[Aavso-photometry] crud on filters

[Richard Miles]

----- Original Message ----- 
From: "Michael Koppelman" <lolife at bitstream.net>

>I think someone else had posted something similar at one point...
>

==> Michael - There's a thread on this subject on the 
CCD-astrometry-photometry e-Group on March 7-8, 2005.  See especially 
Message 2616 for my take on it.

> My photometry went to hell in the V-band. Errors went way up, the  stars 
> looked fuzzy. It was clear something was up. I took the CCD and  filter 
> wheel off and the V filter and to a lesser extent the B filter  had a coat 
> of crud on them. The R and I filters were fine. The crud  looked sort of 
> like salt water had dried on the filter. It was bumpy  and translucent. It 
> would not clean off. I tried pure alcohol and  lens tissue, then switched 
> to pure cotton. I forewent my better  judgment and scrubbed it pretty 
> hard. I couldn't get it clean. I'm  going to try to disassemble it and 
> soak it or something but I suspect  it is ruined at this point.
>
> Although I doubt it is air tight, there is no way dust or  contaminants 
> could get in there that I'm aware of. My observatory  goes through large 
> temperature swings throughout the course of a  year, from -20F to +100F or 
> so. I wonder if the heat made something  bubble up? Why just V and B and 
> not the other filters?
>

==> The problem almost certainly originates from the BG39 component used in 
these filters, which is denaturing.

People have reported this problem with Schuler 1.25" filters - it will also 
happen with some other manufacturers'.  I also have some evidence that some 
batches of BG39 may be worse than others - there are certainly 
batch-to-batch variations in transmission characteristics of this filter 
component at the red end of its range.

If the temperature cycles over wide limits this aggravates the problem just 
as you have observed.

A good solution is to sandwich the filter by cementing it between two 1mm 
thick silica plates.
Anyway you need some sort of protective overlayer to stop it happening.

Hope you get sorted soon,
Richard