[Aavso-photometry] RE: Crud on Filters

[Richard Miles]

Michael K. wrote:
> My filters which went bad it was only one side of the filter. This 
> confirms what others had said and what Arne said in his talk about it 
> being one of the glasses in the V and B filters. Arne suggested that 
> perhaps anti-reflection coatings would stop this from happening  because 
> it isolates the glass in question from the air. This makes  sense to me 
> and makes me think I should spend the extra bucks and get  AR coated 
> filters. Does anyone have reason to believe this wouldn't  work?
>
> The "new" Schuler filters sound pretty good, too, but it doesn't  sound 
> like they will be available for a few months?
>

Michael,

AR-coating will certainly be an improvement but how stable they remain 
depends on a lot of factors; layer thickness, composition, absence or 
otherwise of pinholes, relative humidity in the filter environment, 
BG39/BG40 batch, etc.

As I mentioned, Norman Walker can produce an optically-flat V filter with a 
1mm silica plate cemented to the BG39 component.  Cost for the complete 
filter about 100 UK Pounds.

What is the diameter and thickness of  the V filter you need?

Cheers,
Richard