[Aavso-photometry] crud on filters

[Michael Koppelman]

I think someone else had posted something similar at one point...

My photometry went to hell in the V-band. Errors went way up, the  
stars looked fuzzy. It was clear something was up. I took the CCD and  
filter wheel off and the V filter and to a lesser extent the B filter  
had a coat of crud on them. The R and I filters were fine. The crud  
looked sort of like salt water had dried on the filter. It was bumpy  
and translucent. It would not clean off. I tried pure alcohol and  
lens tissue, then switched to pure cotton. I forewent my better  
judgment and scrubbed it pretty hard. I couldn't get it clean. I'm  
going to try to disassemble it and soak it or something but I suspect  
it is ruined at this point.

Although I doubt it is air tight, there is no way dust or  
contaminants could get in there that I'm aware of. My observatory  
goes through large temperature swings throughout the course of a  
year, from -20F to +100F or so. I wonder if the heat made something  
bubble up? Why just V and B and not the other filters?

I'm a bit confused.

Thoughts?

Michael