CCD-world: Pleas help TASS

[Bruce Atwood]

The following was posted to CCD-world:

At room temperature Zeolite has almost no capacity for air but a huge
capacity for water.  At LN2 temps (you have to be sure the zeolite is
really cold) is acts like a virtual volume of about 10^8 CC/gm for air. 
5X, 8X, and 13X seem to be all about the same for oring sealed LN2
cooled systems.

You can determine if the zeolite is dry or not by putting a small amount
on a balance. If it is dry its mass will increase by 10% to 20% over a
few hours.



Tom Droege wrote:
> 
> 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
> >>
> >> - -- CCD-world -- --
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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
> >Standard replies will go to the list; address personal replies manually.
> >For more information, please go to:  http://www.not.iac.es/CCD-world/
> >
> - -- CCD-world -- --
> 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/

-- 
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
Standard replies will go to the list; address personal replies manually.
For more information, please go to:  http://www.not.iac.es/CCD-world/