[Aavso-photometry] crud on filters

[Shawn Dvorak]

For those of you with SBIG (aka Custom Scientific) filters, how well 
have they held up?  I have gone through a couple of Schuler filters, and 
typically only get about a year out of one here in hot, steamy Florida.  
This year I even brought in all of the equipment from the observatory at 
the end of June to avoid the worst of the heat, but my V filter was 
already suffering some degradation.  I'm planning to get a CFW8 and hope 
that the somewhat more expensive Custom Scientific filters in it last 
longer.

Shawn

Steve Beckwith wrote:

>I can vouch for Richard's statements on the Schuler filter. I had this
>happen to my Schuler 1/25" V-filter and thought it could be some kind of
>glue used in the manufacture of the filter.  I tried everything to clean
>it - denatured alcohol, soap and water, Windex, acetone (nail polish
>remover), etc.  Nothing worked.  I ended up buying another filter from
>SBIG.
>
>- Steve
>
>-----Original Message-----
>From: aavso-photometry-bounces at mira.aavso.org
>[mailto:aavso-photometry-bounces at mira.aavso.org] On Behalf Of Richard
>Miles
>Sent: Tuesday, September 20, 2005 3:08 PM
>To: aavso-photometry at aavso.org
>Subject: Re: [Aavso-photometry] crud on filters
>
>----- 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
>
>
>
>_______________________________________________
>
>Aavso-photometry mailing list
>Aavso-photometry at mira.aavso.org
>http://www.aavso.org/mailman/listinfo/aavso-photometry
>
>
>_______________________________________________
>
>Aavso-photometry mailing list
>Aavso-photometry at mira.aavso.org
>http://www.aavso.org/mailman/listinfo/aavso-photometry
>
>  
>