CCD-world: Pleas help TASS

[Tom Droege]

The following was posted to CCD-world:

Hello experts,

Some time ago I started "The Amateur Sky Survey".  The general goal is to
build a bunch of 2k x 2k cameras and survey all the sky all the time.  I am
well on the way to the goal, and have solved an immense number of problems.
  There are 20 dual cameras systems in the pipeline.  Since I am fairly
old, I made the decision early on to take a "Bob Wilson" (The builder of
Fermilab) approach.  The general idea is that you just start the production
line building things as cheaply as you can.  The over designed things give
no problem.  The under designed things stick out like sore thumbs, where
you can give them all your attention.  The idea is that it is cheaper to
throw away the production and redesign a few things than to build
everything with large safety factors.  This, Bob demonstrated, is a cost
effective way of doing things.  (By building Fermilab at twice the design
energy at less than half the proposed cost.)  Since I am paying for this
out of my own pocket, it seemed appropriate to try Bob's approach.  It
involves a lot of failure, so it is not something that you can easily get
by a funding agency.  But I have a tolerant funding agency, me.  There is
also the problem that at 70 I do not have time to build a succession of
prototypes.  

OK, the problem of the day.  It is getting close to the last problem, as I
am taking pretty good pictures with the prototype system.

I am buying CCD 442a's from Lockeed at bargain prices.  Dealing with them
is a pain as they do not seem to care if they sell them to me at $1700 ea.
(2k x 2k grade 3 device) or not.  So I get little help from them and feel
lucky to get any devices.  This includes accurate data sheets.  I had to
learn the pin out by experimentation at the cost of several devices.  (The
diagram said I had a single stage output amplifier when it was two stage,
and the pins were labeled some like the old design and some like the new.)  

I had planned on operating the 442 with the lid sealed on.  Thus I could
cool it and only have to worry about condensation on the cover glass.  It
turns out that Lockheed seals moisture inside the chip.  So if you cool it
down, one gets ice crystals.  Lockheed was not helpful about this.  I then
switched to chips with taped on covers.  The idea is to leave the taped on
cover in place so that this gives moisture a chance to get out.  I do not
have the clean room facilities to keep dirt off the chips, or the
microscopes, etc., to pick off dirt that get on.  

To save costs, I did not design an enclosure to hold vacuum.  I made my own
connectors with epoxy.  Not recommended high vacuum practice.  It is pretty
tight, some have a time constant of order 20 minutes.

The plan was to use a desiccant.  The material I have is called "Drierite"
which is mostly CaS04 with 3% CoCl2.  The design allows a large volume of
desiccant, coupled to the head by a short 1/4" id tube.  The catalog says
"dew point of -70 F.  This I hoped would allow me to go as cold as I could
with a two stage TEC.

The situation:

I start getting ice crystals at -10 C or so.  I added a thermoelectric
cooler to act as a getter which runs colder than the CCD.  This gets me to
-15 C or so.  The cooling system will get to -30 C which is about as cold
as I need to go.  In fact, -15 C is almost good enough for the amateur
locations.  The sky brightness for me near Chicago with the big survey
pixels (7") means that there is always more light from the sky than dark
current.  As you all know there is structure in the dark current above -20
or so.  It would be nice to run there.  I have found a couple of
professional locations.  These would benefit if I could get to -30.  It has
been a struggle to get to -30, so I hate to give it up.

Note that I have tried various cool down procedures, some of which help,
but the above is the best result.  

The questions:

Have any of you run using a desiccant?  What did you use?  Some have
suggested that I fill with dry Nitrogen.  I have a bottle of Argon that I
could use.  I do not have much hope that it will stay in my not very tight
enclosure.  Any suggestion as to what I might do?   Sorry, the camera head
enclosure was not designed to hold pressure, so pressurizing is out of the
question without a complete redesign.

Good advice would be appreciated.  Good advice might be "give up on this
plan and design for vacuum."  Sigh!  I hope it is not.  That would set me
back a year.   

Any help would be appreciated.  I am very close to getting a number of
survey cameras into the field.  This is close to the last problem.  (Sure!) 

Tom Droege


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