Bruce Cloutier wrote:
[snip]
> "Interesting that here we are trying to get rid of excess heat [to help
> Pictor
> reach sufficiently low temperatures] and in parallel threads we are being
> creative in finding ways to add heat to fight dew. Some kind of thermally
> conductive and flexible arrangement might help conduct heat from the 416 head
> onto the OTA. The added heat sink and surface area of the OTA might keep the
> 416 case temperature low enough to reach the -15 deg temps without a fan (and
> the associated power requirement). Meanwhile the added heat on the OTA might
> offset the need for a dew zapper (and the added power requirement)."
[snip]

Sounds like an application for a heat pipe.

For a description of heat pipes, and some vendors, see pages such as:
http://www.indek.com/heatpipe/heatpipe.html
http://www.tokuden.com/principle.htm
http://www.heatpipe.com/index.html
http://www.thermacore.com/ -> "Heat Pipe Technology"
http://tropickool.com/heat-pipes.html
http://gnv.fdt.net/~heatpipe/hotplate.htm
http://www2.txcyber.com/~itoltd/
http://engineer.tamu.edu/tlo/industry/techs/0268.htm
http://www.me.rmit.edu.au/events/5ihps/5ihps.htm
http://www.esdu.com/graphics/volume/htrn3.htm

In our case, the heat source (the CCD camera) will be lower than the
heat sink (the front of the OTA) so experimentally-minded MAPUGers could
build a heat pipe without needing a wick; just plug one eng of some
refrigerator tubing, add some refrigerant, evacuate it except for the
vapor from the liquid, and seal. Such wick-less heat pipe(s) could
carry heat from the camera to the front of the OTA; a heat pipe with a
wick could carry the heat around the front of the OTA.

Notes for anyone who wants to try building a heat pipe:
1. Possible refrigerants include: methanol, acetone, butane, or any
freon, among others. A pure liquid, not a solution. The choice of
refrigerant depends on the range of use and storage temperatures; it
should have, over the entire use range, a high derivative of vapor
pressure with respect to temperature, and a high heat of vaporization; a
suitable vapor pressure over the storage range is also required.
2. Without a wick, condensate on any part of the tube has to flow
downhill to the heat source. (Making an adequate wick for downhill heat
flow is probably tricky.)
3. Air-tight seals are required; the liquid and vapor have to be pure.
4. It might be best to have one straight heat pipe on the OTA, another
one attached to the camera, and a sliding joint of some sort.
5. I've not tried to build a heat pipe myself, but this might be a good
reason to try.

- Chuck C.

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