contaimination by superinsulation

[roger smith x294]

Derek,

Once upon a time, we used aluminized mylar film ("superinsulation")
as a radiation barrier.  We eliminated it from our CCD dewars many
years ago after discovering that it adsorbed water vapour and was
the primary cause of our early contamination problems.  Outgassing
from the adhesive tape was also a concern.  

Our dewars are now built in house from all Aluminium.  We get
similar radiation performance to superinsulation by highly
polishing the inner wall of the dewar and the outer walls of the
nitrogen can, mount ad so on.  Polishing is also necessary to
minimize adsorption onto the Aluminium surfaces.  Anodizing is
forbidden on all surfaces in the vacuum.

We still have some occasional contamination problems.  We use small
amounts of fiberglass in thin walled tubes and the CCD connector,
plus a few nylon bolts, but we feel that stricter handling
procedures and baking are probably all that is required.  Our
colleagues in NOAO/Tucson have their own dewar design which shares
most of the same attributes, except that their inner can is copper. 
I have been using one of their dewars containing a pair of Loral
2x4Ks for the last year (sorry no Pt flash gate), and I am
impressed with how rapidly it pumps down and holds vacuum. ...They
electro-polish and are very careful with handling, and this appears
to pay big dividends.  

Roger Smith,  CTIO