Dusty primary mirror?

[Manok101]

I was wondering what to do about a dusty mirror? I've not used my scope in nearly a year, perhaps more, and the mirror is dusty. What do I do? (I think it's dusty anyway, there are what look like spots on it as well.

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[ronin]

If it is bad and it does sound it then I would use cotten wool pads and something like IPA (Iso Propyl Alcohol).

Wipe gently and you will need a reasonable amount of IPA.

Many will suggest wash in warm soapy water and rinse in distilled water, the reason I have gone the IPA route is that an old mirror may have poor coatings and water and aluminium don't make for a good result. Basically it oxidises. So I say IPA to get it clean and then you can examine the state of the mirror and coatings.

Do it all in a ventilated place and get the cotten wool wet and make sure the sections of mirror are wetted to loosen the crud before you wipe it off.

You could get a soft artists paint brush and attempt to brush off the loose stuff first - you may find it unnecessary to wipe it clean after this if most of whatever is simply lying on the surface and is not attached.

That is my suggestion, others may follow.

Avoid dish washers and car pressure washers. :grin:

Warthog once suggested wire wool and gasoline - I think he was joking.  :eek:

[AstroAdam]

It's actually amazing just how dusty a primary can get before it affects image quality to any noticeable degree.  Even the small spots are unlikely to do anything.  

If you're determined to clean, I used water with a tiny amount of detergent, with cotton wool - wiping just once with each piece, and disposing of it after.  I just dragged the cotton wool across the surface under it's own weight.  You should never 'rub'.  I then rinsed with deionised water and left to dry naturally.  Result?  Great clean mirror with no streaks or drying marks.

I used this video as my overall guide...

Again though, you should think twice before cleaning, unless it's REALLY bad!  

[Uranium235]

I cleaned my secondary recently, and though it might be more difficult to do with a primary - I found a really good way of drying it off is to give it a good shake, the water flies straight off - leaving a perfectly clean surface (as de-ionised water still contains contaminants). Needless to say, you have to be careful when doing it

[Uplooker]

Best to use RO (reverse osmosis) water to rinse off. You can get 5 litres or so from any aquatic / fish shop.

Ian

[AstroAdam]

The RO stuff is fine.  However, I've also used deionized water with no ill effects.   I tend to use the corner of a piece of kitchen paper to remove the last few drops when you rinse.  Giving a primary mirror a good shake probably isn't the easiest thing to do  .  Fine for a secondary though. 

[kirkster501]

Don't clean it unless it is truly filthy.  There are alway risks when cleaning a mirror no matter how careful you are.

My 2 penneth

[Uplooker]

The RO stuff is fine. However, I've also used deionized water with no ill effects. I tend to use the corner of a piece of kitchen paper to remove the last few drops when you rinse. Giving a primary mirror a good shake probably isn't the easiest thing to do . Fine for a secondary though.

I use my bulb blower to expel the last few droplets, even after using RO water ;-)

Ian

[acey]

The spots are from dried-out dew, and the dust is... well, dust. My primary also has tiny bits of stuck-on grass, dead flies and goodness knows what else - none of which makes a noticeable difference at the eyepiece. I'll get round to cleaning it eventually - what puts me off is the effort of lugging out the primary cell.

In the past I've used cotton wool and isopropyl alcohol. Next time I may do it the same way I clean my spectacles every day: tap water, washing-up liquid, and my fingers to wipe the muck off the surface. As long as the coating is old enough it will be hard enough to withstand cleaning. Years ago I ill-advisedly cleaned an 8" mirror that was only about six months old, and left a scratch right across its surface. Made no difference to the views but caused a lot of expletives.

Dewspots can only be avoided if you can avoid dew, but no matter what I do, I nearly always find my primary dewed next morning when I inspect it after a session, and once dry it leaves those little spots. So even if you clean the primary, expect it to look spotty again very soon.

[kirkster501]

I think obsessive, compulsive tendencies can surface when it comes to the cleanliness of telescopes, a compulsion to want the stuff dust free and immaculate. I keep a very clean house and I am always srubbing, cleaning and tidying.  It's a big mistake to extend this habit to telescope mirrors, lenses and corrector plates!