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March 1999 Entrance Filter Failure Event

Image name: MAR_99_Filter_Failure.png (click image to enlarge)
Image size: 10.8140 KB (512x512)
Date submitted: 03-Aug-2001


                 L. Acton, 3-Aug-2001

The figure demonstrates that the March 1999 entrance filter failure
took place during a Yohkoh/SXT data gap.  The above curve is the signal
in a strip across the bottom of long-exposure AlMg FFIs.  The exposures
for these images was DPE=19 for QR and DPE=25 for HR.  The area summed,
after QR images were rebinned to HR, was [1:510,0:50].  The data points
with diamonds are from the HR images, showing that there was no HR/QR
bias in these data.  The images were prepped with /dc_orbit_correct but
without leak correction.

There is an obvious bifurcation of the straylight signal in these x-ray
images both before and after the entrance filter event.  The few
parameters I've checked, i.e., FMS, pointing, cannot accout for this

The lower curve is made from the signal in all useful NB straylight
monitor images.  It clearly shows the entrance filter failure, too.

Because of the paucity of data at this time it is impossible to
determine the exact time of filter failure.  For SXT data analysis
purposes a failure time of

	12-Mar-1999 02:00

is a satisfactory time to enter into yo_dates.  This time is indicated
on the figure by the dotted lines.

Until better information is available we should asign a complete sector
of 1/12 (0.083) as having opened up at the time of this event for data
analysis purposes.

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