April 2001 anneal report for program 8841.

There was an anneal of the CCD on April 7, 2001, 27 days after the previous
anneal on March 11, 2001.  A homogeneous set of data was obtained from
proposal 8841:  The CCD Hot Pixel Annealing.

Previous trends continue.  The growth rate of hot pixels continues within the
range seen for previous months.  The anneal worked well to get rid of 0.1 e/s
and 10. e/s hot pixels, but not as well at the 1.0 e/s level.

1) The number of post-anneal hot pixels has decreased at the 0.1 and 10 e/s
   levels, but increased at the 1.0 e/s level.

   > 0.1 e/s: 17042 pixels on Apr 7, 2001
   > 1.0 e/s:  3473
   > 10. e/s:   389

   > 0.1 e/s: 17151 pixels on Mar 11, 2001
   > 1.0 e/s:  3452
   > 10. e/s:   397

   > 0.1 e/s: 16565 pixels on Feb 10, 2001
   > 1.0 e/s:  3372
   > 10. e/s:   382

   > 0.1 e/s: 16286 pixels on Jan 14, 2001
   > 1.0 e/s:  3364
   > 10. e/s:   371


2) The *raw* post-anneal dark countrate for this anneal is 0.02618 cts/pixel.
   This compares with 0.02685 cts/pixel on Mar 11, 2001, and 0.02245 cts/pixel 
   a year ago in April 2000. 
   
3) The iterated median count rate (after iterating up to 30 times by
   eliminating pixels +/- 5*sigma) is 0.00425, down from last month.

4) The creation rates for hot pixels over this period were within the normal
   range of fluctuations, on the low side actually.

   > 0.1 e/s:  12.8 pixels/day in Apr 2001 
   > 1.0 e/s:   0.2
   > 10  e/s:  -0.9

   > 0.1 e/s:  53.0 pixels/day in Mar 2001 
   > 1.0 e/s:  10.2
   > 10  e/s:   1.1

   > 0.1 e/s:  41.3 pixels/day in Feb 2001 
   > 1.0 e/s:   4.5
   > 10  e/s:   0.3


   > 0.1 e/s:  14.4 pixels/day in Apr 2000 
   > 1.0 e/s:   1.0
   > 10  e/s:   0.0


5) The rate at which new hot pixels annealed out was within the normal range of
   fluctuations.

   > 0.1 e/s: 132.8% annealed away in the Apr 7, 2001 anneal.
   > 1.0 e/s: -425.%
   > 10  e/s:  65.0%

   > 0.1 e/s:  60.5% annealed away in the Mar 11, 2001 anneal.
   > 1.0 e/s:  71.9%
   > 10  e/s:  50.0%

   > 0.1 e/s:  74.0% annealed away in the Feb 10, 2001 anneal.
   > 1.0 e/s:  93.1%
   > 10  e/s: -22.0%


   The usual plots are attached.