Observing Efficiency (September 2001) 

Readout of the NSFCam array requires a minimum of 3 separate reads (for
NDR=1), each requiring the minimum integration/readout time.  The first of
these reads resets the array, the second reads the pedestal level, and the final
read is the data sample.  In the output image, the value at each pixel is the
difference between the sample and pedestal values.  When using the very short
integration times necessary for imaging at thermal (L- and M-band) wavelengths,
this multiple sampling of the array will significantly increase the real time
needed for each integration, thus reducing the observing efficiency.

ITIME is defined as the time between the start of the pedestal read and the
start of the sample read.  For any ITIME, the real time 
(actual clock time) needed to complete a single integration is given by:

     Real time = (reset time) + (ITIME) + (number of NDRs * read time)

Assuming the default setting for SLOW CNT = 1, the readout time for 
the full array is 0.077 sec. for each NDR.  Thus the minimum ITIME possible
is 0.077 sec.  The real time required to take this minimum integration is:

     ITIME = 0.077 sec.  NDR = 1 (set automatically)
     Real time = 0.077 (reset) + 0.077 (ITIME) + 0.077 (1 NDR) = 0.23 sec.
     Efficiency = 33%

Under normal operation, as the ITIME is increased, the number of NDRs
will increase up to the default maximum of 8.  For example, the minimum 
for 8 NDRs is 0.077 x 8 = 0.62 sec, so if ITIME=1.0 sec, the number of
NDRs will automatically be set to 8.  The real time required for 
this 1.0 sec. integration is:

     ITIME = 1.0 sec.  NDR = 8 (set automatically)
     Real time = 0.077 + 1.00  + (8 * 0.077) = 1.69 sec.
     Efficiency = 59%

The maximum number of NDRs can be manually set in the Setup window (click on
the "Setup" button in the "Observing Parameters" window).  For example, if the
maximum number of NDRs is manually forced to 2, a 1.0 sec. integration would
actually take:

     ITIME = 1.0 sec.  NDR = 2 (set manually)
     Real time = 0.077 + 1.00  + (2 * 0.077) = 1.23 sec.
     Efficiency = 81%

The time to readout a subarray is shorter than that needed for the full array.
For example, the readout of a 128x128 pixel region (1/4 of the array) takes
0.020 seconds.  An integration with ITIME = 1.0 sec and default of number 
of 8 NDRs will take:

     Subarray of 128 x 128 pixels
     ITIME = 1.0 sec.  NDR = 8 (set automatically)     
     Real time = 0.020 + 1.00 + (8 * 0.020) = 1.18 sec.
     Efficiency = 85%