Washing the mirror of a 200mm f5 Helios Newtonian

I bought a demo Helios Dobsonian mounted Newtonian telescope (Diam. = 200mm, F/L = 1000mm, Aperture = f5) in Spring 2004. Almost immediately I realised that the mirror was rather more dusty than I'd expect for a demo 'scope which the vendor claimed had never seen the stars. It worked well enough, and initially I decided to leave well alone.

Click on any photo to see a larger version   

04668 Before cleaning.JPG

04672 In the wash.JPG

04673 After the wash.JPG

04674 After cleaning and drying.JPG

However after I found a spider making its web deep down the tube, I decided action was necessary and that I'd follow the advice on the internet and get involved in gentle laundry.

Removing the mirror assembly was straightforward, just 6 short screws, and the mirror end of the 'scope was removed. (Before disassembly, I marked the mirror mount and tube but this proved to be not necessary.) A small amount of the dust was easily blown off the mirror, but tests on that near the edge showed that it was stuck to the coating, so I filled the sink with warm water with a few drops of detergent, and left the complete mirror assembly in the solution for 30 minutes. (Warm water to help with softening of the deposits and to aid later drying.) After the soak I swished the mirror around and checked for removal of deposits. Slight improvement, but too much was still adhering to the mirror. So I gently swabbed the surface with a wet tissue (as far as I know, not made from recycled paper...), noting that the deposits had now been removed, and then rinsed in plenty of tap water, followed by de-ionised water and a little photographic rinse-aid.

Image 04673, showing the mirror surface after initial rinses, shows a multitude of small fibres from a towel (hanging above the sink and which I'd neglected to remove before the wash) - these were easily blown off the mirror. The final image attempts to show the difference from the first image before I started the cleaning. (Despite the dust showing on the photo it really is much cleaner than when I started.)

Re-assembling the telescope was easy, as the screws fitted their holes accurately, and there was only one way in which to re-assemble due to the seamed nature of the tube which fitted into a mating cut-out in the mirror mount.

Is there any improvement? I think so, but then I would say that wouldn't I ? ...
My main worry was that I might have upset the collimation, but checks on this on the first clear night since the wash have been very encouraging. On focused and defocused stars, it's certainly no worse than before, and pointing towards the moon I was definitely suffering temporary blindness. I'll now have to buy a moon filter, which I certainly hadn't felt necessary before. Looking at the sky with both 'scopes during the same evening, it was now very obvious that the 200mm Dobsonian was appreciably brighter than the ETX-105.

Will I now try washing my ETX-105 mirror? I don't think so.

Malcolm Stewart
Milton Keynes, UK
May 30th. 2004