5.3 Valid Exposure Times

The minimum COS exposure duration is 0.1 s (but FLASH=YES TIME-TAG exposures impose a longer minimum; see Section 5.7.1), and this is enforced by APT. The maximum is 6500 s. Between these extremes COS exposure times must be integer multiples of 0.1 s. If the observer specifies an exposure time that is not a multiple of 0.1 s, its value is rounded down to the next lower integer multiple of 0.1 s, (or set to 0.1 s if a smaller value is specified). Exposure times larger than about 3000 s are normally appropriate only for visits with the Continuous Viewing Zone (which is a special requirement that must be requested and justified in the Phase I proposal) because the visibility period of a typical orbit is ~50 minutes. See the HST Primer for information about HSTʹs orbit and visibility periods.

For TARGET=WAVE exposures, DEF (default) must be entered as the exposure time. The value appropriate for the optical configuration will be chosen from a table established at STScI for best performance. At present TARGET=WAVE flash durations are identical to those given in Table 5.2.

Table 5.2: TAGFLASH Exposure Durations.