How Do BIG Telescopes Get Cleaned?

[H3LL]

My brother and I were sharing each others general ignorance on telescopes and wondered how the reflectors were cleaned.

Factoids we think are right but might not be:

Telescopes(?) such as the VLT use a large reflector(s) accurately positioned to very-small-measurements-probably-microns-or-even-smaller.

The reflective surface is not covered by glass or any other material as you would find in household mirrors etc.

The reflective surface is very thin and delicate.

Large telescopes do not appear to be protected from dust etc. when the big doors open for them to have a peek.Thus the question. How do they clean the reflectors in large, precision built, telescopes?

Thanks.

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[Cuddles]

http://adsabs.harvard.edu/abs/1997SPIE.2871..416G
http://spiedl.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=PSISDG004096000001000082000001&idtype=cvips&gifs=yes

Unfortunately pretty much all the links I can find won't let you read any more than the abstract. The most common method is CO2 snowflake cleaning. If your google-fu is stronger than mine you might be able to find something a little more useful.

[Dr Adequate]

With an enormous chamois leather.

[RecoveringYuppy]

I think that water is more dangerous than dust and most telescope enclosures are dehumidified or built in dry areas. I also think that most high performance reflective telescopes get resurfaced every couple of years.

ETA: http://www.journals.uchicago.edu/PASP/journal/issues/v117n829/205024/205024.html

[Magic 9-Ball]

I do think it would take a VERY long ladder to clean the Hubble telescope.

I know an astronomer, and it's a good question I'm going to ask him next time I see him.

[rwguinn]

I do think it would take a VERY long ladder to clean the Hubble telescope.

I know an astronomer, and it's a good question I'm going to ask him next time I see him.

And its not like space is a dust-free (http://antwrp.gsfc.nasa.gov/apod/ap070813.html) emvironment, either

[YouBelieveWHAT?]

Carefully - very carefully?

[EvilBiker]

I went to visit the SALT scope in Sutherland while it was under construction last year:

http://www.salt.ac.za/telescope/subsystems/mirror/

They have an intricate setup to remove individual mirrors for cleaning and resurfacing (if necessary).

If I recall correctly, they blow the dust off the mirror surfaces using an inert gas like nitrogen...

[Kevin_Lowe]

It requires as many as fifty people to spit on a hankie the size of a bedsheet, which is then rubbed over the surface with a large padded stick scientifically designed to mimic a human finger.

[wollery]

Dry ice snow seems to be the preferred method, and they've actually done research at Subaru which shows that it performs better than inert gas blowing. The ESO website (http://www.ls.eso.org/lasilla/sciops/ntt/telescope/esontt.html) notes of the New Technologies Telescope that, The main mirror is frequently cleaned using carbonic snow (weekly) and water (every six months), maintaining its reflectivity to an optimal value of ~88-90%.

The VLT (http://www.eso.org/org/tec/TechTrans/DevelContract.html) site has this,
Peel-off cleaning technique for telescope mirrors

Following discussions with Bayer AG about ESO's requirements for cleaning telescope mirrors, Bayer developed a trial product which they believed would meet ESO's requirements. ESO tested this product on various surfaces and sent the results back to Bayer. After four additional experimental iterations, a product was finally selected for commercial production.

The resulting peel-off cleaning product, "XL Clean 5", is produced commercially by IRSA Lackfabrik GmbH.

But also,
Laser Cleaning of large optical surface

The techniques of using UV lasers to clean optical surfaces were known but not well adapted to the in situ cleaning of large surfaces, such as telescope mirrors.

An ESO study and development contract allowed Laser Laboratorium Göttingen to develop the technique to be applied to large optical surfaces in non-laboratory environments.
So I'm not sure which technique they actually use.

Additionally the mirrors of optical telescope need to be resurfaced and polished every few years as well!

ETA Magic 9-Ball, being an astronomer doesn't mean that your friend will know this, I'm an observational astronomer and I had no real idea how it was done. :D

[hoemaco]

Actually, at the public observatory I've been working for, most often we refrained from cleaning the lenses/mirrors, because applying a sponge or similar would make the dust scratch the surface, which would be even worse than letting it be a little bit dimmer. Though when idiotic visitors tapped them too much, I had to use the deer leather (is that the chamois someone mentioned? not good at foreign languages) to remove the grease/fat whatever. Really, it was not so much of a problem with a telescope that survived a hundred years. Yeah, those guys in the early 20th in Germany surely did a good job. No plastics, only steel, copper, and a little wood for the knobs. And it's working! Even the gravitational clockwork can be adjusted to be as exact as an electrical one. (I was really shocked when I found out that the newer ('80es maybe) Zeiss telescopes had a plastic casing for the metal screw (how do you call that thing... time for an online technical hun-eng dictionary), which was eroded by the screw, though it did last for quite a few years. nowhere a hundred, of course.

[wollery]

Ah, but can it take a 1 to 1.5 micron spectrum with a S/N of 100 for a star with a magnitude of 15 in the H band?

[H3LL]

Ah, but can it take a 1 to 1.5 micron spectrum with a S/N of 100 for a star with a magnitude of 15 in the H band?

That's funny. I was going to say exactly the same thing. :D :D ;)

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[H3LL]

Thanks for the info...Keep it coming.

CO2 snow, distilled water, optical cleaners and methanol seem to be among the methods.

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[wollery]

The last thing you want to do with a modern telescope mirror is touch it with a cloth once it's finished being polished. The surfaces are very thin, and extremely finely finished in a dust free environment. Any cloth could produce scratches by rubbing dust and dirt into the surface, which would reduce the reflectivity.

[Magic 9-Ball]

Sorry for the delay, but I have an answer!

How do they clean them? They usually don't! They may ocasionally blow dust off with air, but the likely way to "clean them" is to replace the covering, or realuminizing. At great expense every 18 months to 2 years they take the mirrors off, remove the old reflective surface, then apply a new reflective surface, called realuminizing. The telescopes are offline for about a week.

There's an answer directly for an astronomer! Sorry for the delay. I hope someone sees this... ;)

[wollery]

Magic 9-Ball, see my answer above (post #10).

They do get cleaned, on a regular basis. A lot of research has gone into the best ways to do it.

[Magic 9-Ball]

Magic 9-Ball, see my answer above (post #10).

They do get cleaned, on a regular basis. A lot of research has gone into the best ways to do it.


Ah, yes. Here I was finally with an answer to post and I neglected to review the messages. Another Lesson Learned. :blush:

Thanks.

[Michael Redman]

They use an electric charge to repel dust on some small optics. I wonder if that would be practical for a large telescope.