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Rate for NICMOS time conversion in error

Rate for NICMOS time conversion in error

In September 1997, a drift of about 4 seconds/day was discovered in the
NICMOS HST time keyword.  This resulted in incorrectly Modified Julian Date
(MJD) and UT times in the science header.   The problem was traced to an
incorrectly specified scaling factor used by the ground system. The NICMOS
clock is actually a counter. Every time the NICMOS transitions from BOOT to
SAAOPER the counter is  reset  to zero and a time code update is performed
which sets the NICMOS clock to the space crafts clock's value.

If you have NICMOS observations before November 10, 1997 00:00 UT,
then the times in
your science headers could be off by ~4 seconds/day since the last
time NICMOS time code was updated.

NICMOS time code updates establish a fiducial reference from UTC
to S/C time.  An internal elapsed timer is reset at this time to
zero and continues to count once the new NICMOS Major Frame value
is received.  The S/C and elapsed times are read and written to the
embedded engineering attached to each observation.

The OPUS pipeline obtains the S/C clock time from the engineering mnenomic
NHSTTIME (32-bit word) and converts it to MJD and adds the value of the
NICMOS elapsed time.  The elapsed time value is stored
in the engineering mnenomic NELAPTIM (48-bit word).

The cause of the ~3.32 seconds a day drift was strictly
due to excessive rounding of the Least Significant Bit (LSB)
scaling factor when converting the NELAPTIM value to days.  OPUS
was using 9.766e-07 seconds/LSB, which
has been changed to 9.765625e-07 seconds/LSB.  This new
calculated scaling factor results in no significant
drift over a 1 week period.  However, due to uncertainly in the
engineering data collection, there is an inherent uncertainty of
+/- 0.064 seconds when calculating elapse time.

The change in scaling factor is documented in Document Change Request
(DCR) 1428, "Correct Drift in NICMOS Spacecraft-Time Update",
against document ST-ICD-08.
DCR 1428 Description of Change:

On page IX-26, in table IX-1, for entry HST Time, in Description column,
last two lines, change the time represented by the LSB of the 48-bit
integer from "0.9766 microseconds" to "0.9765625 microseconds".


The following is a procedure for correcting HST time
keyword for NICMOS observations obtained before

1) Determine when the last time the time code update was performed.
   A list of those times is provided below.


2) Use the TCU_TIME as your zero reference time. Subtract the
   erronous HST time keyword value (HST_TIME_ERR) from the TCU_TIME.


3) Take the delta time in seconds and compute delta ticks.


4) Take DELTA_TICKS and multiply by drift rate to compute.
   a correction term.


5) Subtracting correction term from the HST_TIME_ERR will result
   in a correct HST time.


List of NICMOS time code update