Time Constrained Observations
Time constrained observations with JWST are observations required to begin within a specified date and time interval, or specified phase for sources with known periods. They constrain the JWST schedule to an extent dependent on the length of the window for the start time.
Time critical observations are those required to start within a constrained window that is less than 1 hour. Due to their impact on the schedule, time critical observations will incur an additional overhead of 1 hour per visit. Observations with execution windows greater than or equal to 1 hour are not considered to have a significant impact on the scheduling, and therefore do not incur any additional overheads. See JWST Observing Overheads and Time Accounting Overview for a description of accounting, including Smart Accounting, and overhead terms.
There are several kinds of time constrained observations that could be considered time critical in some way. Some scientific examples might include observations of specific phases of variable stars, many transiting exoplanet observations, and some solar system observations. Observations that require a particular telescope orientation (or position angle) are implicitly time constrained; annual visibilities at a specific orientation are typically limited to 10 days or less. The JWST Target Visibility Tools and/or JWST Moving Target Visibility Tools may be useful in determining these time constraints on a fixed orientation at a given date of observation.
Coordinated JWST observations with other observatories are by definition time constrained observations, which may or may not be time critical. Linked subsequent observations (specified using the SEQUENCE OBSERVATIONS NON-INTERRUPTIBLE timing special requirement in APT) do not necessarily incur additional overheads, unless they are also specified as time critical visits with critical scheduling windows. Linked observations that are scheduled to occur within 4 hours of a previous observation will be considered time critical observations, incurring the additional overhead. Proposers can only request SEQ NON-INT with a clear scientific justification. Unjustified requests will not be permitted.
Proposals may request time constrained observations for a specific date or range of specific dates, when scientifically justified, and can be specified in APT with Timing Special Requirements. See JWST Time-Series Observations for planning monitoring sequences.