[Aavso-photometry] Where do the professionals source

[Richard Miles]

----- Original Message ----- 
From: "Tom McDonagh" <tom_mcdonagh at yahoo.com>


> One suggestion that may work is to store filters in a
> dessicated, nitrogen gas environment when not in use
> for extended periods of time.  I'm lucky enough to
> have this option available to me.
>

Tom,

That's good advice re. storing of filters and filter glass.  One is well 
advised to store V and B filters in a closed container with dessicant such 
as dry silica gel.

I discovered the problem with BG39 glass a long time ago now.
I had wrapped it in tissue paper and left it in its box in a drawer in the 
observatory.
2 years later, when I went to use the filter glass I found that both sides 
were coated with a residue which was impossible to clean off without 
damaging the surface of the filter.  BG39 glass is also not very hard and is 
easily scratched.

Unfortunately using a mix of other filter components to produce a V filter 
will yield a poorer match to the standard system.  Alternatives that roughly 
match V and which are immune from degradation are VG12 or VG14.  Just a 
single 1mm thick piece of either is all that is needed.  However, the 
transmission in the V is noticeably less compared with the standard 
component mix for V using BG39.

Cheers,
Richard