CCD-world: Pleas help TASS

[Tom Droege]

The following was posted to CCD-world:

Bruce,

Thank you very much for the advice on Zeolite.  I went ot the web and the
first thing I found was a table of 128 different types of Zeolite.  Then I
went to the Cole-Parmer catalog and found only one.  So I ordered some.  It
is listed as Na2O.Al2O3.xSiO.yH2O is this possibly the right stuff?  Do you
know if it will come dry, or will I have to bake it?  Hmmm!  350C counds
high, I wonder if a home oven on "clean" will do it?  Others have indicated
that they use Zeolite at LN2 temperatures.  I assume it works at room
temperature?

Thanks again for the help.  I am almost there!

Tom 

At 03:44 PM 5/5/00 -0400, you wrote:
>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)
>- -- CCD-world -- --
>CCD-world is fully moderated. Send posts to CCD-world at astro.ku.dk
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>For more information, please go to:  http://www.not.iac.es/CCD-world/
>
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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/