|Space Telescope Science Institute|
|STIS Instrument Handbook|
• Take a series of multiple, short, identical repeated observations (described in Section 11.2.4) of your target in ACCUM operating mode with either the CCD or the MAMAs.Both the data products received (an event stream in the case of TIME-TAG and a series of individual images from each ACCUM mode exposure in a repeated sequence) and the basic parameters of the time-resolved observations (e.g., sample time, interval between samples, total number of samples or equivalently duration) differ dramatically in these modes. In Table 12.1 we summarize and contrast the ranges of parameter space covered by the different methods of obtaining time-resolved observations.The information presented in Table 12.1 can be summarized into the following set of guidelines for performing time-resolved observations with STIS:
• If you wish to observe variability on second-or-less timescales, observe in the UV using TIME-TAG mode. Figure 12.3 shows an example of a time profile of the Crab Pulsar obtained with STIS G140L in TIME-TAG mode overlaid on prior HSP observations.
• In the UV, variability on the several-minute or more timescale can be observed by multiple ACCUM mode exposures with the MAMAs or using TIME-TAG (subject to count rate limitations).
• Single MAMA visits are limited to five orbits (see Chapter 2) so continuous variability can be tracked for only that duration with the MAMAs.
Total Duration of
Uninterrupted Time Series1
full frame: 45 second
1060 × 32 subarray: 20 second
For τ > 3 minutes, no limit
For τ < 3 minutes:
dur = (τ+Δτ) × 7, full frame (1062×1044)
dur = (τ+Δτ) × 256, 1060×32 subarray
30 second for τ > 3 minutes
2 minutes for τ < 3 minutes
125 μsecond 6.0 ×107 / R seconds if R < 21,000 counts/s
4.0 ×106 / R sec if R > 21,000 counts/s
where R is rate in counts/s,τ = duration of an individual exposure.Note, with BINAXIS1=BINAXIS2=2, 7 images separated by 1 minute and each of duration τ < 3 minutes can be obtained.
Figure 12.3: Crab Pulsar Observed Using the STIS FUV-MAMA with G140L in TIME-TAG Mode.The resulting integrated time profile is shown superimposed on the early results from the HSP. The full period of the Crab pulsar is 33 milliseconds. (Figure courtesy of Don Lindler and Ted Gull, see also Gull et al. 1998, ApJ, 495, L51).