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 these times.

    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, are included.

    is the total "good" time (in seconds) on 'source'. If a 'Good Time Interval' (GTI) table is provided, ONTIME 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 LIVETIME/ONTIME 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 (i.e. LIVETIME/ONTIME), and lies in the range 0.0-1.0. Thus the multiplication of this value by the ONTIME value gives the 'effective' integration time or LIVETIME (TIMEDEL in 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 DEADC 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. The ERRCOR keyword contains this extra value. The value is unitless. Example:
    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 VIGNET 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.