From SCHUEHLE@sumop1.nascom.nasa.gov Tue Nov 18 16:41:44 2003
Date: Fri, 30 May 1997 14:37:51 GMT
From: SCHUEHLE@sumop1.nascom.nasa.gov
Reply-To: sumer-soft@esa.nascom.nasa.gov
To: sumer-soft@esa.nascom.nasa.gov
Cc: SCHUEHLE@sumop1.nascom.nasa.gov
Subject: SUMER-SOFT: instrumental resolution with B-detector

      Instrumental resolution with the B_Detector
      ===========================================

      
      In the course ot measurement of the widths of spectral lines we have tried
      to determine the broadening due to intrumental resolution.  Jongchul
      Chae found that the intrumental broadening was varying with time
      particularly with the use of the B_Detector.
      I expected that this might be caused by low gain of the microchannel
      plates which also varies over time as the scrubbing goes on and the
      high voltage is being raised in a regular but not steady way.
      I therefore undertook a few measurements to determine the spectral
      resolution as a function of gain, e.i. as a function of high voltage.
      Since we normally raise the high voltage by not more than 100 Volts the
      variation should be inside this range. So the measurements were done
      starting at 50 Volts higher than the nominal value at that time (5300V)
      and stepped down in 20 V steps.  The line width was measured for the
      C I line at 1249 A and the O V line at 629 A (which has twice the photon
      energy).  The results are the following:

      The line width - and thus the instrumental resolution - increases
      as the voltage decreases.
      The increase starts at a higher voltage setting as the nominal voltage.
      That means, at the nominal voltage setting, which we have decided to
      operate at when we commissioned the B_detector last September, the
      resolution is not optimal and it degrades as the gain degrades.  Since
      the photon energy may be different by more than a factor 2, this effect
      is not wavelength independent.
      Quantitatively:
      -  The resolution would be optimum at 50 Volts higher. Plus a margin of
         100 Volts would be needed for gain loss between voltage updates.
      _  The change of resolution for the C I line increases linear with
         voltage, and the variation of line widths is not more than 0.5 pixels.
      -  For the O V line the variation is about twice as steep but levels off
         at lower voltages so that the line width increase is no more than
         0.8 pixels.

      Thus have to expect that the instrument resolution is different
      if we compare measurements at different times of observation depending
      on the gain situation on the particular day and location on the channel
      plate.  We must expect differences of resolution of less than 0.8 pixels.
      
      Since the A_Detector has always been operated at a higher gain level,
      these effects are not expected in the data of the A_Detector.
      

      Udo Schuehle, GSFC, 30-May-1997