There has been lots of info on several astronomy forums on how to clean a mirror but so far I haven't seen any info on WHEN to lean a mirror. Could someone take some photos of a mirror in various stages of filth and post them? Then the forum members can comment on which photo depicts when the mirror should be cleaned. Mr Q

Never look at a mirror in the daylight, it will look dusty even if you just cleaned it.

I wait until it is really really dirty. I have had a 10 inch dob for more than a decade, and I have cleaned the mirror once.

__________________
"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender

I clean my primary about every 2 years, which may in fact be more often than necessary. Although, the last time I cleaned it, there was a film of some sort on the mirror (which cleaned right up) as opposed to the previous time I cleaned it, where there was just a layer of dust on the mirror. It takes quite a bit of dust to actually start to have an effect on the mirror.

Perhaps you could post photos of your mirror, how dirty is it?

Yeah, according to the manuals Orion has for their scopes, you're only supposed to clean the mirror once a year, maximum. Otherwise, there's too much risk of repetitive cleaning damaging the coatings. If the performance of the scope has not degraded, and the mirror itself looks fine upon a close inspection, don't clean it.

Once a year! I can't imagine one needing it that often. Those at Hyde Memorial Observatory are cleaned about every 12 to 15 years. I doubt I've ever cleaned one of mine more often than once every 10 years or so. I store the scope mirror end up with shower caps over the ends and a film can in the focuser. Secondary may need it every 5 years or so as dust falls down on it.

My general rule is that if you can't be certain it needs cleaning it doesn't need cleaning. Even after more than 10 years I rarely see any change in the view. Maybe a slight contrast increase but you have to look for it.

Rick

Hey - I had a post here last night which has now disappeared since the server change! I doubt the mods deleted it (no reason to) so it must be a casuality of the switch-over.

-Veeger

Veeger - Can you repost your reply? Thanks, Mr Q

O.K. I do clean my mirror very seldom (only several times in the past 25 years) but since I moved from a humid climate to this very dry and dusty climate, I'm sure the mirror will collect much more dust than before. This was the basis of my question and also to educate beginners out there on how often to do it. But keep on with the replies! The more opinions I get, the better the final answer will be, I'm sure. Mr Q

My previous comment - paraphrased:
I agree that cleaning the primary mirror should only be done when it is obvious the quality of the image is being affected. In most reflectors, the mirror must be removed to be properly cleaned and this presents all sorts of risks of physical damage, and improper alignment when reinstalled as well as the obvious degradation of the surface from wiping it with tissues or cloth. Probably the safest way to clean a mirror is a spritz of clean, dry air, but there is no easy way to do this to an objective that is at the far end of an optical tube, unless someone has developed new cleaning methods of which I am not aware. I cleaned my mirror, probably twice in about 10 years and mainly because I was not conscientious about storing it properly. My currect telepscope is covered when stored and I have never touched the optics (it is about 8 years old).

-Veeger

You should find old threads here about how to properly clean a mirror, and I remember posting about the danger with compressed air (getting the propellant on your mirror, you need to be careful).

There are books and articles that describe good techniques, so I'd suggest looking for that.

__________________
"I'm as accurate as any psychic. And I'm a cartoon!" -- Squidward

"Arrrgh, the laws of physics be a harsh mistress!" -- Bender

Yes - thanks for reminding everyone of that. Most compressed air is not clean as it either had some kind of propellent, oil or other contaminant. I was thinking along the lines of a squeeze bulb used dust off lenses.

-Veeger

I use filtered compressed air at about 5 pounds per square inch. Works well. Once the big crap is blown away I up the valve to a 10 lb and that gets the finer stuff. There's nothing wrong with compressed air. The problem is with canned pressurized liquid gasses you buy commercially. That's not compressed air and in fact will kill you by replacing the real air in your lungs if you tried to breathe it. Get that liquid gas (it's not air but a man made chemical) on your mirror and its coating is ruined in that area.

Filtered compressed air, however, is perfectly safe to use if the pressure is controlled so you don't sand blast the surface with the dust you are trying to remove. Often a quick air treatment is all I need to remove dust mots from mirrors or filter surfaces in my camera and filter wheel. I've used it for years with excellent results and no harm to an optical surface.

I use an oil free compressor. I'm not sure if the others that use oil are safe or not.

Rick

look at this for a "dirty" and clean mirror and apparently one of the better methods of cleaning...

http://www.ghg.net/cshaw/clean.htm

W

Never heard of cleaning a mirror this way but if it works. Guess I'm over-rating the dirt amount when thinking my mirror needs a cleaning. Thanks for the photos (although I was hoping for a mirror in various stages of dirt accumulation). Maybe someone out there can provide them? If so, Mr Q

Do you have pics of your mirror? Perhaps we can advise you as to whether or not your mirror needs a cleaning.

No, I don't. I know a noticeable amount of dust on the mirror will not degrade the images but I'm concerned with the idea that this dust can absorb moisture with who knows what pollutants that could etch into the mirror surface. A set of pictures with varying amounts of dirt on a mirror would help immensely in determining when to clean it, as a general guide. Saying a mirror is "dirty" can be very subjective as to the actual amount of dirt present - hence the several pictures to clarify this dilemma. A Mr Q