[Aavso-photometry] RE: Crud on Filters

[Laurence Marschall]

At the Gettysburg College Observatory we've had the same problem with
two sets of 2 inch square B and V filters over the past 10 years, one
from Custom Scientific and one from Optec (the latter, I believe,
supplies the Custom Scientific filters). In both cases they were way
beyond redeeming by any means---and I had to replace both sets. The
current set, also from Optec seems to be holding up OK, but I'm not
holding my breath.  
      So buying expensive filters is not the solution---it's chemistry
and physics that are to blame, and the suggestion that it's the BG39
seems as good to me as any.  I had earlier thought it might be the
anti-reflection coatings which I have on the filters, but I couldn't
understand why it happened only to the B and V. I and R are good as new.
It's small comfort to hear that we're not alone :-)

  Cheers,
Larry Marschall