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Epic Mirror clean

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2-3 years ago I acquired a solid-tube SW 200p f/5 newt. It seemed in good condition and I have enjoyed using it. However it’s not been without some problems. The geometry was all wrong: the position and the size of the secondary mirror means that the secondary is significantly undersized. IE that the converging light-cone is bigger than the outside edges of the secondary. It means that the scope is acting not as a 200mm scope, but rather as a 178mm (by pushing the primary back as far as possible, and the spider and focuser as far "up" as possible without surgery, I've eked out another 5mm of effective aperture). Its also very "non-stiff", requiring recollimation of the secondary tilt for any big change in altitude. Anyway, the purpose of this account is not to address those, but rather to talk about how dirty it's been, and how I've cleaned it.

When it arrived, it had rather a dirty primary mirror, well beyond my own criteria for requiring a clean. Lots of dust of varying sizes, but also a layer of something very fine, causing a marked dullness. So I cleaned it according to my method then, which was simply soaking it for a while and slooshing it in detergent and warm water, and using many cotton-buds to gently wipe the surface.

What emerged was undoubtedly better, but that fine layer was mainly still there. You could see some evidence of wiping on it, but it was barely affected by my clean. I ignored it and have used the scope many times since. It’s given me much pleasure.

However, a year or more on, and I’ve decided to give it another go. It’s acquired enough other dust and crud to need a routine clean anyway. In the pictures below, you can see the not-removed fine layer with its wipe-marks from the old clean under the dust.

I now clean my mirrors differently. I soak and sloosh in warm detergent-water, as before, and vigorously run warm water over the mirror to lift and rinse off actual dust particles. But I have made up some home-brew ROR (Residual Oil Remover), and my next step is now the “fingertip method”. After doing this to my mirror yesterday, using this method with my ROR, I thought “nothing will survive this combination of solvents”. Final rinse is first with softened water from the tap, then a pour of de-ionized water, and finally a sloosh with laboratory-grade purified water.

Well the mirror emerged apparently very clean, EXCEPT FOR THE FINE LAYER. It had made not a bit of difference to that, it was still there! I had as a “target” the bits of the mirror which had spent their lives under the edge-clips: they were pristine bright and shiny, and by comparison the rest of the mirror was not!

I started over again, this time using a little more fingertip-pressure. Still no difference.

I had one weapon-of-last-resort in my armoury: pure Acetone. This time I used a “wad”, a rolled-up piece of kitchen-cloth dabbed in the Acetone. I tested it first on a tiny section of the mirror just next to one of the mirror-clip “clean bits”. It worked! So using @markse68 ‘s method of successively dabbing, wiping, and snipping the rolled-wad (snipping to remove the contaminated end), I wiped the whole mirror in pure Acetone. The snipped-off bits when discarded were rather dirty! Remember this was AFTER doing a “normal clean” 2-3 times! Then more final-final rinses in my succession of water-types.

I was very happy with the end result, and the pictures below should show the difference. To be sure, you can still see evidence of the contamination remaining, but the vast majority has gone, and the pictures do I think show a marked difference in contrast. When taking them, I tried to use identical exposure settings and I processed them out of the camera using the same “load settings” for each pair of pics.

As for what the contamination was (is), I have no idea. My theory is that some previous owner was a smoker.

Anyway, I look forward to trying it out, perhaps even tonight.

Cheers, Magnus


Before and after, from the side. Note the streaks on the "before" and the small amount remaining after cleaning (in a radial pattern). Not also the difference in contrast of the window-frame behind (the bright white spots on the clean pictures are spots of water, lab-grade-purified, as-yet undried):



From directly above:



And my control, giving me an idea of what I was aiming for:


Edited by Captain Magenta
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Nice one Magnus. I too subscribe to the fingertip method! I let the mirror soak for quite a while, and my fingers too so they are nice and soft. It seems to work better than anything else with stubborn dirt.

Are you going to market your new solution?

Magnus’ Magnificent Fluid perhaps?

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4 hours ago, Stu said:


Are you going to market your new solution?

Magnus’ Magnificent Fluid perhaps?

Haha I’d have to add something, all I’ve done so far is find ROR’s regulatory filing and more or less copied that recipe. Perhaps an extra drop or two of “Eau de Magnus” to make it uniquely mine. Yuck.

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