The Joys of Cooling

[Stupendous Man <richmond@a188-l009.rit.edu>]

  I've been experimenting with the Rochester Mark IV.  Last night,
I ran a test that gave some strange results.  I have some ideas,
but hope that others can give me advice.

  The Mark IV has two CCD cameras.  Each one has a TEC inside to
cool the CCD chip.  Tubing runs through the camera heads; one can
circulate a liquid through the tubing to carry heat away from the 
TECs.

  Last night, by mistake, I forgot to turn on the pump which forces
water through the tubing.  I ran the cameras with the TEC units turned
on to an intermediate voltage ("3", according to Tom's control
program).  The ambient air temperature was around 15-20 C, and
the cameras cooled to about -5 degrees.  

  Then I realized that the pump wasn't on.  So, I turned on the 
pump, and watched as:

        - the V camera dipped by about 1 degree C for five minutes,
                then returned to its previous temperature

        - the I camera dipped by about 3 degrees C for maybe 20 minutes,
                the gradually returned to its previous temperature

  Huh?  Now, it's true that the fluid was probably warmer than the air,
since it had been heated all day by the Sun (it sits in a garbage can
outside).  But I expected some advantage with the fluid.

  Later that night, I tried to drop the temperature of the fluid.
The garbage can contained about 15 gallons of water and antifreeze.
I placed 14 pounds of ice into the water, while the fluid was
circulating, and watched the CCD camera temperatures.  Over
the next 20 minutes or so, the CCD temperatures gradually dropped
by about 1 degree C (both of them), and then they both rose
back to their previous levels.  The ice hadn't fully melted
(or even close to it) when the CCD temperatures were back to their
pre-ice levels.

  Hmmmm.

  My guess is that the thermal contact between the block through 
which the water flows, and the finger running to the CCDs (or the
finger-CCD connection), isn't as strong as it ought to be.  
If I'd been smart, I would have monitored the "Water Temperature"
reported by the Mark IV, which I _think_ is the temperature of the
block through which the coolant flows.  All I know is that
the coolant 

      - started the night at 23 degrees C,
      - was at 21 degrees C about 90 minutes after adding ice
      - was at 19 degrees C about 130 minutes after adding ice,
                 when I gave up and went home

  It may be that the flow of coolant is too small to cool the
block and finger effectively.  Because I'm using lots and lots
of narrow tubing, only a trickle moves through the block --
I measured it last January at something like 4-7 gallons per
hour.  

  Tom, can you tell me exactly where the thermistors for the
"Water Temperature" and "CCD Temperature" are connected?

  Oh, one more thing: the tracking is fine when the telescope
points to the East or South.  Once it moves about 45 minutes
West of South, it starts to stick, so that 60-second exposures
show short trails, often with "dots" (meaning that the telescope
tracked well for a while, then stuck for a moment, then tracked
well again afterwards).  Perhaps this is due to dirt on the
portion of the threaded rod which touches the drive mechanism
when the telescope points West.  I plan to clean the rod, and
_maybe_ to lubricate it, before the next night's run.


                                         Michael Richmond