Rochester status report, Jan 2, 2002

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

  I continue to prepare the Rochester Mark IV for regular operation.
Most of the recent developments deal with the cooling system and
dark frames.

       1. drilled holes in the sides and floor of the shed,
              so that coolant lines can enter the shed even when
              the doors are fully closed

       2. added 2 gallons of antifreeze to the coolant, and put
              insulation around the tubing which is exposed to the
              outside air (these spurred by discovering a 
              layer of ice 3 inches thick on the top of the 
              coolant a few days ago)

       3. changed dessicant in the air tubing, so that moisture
              won't build up in front of the CCDs

       4. labelled the switch controlling the air pump with "ON" and "OFF"
              (I discovered it was on when I thought it was off)

       5. covered the (outdoot) electric outlet to which the coolant pump is
              connected with an inverted bucket, so that I can 
              keep it plugged in at all times (even when it's raining)

  I ran some tests of dark frames, and discovered a number of things:

       - it takes about an hour for the temperature to stabilize after
             the TE coolers are turned on

       - the stdev of pixel values in a dark frame drops with temperature:

                  T =  +9 C         stdev = 65 DN
                  T = -12 C         stdev = 42 DN
                  T = -20 C         stdev = 14 DN

         The histogram of pixel values shows a small secondary peak
         at T = -12 C, but it disappears at -20 C.  I judge -20 C
         to be a reasonable CCD operating temperature.

       - the first frame read out after prolonged inactivity 
         (an hour or more?) usually has garbage in the first few hundred
         rows -- even if the chip has been read once to clear it

       - the "F" (V-band) camera shutters don't quite meet when they
         close.  This causes a light leak during the daytime.  At night,
         with the shed roof closed, it doesn't appear to lead to any
         perceptible signal.


  I spent some time trying to understand the pixels near the edges of
the frames -- the first and last few columns.  What I found was:
(I use "active" to mean a pixel exposed to light in the usual way)

       - cols 1, 3, 5  all have the same, fixed value (bias, I guess)

       - col 2         is higher than 1, 3, 5
                             its value doesn't change with exptime         

       - cols 0, 4     are higher than 1, 3, 5, but not as high as 
                          the "active" pixels
                             their values DO increase with exptime

       - col 6         is a tad lower than "active" pixels
        
       - cols 7+       are "active" pixels

  And, at the other side of the frame, the edge cols are

       - col 2038      is similar to cols 0, 4
                             value increases with exptime

       - cols 2040, 2041, 2042  are similar to 1, 3, 5

The behavior of these edge pixels may change with newer versions
of the electronics and/or STAMP code.  I'm using an old version,
so Tom will probably chastise me :-/

  I was hoping to find a "dark" column, which would be shielded from
light, but give a reasonable estimate of the "dark current" for 
active pixels.  I'm not confident that there is such a column;
cols 0, 4 and 2038 look like the best bets.  One might want such
a column if the temperature varied slowly throughout a night, 
with consequent variations in the dark current: dark frames taken
during the day might not have the same temperature as images
taken at night.  One could use the "dark column" to shift the
daytime dark up or down as necessary.  

  I need to run more tests of the cooling system to see how 
stable the temperature remains over a 24-hour period and day/night
cycle.  I also need to see if there's a close relationship between
the values of, say, column 4 and the mean value of active pixels.
More work...

  Finally, I discovered that my current pump sends only about
4 gallons per hour through the cameras.  That's not enough to
keep them as cool as possible -- I discovered that one of them
can be chilled to only -20 C, while the other keeps going to about
-30 C.  Tom suggested that the downstream camera is getting
warmer coolant, which can't carry off enough heat to cool it
further.  He estimates that 10 gallons per hour is required to
cool both cameras fully.  

  I went to the hardware store today and found that the cheapest
pump was NOT an improvement over the current one :-(  Rats.

  One of my colleagues here suggested I re-wire the coolant system
so that it runs through the two cameras in parallel, not serial.
That would be cheaper than buying a big (1/6 HP) new pump, but
would take more work outside in the cold without gloves.  Sigh.
I'll let you all know what happens.

                                              Michael Richmond