Understanding UVOT Exposure Time Keywords

Added November 13, 2006

UVOT science data files include a number of special keywords that are used to calculate the total exposure time. These keywords contain information about the CCD read-out time, as well as the amount of data lost due to several possible effects. The keywords, and their role in determining the total exposure time, are described below.


This is how TIME keywords for UVOT raw images are defined
as of the UVIT2FITS telemetry conversion software version V3.15+.


TSTART = The actual start time (from the exposure report packet in the telemetry)

TSTOP = The actual end time (from the exposure report packet in the telemetry)

TELAPSE = TSTOP - TSTART

FRAMTIME = The calculated frame exposure interval which is a function of the
           UVOT detector hardware window size. This is usually the full 2048
           by 2048 but can be smaller for exposures in the White filter, to
           reduce telemetry volume.

BLOCLOSS = The time lost if the exposure began with the BLOCKED filter in place
           but was later commanded to a different filter during the exposure.
           Effectively a dead time. Could be in error by 0-10+ sec
           due to the timescale on which filter position is available in the
           housekeeping data. This keyword is present only if the value is nonzero,
           and should only rarely be present. It is nonzero for some grism observations
           of RS Oph.

TOSSLOSS = Time lost when the onboard shift-and-add algorithm, which corrects image
           positions for satellite drift, tosses event data completely off the
           image. Effectively a dead time. Loss of Aspect Following packets
           in the housekeeping stream can cause this estimated value to be incorrect.


STALLOSS = Time lost when the DPU stalls because the count rate is too high.
           Estimated from ((TELAPSE/FRAMTIME) - "EXP Report total frames")
           times FRAMTIME. Set to zero if less than 1 sec.

ONTIME = TELAPSE
       - TOSSLOSS
       - STALLOSS
       - BLOCLOSS (As of UVOT2FITS V3.16)

BLOCLOSS was accounted for in the DEADC value for UVOT2FITS v3.15. 
         It was removed from the DEADC calculation and treated like 
         all other time losses as of UVOT2FITS V3.16.

LIVETIME = (ONTIME  * Calculated dead fraction) - BLOCLOSS for UVOT2FITS v3.15.
LIVETIME = ONTIME * Calculated dead fraction as of UVOT2FITS V3.16.

EXPOSURE = LIVETIME

DEADC = EXPOSURE / ONTIME

EXP_UNC = The uncertainty in the EXPOSURE keyword value. This attempts
          to account for uncertainty sources from the telemetry.
          The major source of uncertainty in the EXPOSURE keyword comes
          from lost telemetry during shift-and-add. When onboard shift-and-add
          is turned on and telemetry loss is indicated, EXP_UNC is
          1.0 - ("Found Shift&Add Intervals" / EXPOSURE).

CNTEXP =(GIMAGE/NEVENTS)*NFRAMES*CalcFrame*CalcDEAD as of UVOT2FITS V3.16 when
    the image is 2048x2048. Parameters are defined by->
    In sw*uct.hk, take the columns NEVENTS, NFRAMES, GIMAGE. Get values of
    CalcFrame, and CalcDEAD for the standard window.

    NEVENTS = Number of events handled by DPU during exposure.

    NFRAMES = Number of exposure frames handled by DPU during exposure.
              This figure automatically accounts for STALLLOSS since stalled
              frames are not counted in this number (we hope).

    GIMAGE = Number of events found in the image on the ground.
             This may be in error if there were lost rows in the telemetry.
             This number SHOULD be identical to NEVENTS in 2048x2048 images
             but often is smaller by fractions of a percent. The reason for
             this is unknown and helps make CNTEXP a fuzzy value. One possible
             source is fractions of images shifted off by S&T.

    CalcFrame = The time for 1 exposure frame based on the hardware window.

    CalcDEAD = The 1 frame DEADC fraction which actually is 1.0 - dead fraction
               and thus is the live fraction (I didn't make this up, it is a
               standard). This value can only be applied to actual exposed to
               sky time. For example, TELAPSE*DEADC is meaningless unless TELAPSE
               = ONTIME.

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National Aeronautics and Space Administration

Goddard Space Flight Center

Understanding UVOT Exposure Time Keywords