO2 layer very fast and it is a grey not a reflective surface. The layer is however stable and so prevents the remainder of an aluminium bar just oxidising (rusting) away. Did you never do the aluminium powder experiment at school? Take Al powder, add an oxidiser and set a flame on it. It burns fast and hot. That is Aluminium.

Would not think that a lens can be so protected. The AR aspect calls for material properties of the various layers - mainly the refractive indicies and the material is chosen for the refractive index not the hardness. In effect a different requirement. The mirror is not AR coated just protected from the atmosphere. So they have different criteria at depositation.