Condensation on AR coatings?

[roger smith x294]

On Jan 10, Ralph Florentin wrote:

> 	Finally one word of warning: One certain way to ruin the AR coating of
> a thinned CCD is to let non-dry air into the cryostat before the CCD has
> reached room temperature.  The water vapour will settle on all cold surfaces -
> including the CCD - as little ice crystals. A new coating costs a lot of
> dollars!

---------------------

Ralph,
	Can you (or anyone else out there) comment further on the damage done
to AR coatings by condenstaion of water or (heaven forbid) other contaminants.
The reason I ask is that I believe most CCDs see more contaminants on their
surfaces than people would expect.  Unless your dewar is very clean and
is kept that way while in service, then there will be some condensation
occuring every time it warms up to room temperature.

	After liquid Nitrogen runs out the flask and molecular sieve tend to
warm faster than the massive and thermally isolated detector mount.  Of course
radiative loading onn the detector helps initially but not when the
temperature gets closer to ambient (goes as T^4). So the sieve which has
soaked up all the contaminants dumps a fair fraction of it back into the
vacuum and other material is liberated from the dewar walls.  All this then
goes and condenses on our precious detector mount.  Of course when the
detector warms up too, the contaminants will evaporate again into the vacuum.

	On cooldown, the can and sieve cool first and do their job soaking up
the contaminants, so the detector looks acceptably clean.  Thus CCDs which
don't need UV flooding can survive for many months and many thermal cycles
with little problem.

	As a reminder that this process is going on we have an incident about
once a year where someone is late refilling the dewar.  ...They find it half
warmed up and hurriedly refill it.  This is a bad move since it results in
freezing contaminants which have condensed onto the surface.  While this frost
more than halves the QE and makes flat fields look grainy, complete recovery
is acheived by warming completely and cooling again.  (Its good to pump the
dewar out, but this can be omitted in emergencies.)

	I would like to hear more about AR coating vulnerability....

Roger Smith

Senior Electronics Engineer
Cerro Tololo Inter-American Observatory (CTIO), La Serena, CHILE.

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