Newtonian Mirror Cleaning

[§AB]

Ok I have decided to take the plunge and clean a newtonian primary mirror for the first time ever. Over the months, some crud has gradually built up so I thought it was time. I've read other stories of people cleaning theirs and seeing a marked improvement in image quality, so that's my major incentive to do it. I also agree with Mike, If you can see a film of dust/crud, there's no way it wont affect image quality.

I plan to follow Mike's (Iceman) routine....

1. Rinse it with luke warm water first
2. Let it soak for 15-30 mins in luke warm water with a drop of detergent
3. Rinse it with warm water
4. Rinse it with distilled water.

I don't know if I should drag the cotton buds across the surface, the mere thought of doing that and leaving scratches and streaks has me turning pale. I think just for the sake of my mental stability I'll leave that part out.

Just a couple of questions: does it matter if the detergent is like "lemon flavour" or anything like that, and is it okay to clean the sink before hand with bleach and then wash the sink thorougly or is the possibility of residual bleach a risk to the mirror's coating?

[Starkler]

I have had discussions with Mark Suchting and others which indicate that the paranoia associated with mirror cleaning is unfounded if your mirror is overcoated, as just about all modern mirrors are.

The overcoat has the hardness of glass and wont be damaged by the gentlest of wiping with soft cotton underwater, given there is no grit there being ground in. In any case you might fing a good rinse is good enough, with the small difference gained from being wiped being lost after one or two uses anyway.

[Night Owl]

http://www.gemini.edu/index.php?option=content&task=view&id=60

I'm going to experiment on something first.

[ngcles]

Hi §AB,

Yep, I find mirror cleaning scary too -- even after all these years. I really only do it when I have to. But ...

Little harm will be done with a wad of clean cotton wool if it is dragged across the mirror only with its own weight (after a really good rinse). Work from centre in straight lines toward the edge. If there is a spot on the mirror that refuses to budge after a good rinse and a very gentle wipe with the cotton -- leave it.

Look for a deteregent that is free of additives and is therefore pretty much pure soap -- probably like the horse soap. The additives are usually there to soften hands etc and are more likely to leave a residue on the mirror. As a rule of thumb, cheaper detergents are better. If the mirror is overcoated there is very little likelyhood of you doing damage to the surface if you are reasonably careful.

As a last step, consider dousing the miror in isopropyl alcohol because it drives off the remaining water, dries quickly and does not leave dust-rings on the surface (as water almost certainly will).

Best,

Les D
Contributing Editor
AS&T

[coldspace]

In regards to cheap detergent having less additives in it, this is true. Use 1-2 drops of trix brand as this has the less additives in it.

Matt

[§AB]

Well I've done the cleaning, and the vast majority of the dust and crud actually came off. I used a couple of drops of cheap home brand detergent.

But there's one thing: the mirror appears to be covered in a hazy substance with a bluish/smokey appearance. You can see it easily when you shine a torch onto it. The mirror is probably around 4-5 years old btw. Don't know if that was there before (all the dust and particles probably hid it!) and will it effect my images? Or is a re-coat on order?

[skwinty]

Hi All
Attended a lecture by Dr David Buckley the scientist in charge of the SALT telescope in sutherland South Africa.He is a New Zealander who studied at the university at Siding Springs in Oz.
He was explaining how the mirrors on SALT hadnt been cleaned since they were manufactured. The dust was so bad after 2 years you couldnt see your own reflection. The telescope still operated albeit with slightly degraded efficiency. The new mirror cleaning system is almost complete and it is basically a robotic arm that sprays highly filtered and 99.99% CO2 over the mirror. The pressure and flow rate is quite high and the CO2 forms snowflakes. The arm starts at the top and works its way down and the arm obviously does not touch the mirrors. Apparently this is very effective and the tempereture does not affect the mirror. I suppose you could use a CO2 fire extinguisher provided you used an efficient filter to prevent any particulates getting to the mirror.
Just a thought.
Regards
Steve

[§AB]

Oh btw, this haze is easily visible when I tilt the mirror toward a light when i had it out of the cell.