Cleaning Objective Lens

[Visionary]

I have read many and differing accounts on the where and where'fors of cleaning primary and secondary mirrors. My question is.... Given that the advantage of using aluminium in marine environments is that it "skins" and dulls the moment it is exposed to the atmosphere.
How is it then that telescope mirror with its reflectivity in the range of 99% is achieved via their aluminium surface coatings, how is it that the coating does not immediately oxidise? Is a second protective surface applied? How is it done?

[mental4astro]

The aluminium coating is prevented from oxidising by the application of a protective silicon oxide coating. It keeps the reflectivity at a very high percentage over many years. It is not a perfect seal, and the aluminium coating does slowly degrade.

Professional instruments, such as the 4m AAO puppy, is just the naked aluminium. As a result, the coat of Al degrades over a couple of years, and needs to be recoated. This is done with facilities built into the observatory.

There is a really good set of before and after series of photos about the recoating of the 200" Hale Telescope. The recoating of this 5m mirror is also done every two years. The difference in the appearance before and after is quite striking:

http://www.pfonline.com/articles/mirror-mirror

The 4m AAO instrument, and its smaller siblings also undergo the same recoating regime.

Now, recoating OUR mirrors is more problematic. Non-overcoated mirrors are have a sodium hydroxide solution put over them to remove the aluminium. The Pyrex glass is cleaned, and the aluminium is reapplied.

Silicon oxide overcoated mirrors need to have this protective coating removed. Problem here is the substrate, the parent glass the mirror is made of, NEEDS to be Pyrex, or SuperMax, not BK4 or BK7 glass types, and certainly not plate glass. The chemicals used to remove the overcoat will also dissolve these glass types. In these situations the overcoating needs to be mechanically removed by grinding/polishing. Recoating overcoated mirrors isn't cheap, and more so those using chemically sensitive substrates.

Alex.

[Visionary]

Alex,

That is an excellent answer to a question that has been rolling around in my mind for some time. A protocol exists for larger scopes however for our smaller scopes re-coating is extremely difficult and impractical.

Many thanks

David

[dannat]

Silver is highly reflective, & an alt. To Al for mirrors. The formation of oxide can make the mirror slightly more durable
Quartz overcoat is quite hard, and while may need to be removed should last a long time provided you look after the mirror, eg avoid moisture
The top coat can be applied diff ways, eg ion deposition

[mental4astro]

No, not extremely difficult or impractical in all cases. It requires special consideration of the material the mirror is made from.

There is also another thing to consider. With the standard of mass produced mirrors being quite good today, and their cost greatly reduced too, the quality of mirror that would have justified recoating 20 years ago has been superceded by cost of the recoating process versus the cost of a new, cheaper and relatively better mirror. Today, what makes recoating a practical consideration is for the high end mirrors. The cost of these is 5 to 10 times the cost of a cheap mirror. These certainly are worthy of recoating. But there is a significant investment in these mirrors.

Now, lets complicate things even more, :D

While the average quality of mirrors has improved, as in every situation, there will be mirrors that will fall below that average, and some that will exceed that average. And not all mass produced mirrors are made to the same average standard, some higher, some great (I am not talking about the garbage end of the market. Only the serious, fair dinkum end of the market serving genuine astro boys and girls). And, there are also many ways available to amateurs to test the quality of their mirrors. With all of this, there is a good chance that you can have fluked upon an exceptional mirror out of the mass produced ones. Then you have the dilemma of deciding to recoat a bloody good mirror, or chance a new, less well figured one... :lol:

But don't get all worked up about recoating. Protected mirrors these days can last more than 10 years before changes can begin to be seen. And if the coatings are very good, the coatings can last for more than 20. Now this in no way means that after 10 years you will still have 95% reflectivity. No. After 10 you may expect 85 - 80 %. This is still pretty bloody good. And this also depends very much on how you look after the mirrors. Neglect them, over wash them, leave them exposed to sea air, then things go pooh.

[ausastronomer]

Hi Alex,

The 3.9m AAO telescope mirror is recoated once a year. They cannot get more than this out of it due to the sensitivity of the instruments used on it. Some of the smaller telescopes on the mountain can go a bit longer between recoats (up to 2 years). The annual recoat on the 3.9m mirror was delayed a couple of months in early 2014 due to technical problems with the coating chamber ( or the crane, can't remember which ) but normally it's every year for the big fella.

Cheers
John B