JWST Observing Overheads Summary

Various overheads are charged to JWST observing programs: slews, instrument overheads, observatory overheads, and direct scheduling overheads. 

The Astronomers Proposal Tool, APT, charges users time and overheads for their observing programs. These overheads may be viewed in APT at the proposal level, the visit level, and in the exported times report. The total charged time consists of the following components. Terms in bold are as given in APT:

  • Science observing time
  • Slew to begin each visit
  • Instrument Overheads consist of the following:
    • SAMs: small angle maneuvers for dithers, mosaics, target acquisition, etc.
    • GS Acq: guide star acquisition(s)
    • Targ Acq: target acquisition, if any
    • Exp Ovhd: some instruments require an initial reset
    • Mech: mechanism movements, including filter wheels
    • OSS: Onboard Script System compilation
    • Visit Ovhd: visit cleanup activities 
    • MSANIRSpec MSA configuration
    • IRS2NIRSpec IRS2 Detector Readout Mode setup
  • Observatory Overheads = 16% of the total of everything above. This charged time supports observatory activities including calibration, station keeping, and momentum management.
  • Direct Scheduling Overheads, if any, for very tight timing constraints or rapid turnaround Targets Of Opportunity (TOOs).

For further explanation, see the JWST Observing Overheads and Time Accounting Overview.

For quantitative details on slews and SAMs, see JWST Slew Times and Overheads.

For a brief overview of instrument overheads, see JWST Instrument Overheads.

For detailed overhead analyses of example programs, see Observing Overheads for NIRCam Imaging.

For complete quantitative details of all JWST overheads, see Visit Overheads Timing Model.



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