When is a Mirror dirty enough to need cleaning?

[glend (Glen)]

Looking down the tube at my primary mirror (with a torch) I can see dust on the surface of the mirror, not much but its there and without the torch its very hard to see at all. Also there seems to be a light haze on the surface when the torch is shined across it at an angle, and I seem to recall in my reading that others had reported this as well (one attributing it to cooking). The scope (12" Dob) is not kept in the house so cooking is out, and I was leaning towards plastic outgassing (like the haze that builds up on the inside of car windscreens). No smokers at my place so it's not that sort of haze. There plastic tube cap and the shower cap that I keep on the back end when its not being used might be the source of the haze, or maybe the EVA light shield that I made from a camper mat.

The scope is only three months old but it gets used a fair bit (like two to three nights per week weather permitting). I always have the fan running when I am using it to keep the dew away (needed this winter).

BTW my primary mirror in my Celestron 130 is still pristine, but the Dob gets used more these days.

Is this something to be worried about? How dirty is dirty? I read about people saying never clean it, others say clean it once a year, etc. What's the consensus?

[astro_nutt]

Hi Glen.
The question of "when is the primary mirror dirty enough" is a fickle one.
Shining a torch down the tube will show up every speck of dust, but the amount spread over the entire surface wouldn't cover the head of a pin. It looks bad, but that's all. Only when viewing through the eyepiece, you notice a degraded image, (stars not sharp dots, lack of structure in star clusters, the Moon lacking sharpness when looking along the shadows at low magnification, etc. And you know that collimation and seeing conditions are good.)
If the images are still good, then think about cleaning the primary mirror in 12 months time, unless something is spilt on it. There is a good article on mirror cleaning in the "projects and articles" section. I hope this helps.
Cheers!

[Steffen]

Originally Posted by glend

I read about people saying never clean it, others say clean it once a year, etc. What's the consensus?

Well, apparently there isn't one

I suppose you need to clean it once to be happy, to get it out of your system so to speak. Take it out, wash it, rinse it, let it dry. They're all coated these days and don't scratch easily, but be gentle, take your time, think twice before every move. If the mirror isn't centre spotted then fix that as well.

Taking the cell out of the scope and the mirror out of the cell shows you how it all hangs together and gives you more confidence with collimation etc. Putting it back together also lets you make sure the clips (if any) aren't too tight.

Once done you'll find that it was fun but didn't make any difference. You won't clean it again for years to come

Cheers
Steffen.

[GeoffW1 (Geoff)]

Hi,

I'd second that advice to wait on. A mirror has to be quite dirty looking in torchlight before it needs any cleaning from the image standpoint. You would only need to clean it now if you were looking for that last 2% in image quality.

Cheers

[UniPol]

Mice droppings and spider webs are a good indicator

[doppler (Rick)]

Here is a mirror I cleaned for the local high school. It still showed a good image but it was always stored upright, best to park a newtonian horizontal. The diagonal was good because it was upside down in the tube.

[AstroJunk (Jonathan)]

Once upon a time, mirrors had a bare naked film of silver of aluminium on it. It was a very hard thing to clean without damaging the surface and the best advice was leave well alone.

Nowadays, like steffen says, most mirrors have a silicon over coating which achieves two things. Firstly it stops the reflective coating from tarnishing, hurrah, and secondly it provides a robust outer coat to protect the mirror, double hurrah!

Still, always be careful cleaning your mirror, but do it whenever you like. Use very light pressure with soapy water and cotton wool then rinse off with distilled water. Don't scrub, but don't be too afraid!

[knightrider]

Originally Posted by doppler

Here is a mirror I cleaned for the local high school. It still showed a good image but it was always stored upright, best to park a newtonian horizontal. The diagonal was good because it was upside down in the tube.

That mirror looks like it has a lifetime of chalk dust on it and it still show'd a good image? Wow.

It looks much better cleaned.

[Satchmo]

If your mirror is overcoated you can wash it as often as you like.

Sometimes if I am observing only every few months I wash my mirror before each session - the mirror always seems to attract a hazy eucalyptus oil film as I live on the Kuringai National Park. Like having a clean pair of boxers every day , I love to observe with a freshly washed mirror

[glend (Glen)]

I hadn't thought of the bush as a source of the haze. I have a masive bush reserve to the rear of my property. Thanks for that input. I think I will be washing it before I got to Bretti in early September.

[Satchmo]

If you breathe on your mirror and you get a blotchy pattern, with dark spheres of all various sizes, it has oil deposit on it which is very tenacius..I often used methylated spirits first after a rinse to help cut the greasy film . My scope is stored in a shed and doesn't have access to the primary for a close fitting cover .

When the breathe pattern is smooth then the surface is oil free - this is how coaters test for a clean surface putting in the chamber .