From: "Richard Miles" <rmiles.btee
References: <002f01c647c5$87b0d400$0200a8c0
	<007501c64877$e86cf280$0200a8c0
Date: Wed, 15 Mar 2006 22:23:08 -0000

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 
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 dynamic range than is the 

Richard Miles

_______________________________________________