Try acetone if you can make sure it stays away from any plastic surfaces (including plastic elements). Best agent I have ever used.
Alan Chan
Acetone is the best glass cleaner you can find. To clean cemented elements (a doublet for example), it is recommended to dilute the acetone with water 1:1. This recomendation may be true only for the old lenses that used Canada balsam as a cement.
But, in any circumstances, to clean the surface of a group of elements, there is no problem in using pure acetone. It won't reach the cement.
Andre
