How to clean a reflector's mirror

A little while back we received a 10" Dobsonian reflector that had been stored for several years. The mirror was very dirty and the owner was a bit intimidated by having to remove, clean, replace and recollimate it, so he got us to do it.

Reflector telescopes make up probably half of the scopes we sell. Refractors and Schmidt Cassegrains make up the rest, of course. Reflectors, especially Dobsonians, with their large apertures, give their owners a good bright view of the night sky. But yes, sometimes you have to clean the mirror.

When do you need to do it?

New owners of reflector telescopes always ask how often they need to clean the mirror. It's a reasonable question. I recommend cleaning mirrors as rarely as possible, and not until the mirror seriously needs cleaning.

Mirrors, especially large ones, are surprisingly forgiving. As long as they're collimated (that is, adjusted to they're perpendicular to the telescope's axis), they will continue to give a good image even when they're quite dirty.

Eventually, however, cleaning will be necessary. Looking carefully through a scope with a dirty mirror at a star cluster will reveal a light haze over the image - it looks a little like nebulosity that isn't there. At this stage the mirror will also look horribly dirty to the eye.

Because it's way down the back of the tube, the mirror has to be removed so you can get at it. It lives in a holder called a cell, and this has to be removed as well before before cleaning. It's a bit of an operation, but you don't want the cell getting rusty.

Have a look at the photo. This particular mirror was liberally sprinkled with dust and other grit. Apart from that, though, it wasn't in bad condition, the coating looked even and there was no corrosion of the aluminium.

The cleaning procedure

I prepared the area I was going to need, the drying side of a sink. Next, I carefully removed the mirror from the cell, making sure I noted which screws came from which clips.

Then I propped the mirror up so water would flow off it.

Before I wet anything, I used a blower brush to remove any loose grit. Then I tested a tiny part of the edge of the mirror to see if the silvering would tolerate the chemicals I was about to use.

Next, I used lots of cotton wool balls to wash the mirror with a few drops of dish washing liquid in a bowl of water. I used lots of water and gently used cotton wool balls one swath at a time so I didn't take any grit back to the mirror. I washed the whole mirror twice.

Next, I repeated the entire process with undiluted iso-propyl alcohol. This removed any remaining water and then evaporated without my having to touch the mirror.

Last, I dabbed individual drops of liquid of the mirror with the corner of a tissue. I didn't try to get the very last speck off the mirror. This is likely to do more damage than it saved, and in any case the mirror will soon start to get a little dirty anyway.

The final result

Here is the result. You can probably see a few specks left on the mirror - they're fine. Remember any effort I make to remove them risks inadvertently scratching the mirror. All I needed to do now is replace the mirror in the tube and collimate it. The telescope was ready for use again.