I have a set of UBVRI 50mm round filters purchased from you. While taking new flat images, I noticed a very unusual image for my V filter. I removed my camera to inspect the filter and saw that it was fogged over. I tried cleaning the filter with 91% alcohol but the fog persists. Can you recommend the best way to clean my filter? My observatory is in Maryland and I have an automated system,which I leave in the ready state (without the OTA cover on). I have the camera (STXL-6303e) and filter wheel (STXL-FW8G) connected to a PlaneWave CDK 300. The most used filter for me is the V and the system is open but parked in a configuration to prevent dust collection on the mirrors. Could this issue have likely resulted from humidity? I constructed the observatory in Oct 2018 and this is the first summer. I'm trying to figure out how best to avoid this happening in the future.