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Time of August 1995 SXT Entrance Filter Event

Image name: Aug_95_Ent_Filt_Event.png (click image to enlarge)
Image size: 10.9310 KB (640x512)
Date submitted: 26-Feb-1999


This is a redetermination of the time of the SXT entrance filter event
of 16 August 1995.  The revised time of this failure is

	16-AUG-95  08:04:20.

The upper plot shows the dark subtracted signal along the LH side of
all AlMg FFIs, normalized to DN/FRpix/sec.  Night and SAA are filled.
It is clear that the filter event occurred between the first and second
QR images after the sunrise at about 07:50.

The lower plot shows the dark-corrected signal across the bottom of
64x64 half-resolution medium-rate PFIs, all with nearly the same
pointing and corrected for spacecraft jitter.  Night is filled in and
the times of FFI exposures are indicated by the vertical broken lines.
The plus marks are short exposure AlMg images and the marching
downwards after sunrise is caused by exposure normalization of a very
weak signal, not corrected for orbit darkframe effects.  The march
downward is related to the ramping up of the AEC exposure time.  The
longer exposure Al12 PFIs, indicated by the diamonds, are less affected
by low straylight and uncorrected dark signal and are more stable.  it
is quite evident that the entrance filter failed during the fourth Al12
PFI exposure about 15 minutes after sunrise.

                                              L. Acton

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