CCD-world: Pleas help TASS

[Mark Waterson]

The following was posted to CCD-world:

As a possible alternative to Tom's lack-of-vacuum camera enclosures one
might try static fluxh & fill with *dry* nitrogen or other gas (not that
expensive from Airco) to get as much moisture as possible out of the
enclosure. If you basically flush the enclosure for a while (hours?)
then valve it off at static pressure there won't be much exchange with
outside air as long as you don't get thermal pressure changes to "pump"
the leaks.
To get cold, you need to solve aother problem - thermal capacity, since
your TEC's are limited, you need to reduce the heat input - try a good
layer of what we called "superinsulation" at Argonne National  lab -
alternate layers of nylon mesh (the kind ballerina skirts are made from)
and aluminized mylar film. A stack of this 2" thick with an overlapping
wrap will insulate a LN2 pipe without any vacuum, so it approaches the
R-value of vacuum without the mess. Since you aren't trying to get that
cold, less thickness would be ok.

As a getter, Drierite needs to be baked really well and you need to
start pretty dry to get it to work, and more importantly you need
surface area exposure - I'm not sure your .25" tube will give you enough
circulation to dry the enclosure though the tube will be dry :-) Try
enclosing a layer of drierite in filter paper ( to keep the dust in)
inside the enclosure. This surely isn't the approach you'd take in a
cryogenic camera, but this might be "good enough"!

hope the ideas help,
--
    mfw
 Mark Waterson
 University of Hawaii - Institute for Astronomy - Haleakala
Observatories, Maui
 808-876-7600 x108  Office     808-243-5892   Observatory


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