HFWG Recommendation R11:
On the Keywords and definitions relating to 'exposure-times' and their corrections
(Approved: 1994 Jul 06)
Below are two sets of FITS keywords and definitions: List 1 gives the
keywords which should be used in OGIP files to store various 'exposure
times' associated with a FITS dataset; List 2 gives the related
keywords which should be used to store various correction factors to
This set of keywords and definitions was constructed from the most
common usage within X- and Gamma-ray FITS files. The lack of a
consistent naming convention most probably reflects the fact that most
of these keywords were devised by different people in different places
at different times.
It is strongly recommended that these keywords (only) be used to store
the various quantities within OGIP FITS files. It should be noted that
only those keywords/quantities considered necessary must be included in
the header of a given FITS dataset. However, it is suggested that where
possible all known quantities are included in the header.
LIST 1 - "Exposure-time" related keywords
is the time interval (in seconds) obtained as difference between
the start and stop times of an observation. Any gaps due to Earth
occultation, or high background counts and/or other anomalies,
is the total "good" time (in seconds) on 'source'. If a 'Good
Time Interval' (GTI) table is provided,
should be calculated
as the sum of those intervals. Corrections for instrumental
'dead time' effects are NOT included.
is the total time (in seconds) on source, corrected for the
'total' instrumental dead time effect. The ratio
therefore gives the dead time correction value (which hence
lies in the range 0.0-1.0).
is the total time (in seconds) on source, corrected for any
relevant quantity used to calculate the corrected count rate.
The value can include correction which are not directly
related with time (e.g. collimation efficiency or vignetting).
This keyword is a mean value when appropriate.
LIST 2 - Keywords for "Exposure-time" correction factors
is the total correction factor for any dead time effect
ONTIME), and lies in the range 0.0-1.0.
Thus the multiplication of this value by the
gives the 'effective' integration time or
the case of a light curve). Since the total dead time of a
given dataset can be the result of a multitude of
instrumental/processing effects (especially related to the
particular experiment, and/or processing by an onboard computer,
and/or spacecraft operations), it is recommended that as
well as including the total correction factor in the
keyword, instrument-specific keywords are used to keep a record
of the original value for the individual correction factors.
Corrections are usually applied to the stored counts and the
error on those counts. However, in some cases the errors can need
an extra correction, different to that applied to the counts.
ERRCOR keyword contains this extra value. The value is
If the dead time is related to the sampling of the on board
computer, it has been found that different correction are needed
for counts (count/s) and errors. This can be generalized to all
cases in which the error needs to be treated differently compared
to the counts.
is the correction factor for the collimator response or vignetting
function (depending upon the instrument) such as to enable the
scientific dataset (e.g. lightcurve) to be rescaled to that which
would have been obtained had the source been observed at the
angle of peak collimator response (for collimated instruments)
or on-axis (for instruments employing imaging optics).
In both cases the value of
should lie in the range 0.0-1.0,
and thus be the factor by which the scientific dataset should be
divided in order to obtain the equivalent on-axis value.
In both cases (but especially in the case of imaging optics)
the correction factor can be a function of energy, and thus can
only be used if a useful mean value can be defined.
In the case of imaging optics,
VIGNET should contain the
the total correction factor due to vignetting and any
(energy-independent) reduction in collecting area resulting from
obscuration by the near-side of the mirror, support structures etc.
It should be noted that historically these correction factors have
often (naturally) been referred to by different names, and
sometimes as the reciprocal of the value defined above.