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Mirrors, clean or not to clean?

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The mirror has to be really dirty before you detect much difference in performance.

You can use clean and dry compressed air/gas in aerosol cans, I use these to from time to time to blow away the spiders and other bugs that seem to be intent on building a community inside my scopes, but I wouldn't recommend using an air gun as the compressors have a nasty habit of leaking condensed water and compressor oil into the mix.

Clean dust or dirt on a mirror is easy to deal with when eventually necessary but oil spatter is a whole different adventure!


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Not sure I'd use the compressed air in a can. The propellant has a nasty habit of spraying out if you venture a few degrees off vertical.

As has been said, don't go near it with an air line.

I would stick with using a bulb blower.


TBH I would probably leave it well alone unless the mirror was absolutely filthy. You'll be surprised quite how thick with muck a newt mirror can be without the slightest drop in performance.

My 10" is about 4 years since it was clean . It doesn't need doing yet. Sure it's dusty, but compared with some observing buddies brand new 10"there's no drop in performance yet.

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To look at it another way. At least this is from calculations using magnitudes, If a mirror has about 90% reflectivity or 95%, quite a difference, this results in only about a 0.1 change in limiting magnitude, Not a lot. To put that in perspective, a 5 degree variation, in terms of zenith angle and if you are pushing observing in the sort of 20 - 30 degree range above the horizon would significantly outweigh the sort of effects we are talking about.

Now I am not suggesting you use the mirror as a litter tray :D but these sort of calculations give a rough feel for it. I am not claiming my figures are 100% accurate either, but it would be in the sort of ballpark. :)

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  • 1 month later...

Google Kielder scope mirror and look at the image there of a mirror that was/is still in use without any issues. I doubt very much if your mirror needs a clean.


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against general opinion I spring clean mine after each winter, in warm mild washing up liquid solution (cheap one with no perfumes) then rinse with distilled water and dry with blow dryer on cold. Never had an issue and if done carefully doesn't shorten coating life.

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When I finally got around to joining my local club in 2002, I was shocked at the state of some of the members mirrors, and realised that I was OTT in my passion for no dust.

It's not easy to decide when its really needed, it's often said not to check a mirror for dust by shining a bright light down the tube, makes a perfectly acceptable mirror look awful.

Have a look (not for too long :smiley: ) in daylight. If you are not sure it needs a wash, then it probably doesn't.......

Regards, Ed.

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