[csb (Craig)]

It is not actually the specific type of glass that is at risk when cleaning. I think all glass is hard enough too withstand tissue, hair brushes, etc.

However, the lenses have coatings to reduce light reflection & absorption and also increase light transmission. These coatings are at risk if cleaned with too much force or with wrong material/chemicals.

The coatings themselves are quite hardy and can well tolerate being cleaned with tissue, hair brushes, alcohol, windex, water (filtered/deminerlised will leave less residue). Just use common sense and be gentle (don't need to be obsessively gentle).

Last year I had to take the front glass (meniscus lens) off my 8" sct to clean a smudge on the inside (caused by mildew?). I had never done this before but after some reading online, I went ahead.

First I squirted lots of filtered water over the lense to remove any specs of dust. Then I sprayed the lense with windex and held the tissue in a loose ball whule wiping the lens with just enough pressure on the tissue ball to compress it about 1/4 - 1/2 it's size - not a lot but it was enough and the job was done.

I WAS worried when I started in case I did damage but with the assurance of what I had read online I accomplished the task with the feeling I was doing the right thing.

NOTE:
Common alcohols for cleaning are isopropyl & methanol, usually diluted formulations.
I would never use a tissue without fluid. A dry tissue has the potential to push a spec of sand (silicone) across the lense causing a scratch. A hair brush with puffer used dry is ok because it tends to lift the specs off the glass surface.