Windex (the ordinary glass-cleaning kind) is perfectly safe. It's even safe
to use on precision coated optics if you don't do it every day. Unless
there's a delicate anti-reflective coating, you have ABSOLUTELY nothing to
worry about. If there's an anti-reflective coating, you can still use
Windex as long as you don't do it too often. If you need to clean a
delicately coated piece of glass every day or two, half Windex and half
distilled water is a good cleaner.

Clear skies,

Michael A. Covington
Author, Astrophotography for the Amateur
www.covingtoninnovations.com/astromenu.html