[Aavso-photometry] Crud on filters

[Wclark3 at cs.com]

I too am having a problem with crud on my Schuler B and V filters. About 
everything was tried, distilled water and soap, distilled water, isopropyl alcohol 
(91%), and chemically pure (C.P.) acetone. I used q tips and microfiber lens 
cleaning cloths.  The only thing that was accomplished was to smear the crud 
around the glass.  I think the crud is some sort of glue or filler. A small 
piece was pried out of the edge of the filter with a toothpick.  

After all this it was time to seek professional advice.  I contacted Seiler 
Instruments, a small company a few miles from my home that sells and services 
surveying instruments and microscopes, manufactures military fire control 
instruments (lots of lenses), and is the sole US dealer for Zeiss planetariums. I 
took my filters over there and Brian, their optics cleaning expert, took a look 
at them.  He tried acetone and alcohol.  Those didn't work any better for him 
than they did me.  He looked at the filter surface under a microscope.  There 
wasn't any etching of the glass, just stubborn crud.  Next he tried a 
powerful lens debonding chemical but that didn't.  As a last desperate attempt 3-in-1 
oil was tried. That didn't work either.  Harsher chemicals were available but 
he was afraid that they would damage the glass, so they weren't tried.  Brian 
suggested polishing the crud off.  He had seen quartz optics cleaned by 
polishing with tin oxide back in the days when he was running laser shows at the 
planetarium.  ...Anyone know where to get tin oxide?

Wayne Clark