Overhead duration is part of the total time estimated by JWST's Astronomy Proposal Tool (APT). This page describes the components of overhead duration.
SAM time and guide star reacquisition
Small Angle Maneuvers (SAMs) take time to move from one pointing to another. The value of the SAM time is calculated from a lookup table in the PRD, visit_slew_table. This table accounts for very small moves which can be done using the Fine Steering Mirror (FSM) as well as those requiring observatory moves. FSM moves take no time to perform. There is a settle time associated with either a SAM or FSM move based on the distance. This value and the distance at which it applies come from the spacecraft_parameters table. Finally, there is also a time to reacquire lock on Guide Stars given the distance of the move, in the GS_dither_overhead table in the PRD.
SAM_TIME = Move_time + Settle_time + GS_Reacq_time
Target acquisition time
= 1440 (for TA METHOD = MSATA)
Scheduling duration is the sum of the science and overhead components of the duration, and is the time over which a visit must be schedulable. This is the expected time it will take to fully execute the visit as specified, including all science and engineering activities.
SCHEDULING_DURATION = SCIENCE_DURATION + OVERHEAD_DURATION
Direct scheduling overhead
This component represents an additional charge to the Observer due to the potential impacts of particular scheduling constraints on the overall scheduling and execution of JWST observations. Currently this only includes rapid turn around TOOs and tight timing . The limits on when to apply this overhead is stored in the PRD observing_limitations table. The value of the overhead to apply is in the timing_model_overheads table.
Direct Scheduling Overhead is not charged to Commissioning proposals.
= 3600 for tight timing windows (those less than one hour (3600 seconds))
= 1800 for rapid TOOs (those less than 3 days (1209600 seconds))
Observatory overheads is a statistical model of the amount of time that will be needed beyond planned science observations to account for observatory support activities, including calibration, station keeping, momentum management, safings and safe mode recovery. The value of the scaling factor to use is stored in the PRD timing_model_overheads table.
Observatory overheads are not charged to Commissioning proposals.
OBSERVATORY_OVERHEADS = (SCHEDULING_DURATION + SLEW_DURATION) * 0.16
This is the total charged duration for each visit in a program. The sum of the charged duration for all visits defined in the program must remain below the time allocated for that program. The initial allocation is based on the calculation of charged duration by APT. In the course of implementing an approved program, some changes may need to be made to the proposal, but only in so far as they continue to meet this requirement.
CHARGED_DURATION = SCHEDULING_DURATION + SLEW_DURATION + DIRECT_SCHEDULING_OVERHEAD + OBSERVATORY_OVERHEAD