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37.8 Observation Timing

 Knowledge of the exact time that observational activities take place onboard GHRS is not directly available. Instead, we only get timing information when data are dumped to the ground from the onboard computer from which we may infer when things happened. When data are dumped they are given time tags from the NSSC-1 computer; the spacecraft clock has a time resolution of approximately 0.125 seconds. While GHRS can operate on shorter time scales (0.050 seconds), information about these activities does not make it into the telemetry stream.

Nevertheless, as an observer, you may want answers to the timing questions posed in this section. You may find the hst_calib.obsum task helpful in determining timing questions.

37.8.1 When Did the Observation Start?

The closest time tag to the start of most exposures (excepting RAPIDs) is the packet time (PKTTIME) on the first group of the Unique Data Log (UDL), which is the data file with the .ulh extension (see the discussion on data structure in "Contents of Delivered Data" on page 35-1). A UDL is always dumped from the spacecraft prior to the start of a science exposure, effectively flagging the start of the observation. The PKTTIME keyword holds an MJD value good to a spacecraft clock tick (~0.125 sec). This MJD value (for ACCUM mode observations) is stored in the EXPSTART keyword in the science data headers (*.d0h, *.c1h). The accuracy of the start time is limited to the accuracy of the spacecraft clock-about 0.125 seconds.

For RAPID mode observations, the data are dumped as soon as they are taken. The PKTTIMEs represent the time the data left GHRS; calibrated data inherit the PKTTIMEs of the raw science data. The first two and last raw science packets (*.d0h) contain hysteresis information and are thrown away. This is why the calibrated data (*.c1h) will have three fewer groups than the raw science data. The best estimate of when a RAPID observation started is the PKTTIME of the first calibrated science packed (*.c1h[1]) minus the sample time of the observation, which, in this instance, can be found in the STEPTIME keyword. The accuracy in this case is half of a spacecraft clock tick (~0.0625 seconds). Unfortunately, the EXPSTART keyword in RAPID mode data appears to be incorrect.

37.8.2 When Did the Observation End?

For ACCUM mode observations, a second UDL is dumped at the end of the exposure and prior to reading out any science data. Therefore, the PKTTIME of the second group of the UDL can be used to mark the end of the exposure. For observations generating multiple readouts (e.g., FP-SPLITs and repeats) the UDLs come in pairs bracketing the science exposures. The MJD value of the second UDL in a pair should be used as the approximation of the ending of ACCUM mode observations. The accuracy is the same as the start time: about 0.125 seconds. Misleadingly, the MJD value of the final *.d0h PKTTIME is placed in the EXPEND keyword.

For RAPID mode observations, a second UDL is not dumped until the last spectrum, which is a hysteresis measurement, has been dumped. In this case, the end of the observation is merely the time that the last spectrum has been read out and is contained in the PKTTIME for last group of the calibrated spectrum. Note that the PKTTIME for the science data is the tag when the science data are dumped and this must wait until the end of the exposure. Unfortunately, the EXPEND keyword for RAPID mode data is close to being the time-tag of the second UDL packet.

37.8.3 How Long Did the Observation Last?

The extent of an observation is reported in the EXPTIME keyword. This time may not be the same as the simple difference between the EXPEND and EXPSTART. (See "Was the Observation Interrupted?" on page 37-31.) The exposure time in the header is simply the exposure time you requested in your proposal times the number of exposures. To verify that you got the exposure time you expected you can calculate the EXPTIME using available header keywords as shown below.

ACCUM mode

EXPTIME = (RPTOBS + 1) x (fpsplits) x MAXGSS x INFOB x STEPTIME

where,

RAPID mode

 EXPTIME = (groups) x STEPTIME

where:

37.8.4 What was the Exposure Time Per Pixel?

 The exposure per pixel is found in the EXPOSURE keyword in the calibrated flux header. This number is not equal to the EXPTIME because of the multiplicity of step pattern, FP-SPLITs and repeated observations.

This value is calculated by calhrs during pipeline calibration. To double check you may calculate EXPOSURE as follows:

EXPOSURE = INFOC x MAXGSS x (STEPTIME - 0.002)

where:

37.8.5 Was the Observation Interrupted?

 By design, the GHRS was interruptible: a given exposure may begin, be interrupted, and then resume. Observations were routinely interrupted for SAA passages and Earth occultations. The interruption could last anywhere from a few minutes, when skirting the SAA, to about half the orbit for an Earth occultation. In general, this is of no concern. Still, there may be times when you want to know the details of a given observation. Unfortunately, it is nearly impossible to determine when an observation was actually stopped and restarted-this information is just not available in the telemetry stream.

Additional details are available in the OMS observation log files as discussed in Appendix C of Volume I. It is also possible that a given observation may end prematurely. This information is encoded in the FINCODE keyword. We routinely see observations time out due to carrousel resets. If you have additional questions about interruptions, contact the STScI Help Desk via E-mail to help@stsci.edu.


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