The default operating mode of the STIS/MAMA detector is the ACCUM mode that provides a time-integrated image or spectrum of the target of interest. By contrast, the TIME-TAG mode records, as a stream of events, the position and arrival time of each photon on the detector. The time-tag data stream is also integrated in time to produce an uncalibrated ACCUM mode image that is then passed through standard calibration.
Opting for TIME-TAG observations with STIS/MAMA has several advantages and we would like to encourage all PIs of STIS/MAMA Cycle 22 programs to use the TIME-TAG mode for their observations when possible (please see below for count rate limitations). Owing to its higher cadence (0.125 millisecs for STIS vs. 32 millisecs for COS) and lower sensitivity, STIS may be able to observe targets in TIME-TAG mode that are too bright for TIME-TAG observations with COS. TIME-TAG mode also offers the possibility to reduce the effects of geocoronal emission by excluding, during post-processing, badly affected data from the final co-addition. Most importantly, it allows for time-resolved analysis of the target of interest, opening the time-domain window and increasing the legacy value of the observations (see example in Section 12.3 of the STIS Instrument Handbook).
There are some count rate limitations on the use of STIS TIME-TAG mode. Observations that exceed these limits should normally be done using ACCUM mode.
- TIME-TAG observations should generally be limited to count rates below 20,000 c/s for exposures longer than 2x99s, to avoid discarding data, since the minimum buffer dump cadence is 99s. Modestly higher count rates may sometimes be supported for very short observations. See Section 11.1.3 of the STIS IHB for details.
- If more than 30 total buffer dumps are required in any single visit, it may become difficult or impossible to schedule, and so, unless the use of TIME-TAG mode is essential for the science goals, the user should reduce the TIME-TAG usage to keep the total number of dumps from both TIME-TAG and ACCUM observations under this limit. If there is a requirement for more than 30 buffer dumps in a single visit, the observer should consult this with their PC and CS to see if this can be accommodated.
When opting for the TIME-TAG mode, it is important to check that the observations do not violate any TIME-TAG specific count rate or data volume constraints. In particular, one needs to pay special attention to the buffer management and to the choice of the BUFFER-TIME parameter. Details of the MAMA TIME-TAG mode of operations and guidelines to compute the BUFFER-TIME are provided in Section 11.1.3 of the STIS Instrument Handbook. If you have any questions please contact your assigned Contact Scientist.