Why can't you shine a light on a telescope mirror?

[Aussie Observer]

After reading a few posts about marks on mirrors, I saw the comment "Never shine a light on the mirror!" I have never heard this before. Can someone clarify it please?

[John]

You can do it, but you may be horrified by what you see as it shows every tiny dist mote and debris fragment on your mirror

[Parus major]

Yeah, if it's the thread I think you're on about then that's what the person was saying. The opening poster wanted to clean the mirror and he was told not to shine a torch at it, not because it was bad or damaging but because it just looks shocking.

Mine is in that catagory. it looks a smidge dusty but put a torch on it and you wonder why I see anything with the amount of pollen grains on there.

[Aussie Observer]

Thanks for that. I got really worried after reading the post because I have been doing that for years (shining a torch on it that is). My mirror is quite disgusting and in need of a clean and I will get around to it one day. I used to clean it quite regularly- once a year , after taking it away to the beach. Salt air and aluminised mirrors just don't mix!

[Priastro]

Have wondered about it. The mirror seems clean and fine and with light it seems dirty. Even after cleaning it seem not clean with light with som remnants after destilled water!, but perfect without light.

So maybe it will seem worse than it actually is and is nothing to impact to the optical performance.

Martin

[Mr Spock]

A bit of dust won't affect the performance of the scope. Cleaning a mirror too often can damage the coating, so unless it's really filthy I'd leave well alone.