References: <002f01c647c5$87b0d400$0200a8c0
	<005f01c6487f$0a3f12b0$a8b18056
Date: Thu, 16 Mar 2006 10:12:11 +1100
Date: Thu, 16 Mar 2006 10:12:11 +1100

Richard and all,

I agree that parfocal filters, while being desirable, certainly are not a 
major factor in selection for photometry work.

It would seem that the logical solution, if ordering a 'Schuler' V filter 
from Astrodon, is to request that a thin clear glass be added to protect the 
BG39 layer ... which is what Don Goldman offered to do for me

Bernard Heathcote


> I'd echo the point that A/R coatings won't protect against this type of 
> efflorescence of the BG39 layer.
>
> The solution for myself and a few others on this side of the Atlantic has 
> element. No sign of deterioration as yet after a couple of years.
>
> I must get hold of some peroxide to see whether crud on some old BG39 
> glass cleans up as reported.
>
> Just one final point about filters having different thicknesses and hence 
> are not exactly parfocal.  It normally doesn't matter a whole heap that 
> one image is slightly out of focus if you are doing photometry (I believe 
> it is almost the same for astrometry) in that you can make it work for 
> yourself by optimising the focus for that filter that gives the least 
> response (usually the I filter but not if your target is very red of 
> course).  Then you have the advantage of being able to measure brighter 
> stars whilst avoiding saturation, or in other words you get an increased 
> sharp.  Just a thought.
>
> Richard Miles
>
> _______________________________________________
>