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              ELECTRONIC NEWS FLASH OF THE EUVE OBSERVATORY
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Vol 5, No. 2b                1 Mar 1995                    ISSN 1065-3597
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New Standard Data Collection Mode for EUVE
==========================================

   The following memo is being released to Guest Observers (GOs) as it has
direct bearings on the Cycle III observations that began last month.

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To:  Cycle III GOs
From:  EUVE GO Center
Subject:  WSZ Mode

   We are instituting a policy change with respect to the standard data
collection mode for EUVE.  As of March 1, 1995 the standard data mode will be
WSZ and not XY.  Scanner C will be shut off when necessary (for high telemetry
rates).  For a few very bright sources XY mode will still be used.  WSZ will
allow the filtering of pulse heights, which can be used to decrease the
background and increase the signal-to-noise.  The details of which are discussed
below.
   If you have questions or concerns about how the change to WSZ mode might
effect your Cycle III observation, please contact the EUVE GO Center
(egoinfo@cea.berkeley.edu) or call me (Damian Christian) at (510) 642-9908.
   The EUVE spectrometer detectors consist of a stack of microchannel plates
read out by an anode which is partitioned into a separate wedge, strip and
zigzag pattern electrodes.  The digitized W,S, and Z signals for each detected
event can be converted on-board EUVE to X,Y positions and then telemetered to
the ground, or telemetered to the ground directly (i.e., WSZ mode).  The
information which is lost in the conversion to X,Y is the pulse height of each
event.  WSZ mode offers the advantage of being able to filter events based on
pulse height with ground software, since background events have a different
pulse height distribution than source photons.  Tighter lower and upper pulse
high thresholds can decrease the background rates and therefore increase the
signal-to-noise of the spectra.  The drawback of WSZ mode is that it requires
twice as many telemetry slots as XY mode so the telemetry throughput of the
high count rate quadrants (Scanners and DS Al/C) can suffer.  However, the EUVE
telemetry allocation scheme (Primbsching) minimizes the effects for the main
spectrometer quadrants.  Turning scanner C off can save telemetry slots, and in
most cases the total telemetry counts are underutilized.
   The advantages of using pulse height discrimination in WSZ mode have been
quantified using observations of several bright white dwarfs.  Reductions in
background rates for the short wavelength (SW; 70-190 Ang), medium wavelength
(MW; 140-380 Ang), and long wavelength (LW; 280-760 Ang) spectrometers are shown
in the table below.  The background region (BG) was taken from two 100 pixel
regions above and below the spectrum (designated as Source in the table).  The
counts are given in arbitrary units.  Nominal pulse height filter levels
(10,000-35,000) did not produce the expected improvement in the SW.  For this
reason, a narrower filter range of 10,000-30,00 was chosen, and reduced the
background by 26% with an 8% loss of source counts.  An even narrower filter
range of 17,000-31,000 reduced the background by 50% with a 14% loss of source
counts.  Nominal filter ranges in the MW and LW were 10,000-25,000, and
12,500-42,000, respectively.

          Spectrometer      Rate (arb. units)    % Change
          Region          No Filter   Filtered
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          SW BG           244          224         -8%
          SW Source       154          151         -2%

          SW* BG          244          123         -50%
          SW* Source      154          133         -14%

          MW BG           379          241         -36%
          MW Source       27621        26839       -3%

          LW BG           470          344         -26%
          LW Source       67878        66410       -2%

          _______________________________________________
          *Analyzed with 17,000-31,000 filter.
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