Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

5.2 Exposure Time Considerations

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All COS exposure times must be an integer multiple of 0.1 seconds. If the observer specifies an exposure time that is not a multiple of 0.1 sec, its value is rounded down to the next lower integral multiple of 0.1 sec, (or set to 0.1 seconds if a smaller value is specified). The minimum COS exposure time duration is 0.1 seconds (but FLASH=YES TIME-TAG exposures impose a longer minimum; see FLASH section below). The maximum COS exposure time is 6,500 seconds. Bear in mind that exposure time values much larger than 3,000 seconds are normally appropriate only for visits with the CVZ special requirement. See the HST Primer for information about HST's orbit and visibility periods.

If an exposure specifies FP-POS=AUTO, then the valid range for the exposure time is 0.4 to 26,000.0 seconds. This is because the exposure time you enter specifies the total time, and the exposure time for each individual sub-exposure of the four implied by FP-POS=AUTO must be at least 0.1 sec. In other words, if FP-POS=AUTO, the Time_per_Exposure is divided equally among the four FP-POS offset exposures.

For TIME-TAG exposures with BUFFER-TIME < 110 seconds, photon events may be generated faster than data can be transferred out of the buffer during the exposure. In this case, Time_Per_Exposure should be less than or equal to 2 × BUFFER-TIME so that the exposure can complete before data transfer is necessary. A BUFFER-TIME of 110 seconds corresponds to an average count rate of ~21,000 counts/sec.

For target=WAVE exposures, DEF must be entered as the exposure time and the appropriate value for the optical configuration will be chosen from a table that is established at STScI for best performance.