[Aavso-photometry] Crud on filters

[arne]

I'm going to be talking about this problem in the Friday paper session.
Once the paper is given, we'll get it posted on the web as soon as
possible.  The culprit is the BG39 glass that is used as a red blocker;
it is hygroscopic and needs to be protected from the environment.
   Once the damage has been done, the only way to restore the filter
is to repolish.
Arne

Wclark3 at cs.com wrote:
> 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
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