condensation

[tinbergen at ksw.rug.nl]

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+ From:	Jaap Tinbergen		   +
+	Kapteyn Observatory	   +
+	Mensingheweg 20		   +
+	NL-9301 KA Roden	   +
+	Netherlands 		   +
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Intelligent controllers, perhaps, but there may be simpler solutions.
I don't know the details of CCD dewars, but old-time PMT enclosures had 
similar problems. 

Walraven used to argue that you can't ever hope to seal something 
hermetically, so why not construct one large leak deliberately and control
what goes on there. We had very successful multi-PMT enclosures with the
silica gel container at room temperature, as part of the engineered leak.
During cooldown, the moist ambient air would pass through the silica gel,
when cooling gave up the only significant way in or out remained through
the silica gel; perfectly safe for as long as the silica gel was good. 

In a similar vein, is it not possible to design for a guaranteed flushing 
(on warming up) of all contaminants outward rather than on to the CCD? One 
does have a certain volume of cold gas there; this gas will expand on 
warming up, can it drive the flushing? As I mentioned, I am not familiar
with CCD details, but if possible a passive intelligent design is superior
to a controller, however intelligent. Is the interlock that Roger Smith
mentions just such a construction, even if it's not really passive? 

I'll stop, I may be missing the point entirely.   Jaap


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