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Author Topic: Cleaning telescope mirrors  (Read 345 times)

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Offline mechanoid

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Cleaning telescope mirrors
« on: 19:07:58, 05 March, 2017 »
Those of you who read New Scientist will have seen the Aperture feature this week.  It shows two guys in a moveable cradle cleaning off the mirror(s) of the Hobby-Eberly telescope in Texas (one of the world's largest) using a blast of carbon dioxide.  Apparently they do this every two days so as to keep it spotless.

And here was me labouring with soapy water and a scrubbing brush when all I needed was a quick blast from a fire-extinguisher !

Seriously, its a thought. I clean my 200mm Newtonian mirrors about once a year, not a job I look forward to. Knowing how well grime sticks to surfaces, I would have thought it would need a massive blast to dislodge it. I article says that they actually use "solid" CO2 which obviously does the trick.

Dave
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Offline PaulB

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Re: Cleaning telescope mirrors
« Reply #1 on: 20:33:21, 06 March, 2017 »
Personally. I wouldn't touch an amateur telescope mirror.
We all know that it takes a lot of grime to cause any significant light loss.

I have not heard of CO2 being used before. But- I guess they know what they are doing. But every two days.
That's a bit OCD.
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Offline ChrisLX200

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Re: Cleaning telescope mirrors
« Reply #2 on: 23:24:45, 06 March, 2017 »
I don't think it's a process that scales [down] well - the temperature of a small primary of ~12" or so when hit with CO2 @ -40degC is going to drop very fast - the thermal stress would be huge. That doesn't happen with a pro observatory mirror having a large mass, only a small area is treated at any one time and the thickness of the substrate prevents a large temperature drop. In other words - don't try this at home :)

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Offline mechanoid

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Re: Cleaning telescope mirrors
« Reply #3 on: 12:05:39, 08 March, 2017 »
I doubt that thermal shock would be a problem. After all, you can pour boiling water into a Pyrex bowl, the same material as amateur mirrors are made from.  Professional mirrors are probably Zerodur.

Regarding the cleaning of mirrors, in my opinion its not so much light loss as scattering that is the problem. If you can see the mirror surface when a torch is shone on it, its dirty ! Cleaning is quite straightforward with due care, although I had to resort to the internet to obtain distilled water, as nobody sells it these days, not even chemists.  But probably easier to get than solid CO2.

Dave
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Offline petevasey

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Re: Cleaning telescope mirrors
« Reply #4 on: 12:45:39, 09 March, 2017 »
Distilled water?  For those who have a dehumidifier, the resultant condensate is almost pure distilled water.  Certainly good enough for cleaning mirrors.  Just be sure to keep the catchment container clean.

Cheers,

Peter
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Offline Celestron8g8

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Re: Cleaning telescope mirrors
« Reply #5 on: 22:05:07, 18 May, 2017 »
I've been to McDonalds Observatory and I have seen the Hobby Eberly telescope in question . What you may know that might not have been mentioned in your read is that the mirror is not a single big mirror . It's made of 91 Hexagon shaped mirrors that are put together like a puzzle . This type of mirror build allows them to take individual mirrors and sections of the mirror out for cleaning without taking most of the mirror at one time making the scope be down for cleaning for a period of time . This way they can still use the telescope without downtime . Each mirror is I believe approx.  2" thick . These mirrors are not exactly like the mirrors in your small scope but they work the same was as your scopes mirrors . I have to admit this is one huge MAMA of a scope ! 3rd largest in the world ! I think if you tried to clean your mirror the same way you chance breaking it .

https://mcdonaldobservatory.org/research/telescopes/HET
-Ron

Can't learn if you don't ask , First step is the starting point !!

 

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