CCD-world: Pleas help TASS

[Bruce Atwood]

The following was posted to CCD-world:

Zeolite, when properly activated, will do the trick.  Bake it at 350 C
(not F) (a solder pot works fine) and store in a metal can.  It will
soak up an amount of water equivalent to 10% of its weight so 10 grams
will soak up a cc of water.  I've never measured the dew point but the
partial pressure of water vapor above dry zeolite is only a few microns
of Hg.

Tom Droege wrote:
> 
> 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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-- 
Bruce Atwood
Director
Imaging Sciences Laboratory
Astronomy Department
The Ohio State University
140 W. 18th Ave.
Columbus, Ohio  43210-1173
(614) 292 6279 (V)
(614) 292 2928 (F)
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CCD-world is fully moderated. Send posts to CCD-world at astro.ku.dk
Standard replies will go to the list; address personal replies manually.
For more information, please go to:  http://www.not.iac.es/CCD-world/