Times in FITS files may be represented in several ways:
a. Directly as a Modified Julian Day number and fraction (MJD)
It will always be represented in Double Precision (REAL*8) format.
allowing a precision of 15.95 significant decimal digits.
With an MJD daynumber being around 50000.0 this allows a maximam
resolution of about 10^-6 seconds (1 microsecond).
The time column will have the name 'TIME' in this case
b. Indirectly as an integral number of time-units relative to a
certain reference date. In Stage-I processing all of these timea
will be scaled to units of 2^-12 (= 2.44140625E-04) seconds and be
stored in Integer (Integer*4) format. The reference time is given
as in Modified Julian Days as with (a.).
FITS files that have such a time-columns will contain the
following keywords:
MJDREF / MJD of S/C clock = zero
T_SCALE / scale factor of times (seconds)
The time column will be nemaed
WFC_TIME / Space Craft clock time of the WFC
So if the given relative time is TIME_WFC then the actual times in
MJDs will be: MJDREF + (TIME_WFC * T_SCALE / 86400)
c. A combination of a. and b. If only MJDREF is specified in
the priamary header time in MJD will be
MJDREF + TIME
IF T_SCALE is also specified time on MJD will be:
MJDREF + TIME * T_SCALE / 86400.
d. Instrument On-Board time ticks. One tick is defined as 2^-16
second. These ticks will be stored only for the start or end of
observations. Relevant keywords are
SCSEQBEG= spacecraft clock begin (ticks)
SCSEQEND= spacecraft clock end (ticks)
e. As time periods they will be stored in seconds
Used in headers with keywords:
TELAPSE (elapse time from start to end in seconds)
ONTIME (Total uncorrected integrated time ion seconds)
f. In Calendar days and times are stored in character string keywords
and have the form 'dd/mm/yy' (all numerics) and 'hh:mm:ss' or
'hh:mm:ss.sss' Used keywords are DATE-OBS, TIME-OBS, DATE-END and
TIME-END.