Please note that the start time for each individual group should be calculated from
the FPKTTIME of the relevant group and not simply by a recursive addition of
group elapsed time to the start time of a previous group.
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Header keyword 2 Group parameter
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33.1.1 Exposure Time Details
The different times in the FOS headers are:
This simple equation in terms of the basic FOS time and data acquisition terms is given to the full accuracy of the FOS clock by:
expgroup = LIVETIME x 7.8125E-6 xINTS x
In this calculation we have not accounted for the deadtime because we are interested in calculating only the time over which the photons were collected.
This elapsed time can be used to determine theapproximate start and stop time of each group in the data. The procedure is as follows:
The FPKTTIME in the group parameters of the .d0h file is the time at which the FOS memory for that group was read out and is inclusive of all deadtime for the group. Unfortunately, it may also include an indeterminate additional interval of up to 0.13 second caused by HST SDF overheads. In the headers this time is given (to the accuracy of about 0.125 second) in units of modified Julian date, which is the Julian date minus 2400000.5. This time can be converted from modified Julian date to the standard notation using the epoch task. For many FOS datasets in the HST Archive obtained prior to January 1, 1995, the exposure start time is incorrectly populated. The time at which the integration was started is given by:
start time = FPKTTIMEgroup - elapsed timegroup
The group parameters can be determined by running the task grlist to determine all the groups in the d0h file and then finding the value of the necessary keyword using hedit. Note that the FPKTTIME is accurate only to approximately 1/8 second.
In the basic FOS data acquisition units, this is:
exptotal = LIVETIME x 7.8125E-6 x INTS x NXSTEPS x OVERSCAN x YSTEPS x NPAT x SLICES x NREAD x NMCLEARS
Observation start and stop times are determined from the header information with a precision of +0.125 to -0.255 second, while exposure times are determined with a precision of 7.8125 microseconds. This has particular impact on certain RAPID mode timings as described in "RAPID Mode Observation Timing Uncertainties" on page 33-5.
The end time of the integration for each group of data is given in modified Julian days in the group parameter FPKTTIME. The end time of an observation is the FPKTTIME of the last group of the observation. Again, in many FOS datasets obtained before January 1, 1995, the EXPEND keyword is incorrectly populated with LPKTTIME rather than FPKTTIME of the last observation group. You should insure that the FPKTTIME of the last group has been used for EXPEND.
To calculate the approximate group start time, subtract the group elapsed time from FPKTTIME. The group elapsed time (which will be nearly the same for all groups in a given observation) can be calculated in two ways: 1) for datasets obtained after January 1, 1995: as FPKTTIME for the first group of data minus EXPSTART, or 2) for any dataset: from the formula given in the previous section. The start time of each subsequent group is then given as FPKTTIME for that group minus the group elapsed time (see Table 33.1).
As noted earlier, an additional uncertainty of approximately 0.13 seconds exists since the HST Science Data Formatter can be delayed in issuing the command to read out the FOS memory, which is recorded as FPKTTIME.
Exposure Times
The elapsed time for an observation differs from the exposure time (the actual integration time during which counts are accumulated) because the elapsed time includes the deadtime during which the FOS is doing housekeeping (e.g., reading out the diodes) and therefore not integrating. The total exposure time for the entire observation (all groups) is given by the keyword EXPTIME in the data header.
stevens@stsci.edu Copyright © 1997, Association of Universities for Research in Astronomy. All rights reserved. Last updated: 01/14/98 15:41:23