Mirror cleaning

Folks - I managed to get some imaging done a few nights back. Before doing so, I have a quick look at the primary mirror of the C14.

It looks fine but there "appeared" to be a fine layer of dust on the surface - bear in mind that it has been outside for the last 3 years or so.

There certainly weren't any streaks, smudges, obvious "concentrations" of dust or big single particles - just what looked like an overall fine layer of "stuff". But that was also in bright daylight...

I've heard that this is not unusual and should not be regarded as a reason for cleaning a primary. But I'm no expert. That's were ye all come in

So:

1. Should I leave well enough alone?
2. Is there anything I can do to more objectively "test" the "cleanliness" of the primary?
3. If I *do* need to clean the primary, any suggestions?

Cheers
Dave

Dave - I'd be inclined to take the cautious approach and leave well alone untill you are convinced that the reward outweighs the risk involved..

There are web pages - and even videos out there - that walk you through the process. I don't have links but I recall seeing a video of a chap who has cleaned his mirror more than once.

As regards an objective test - I haven't a clue - but if you find a reference I'm sure others will benefit from a link to it.

There is a product used by some proffessional sites that you spray on and peel off (you could do a search trying to target proffessional observatory experiences).

Just did a quick search - and found this from the amateur community:
stargazerslounge.com/equipment-discussio...r-first-contact.html

If you are careful it can be done (mirror cleaning generally) with minimal risk.

However, I think you are absolutely right in doing some research before you attempt anything.

Good luck!

Mark C.

One quick tip I can offer Dave is a camel hair brush to gently sweep the mirror. You know the dangers yourself of using compressed air and it doesn't sound like it needs a wash. Then again, Mark's first line above is also good advise.

Seanie.

Its something you should do only IF you really have to Dave.
I have a 12" SCT which I got in 2000, last year I finally had to clean the primary mirror.
What I did was remove the corrector plate very carefully (making note of its rotational orientation) and been very consious of how youre handling it and where it is, one chip on this and its sayonara for all the optics.
You need to use surgical cotton and a bottle of distilled water from a chemists.

Then using wet cotton, very carefully dab any big particles off the mirror, do not rub or its gonna scratch the coating, usually you'd drown a mirror with water before you rub it buts thats v. diffucult in a SCT unless you're gonna take the mirror out of the OTA. Personally I wouldn't.
Once youre happy that any visible grit is off the mirror, put some of the water into another container and then put in literally one drop of washing up liquid and give it a good stir, any more washing up liquid will leave streaks on the mirror. Use this water and cotton to give the mirror a clean, rub from the centre to the edge, dont rub in circles otherwise any scratches will be long ones. Also dont be skimpy on the cotton, in theory once you've rubbed the mirror with a piece of cotton, you shouldnt use it again, that all depends on the cleanliness of the mirror.
Then using dry cotton, dry off the mirror.
Thats it, its fairly easy, you just need to be carefull.

I've had to clean a couple of mirrors in the past year (10 years of dust, some web like fungus, and a spiders web!). Heres my experiences, follow at your own risk!

I was able to take the mirrors out of the OTAs which was a big help. I'm not sure this would be a good process for SCTs.

I learned that touching a mirror with anything dry will scratch it. Surgical cotton (even wet), camelhair, expensive photo brush... you are taking a risk on a dry mirror. I suspect its the dust underneath rather than that brush materials. The scratches may not be obvious in fact you might need to catch a light in just the right way to even get a hint of them (shine a laser pen on the surface and watch the reflected beam on a wall at the other side of a room, that'll pick up scratches you wont see with your eye). luckily I found that out on the old mirror.

Cleaning solutions are a bad idea (were talking drops of stuff per liter of distilled water here). Again learned this on a small old mirror. Unless you know exactly what the coatings on the surface are and have researched very well what is safe to use, don't. I ended up with a nice foggy mirror. Acetone got rid of that, but probably took the coating with it. And whatever about going into a chemist and asking for distilled water and surgical cotton wool, you get some strange looks going in the next day looking for half a liter of nail polish remover!

So for the mirror I care about heres what i did. Take it out of the ota and put it in a big sink. Turn the cold tap on, fairly gently at first, and let the water flow over the mirror and this will remove most of the big particles. Depending on the dust you might be happy enough to stop here without even touching the mirror.

Next get some cotton balls. Get the best surgical stuff they have in the chemist, it will be purer than the cosmetic kind. With the tap still running gently, and I mean gently drag the wet cotton wool over a small part of the surface. (I can hear gasps from you all!). This isn't as destructive as it sounds. the flowing water immediately carries away any particles and since it is wet and you are not applying pressure you don't damage the mirror. as Dave said don't skimp on the cotton. In one place a bit of dirt was stuck hard to the mirror, may have been fungus actually, I found was was able to lightly scrub the area with the cotton wool under running water and could not detect any scratches later.

To finish you need to rinse the mirror using distilled water. Impurities in tap water will leave streaks when it dries. Pour a couple of liters on to rinse. You can then take the mirror out of the sink and leave it to dry. I wouldn't use anything to dry it, cotton or a hairdryer could easily cause scratches now, just leave it be for the remaining water droplets to evaporate off. NB put the mirror somewhere that it wont end up with things falling on it!

Thanks Al/Dave/Mark - all good advice.

I've now put away my brillo pads and lighter fluid - not one of my better ideas...

I think I'll hang fire for the moment. Perhaps its not as big a deal as I thought.

I have, on occasion, cleaned the corrector plate (with some nifty and expensive AstroPhysics cleaning solutions that Jeff kindly bought for me plus their supplied lint-free paper-like squares) and know of the trauma involved.

If the performance of the OTA is poor this season, I'll have a go or consider sending it to someone with more nerve than me.

Thanks again folks - appreciate it.

Dave