[Aavso-photometry] RE: Where do the professionals source

[Dennis Hohman]

I have been using 48 mm Schuler filters and noticed the "goo" problem on the
V filter only.  Managed to clean it off with a distilled water/alcohol/
detergent solution this past spring. I examined the filter today and see
that the deposits are reforming. It looks like the surface coating is
raising up in a radial pattern that is slowly creeping inward. It occupies
the outer 10 % of the filter

My filters stay in the filter wheel in a unheated observatory and are
subject to relatively high humidity but it is not of a condensing nature.

Dennis Hohman
Stone Edge Observatory,
Orchard Park, NY