Visit Timing

Visit timing is part of the total time estimated by JWST's Astronomer’s Proposal Tool (APT). This page describes the components of visit timing.

General visit overhead

 Visit overhead is described in Table 1.

Table 1. Visit overhead time

ActivityTime (seconds)Comment
OPE constraint scripts20
Disable fine guide30if visit has fine guiding

The following templates have additions to visit overheads:

Note that mechanism moves performed at the start or end of the visit are included in visit overheads but are not identified as mechanism overheads in the APT times report.

Visit timing model

Several intermediate values for visit timing need to be reported in order to support other systems and user interest. These include:

Figure 1. Visit timing model
Visit timing model

Science duration

Science duration is defined as the sum of the Photon Collecting Time of the exposures in the visit. This value is based on the instrument mode, subarray, readout pattern, number of integrations, and number of groups, as defined in the Proposal Instructions, Appendix D. It does not include the internal exposure overheads of the Frame Synchronization, Frame Resets, and Exposure Setup and Clean up. Those values are considered Exposure Overhead (see Exposure Timing).


Initial overhead    

The Initial Overhead is the time between the end of the visit slew activity and the start of the first Science Activity. It includes the Guide Star Acquisition, Target Acquisition, and the initial movement of any filters, pupils, or other mechanisms. This time is used when setting scheduling windows to allow timing requirements to specify a time when science activities will start.

INITIAL_OVERHEAD = GS_ACQ_TIME + TARGET_ACQ_TIME + [FILTER_MOVE_TIME, if first science filter is not = TA filter]

Table 2. Initial overhead

ActivityTime (seconds)Comment
OPE constraint scripts20 
GS Acquisition284if visit has fine quiding
Target Acq[1]300if visit has TA

 Certain templates have additions to initial overheads (see Instrument Specific Overheads).

Overhead duration

(also see Overhead Duration Components)

Overhead duration is meant to account for all of the direct, non-science activities that occur in the visit to support the exposures. As such, it includes the initial overhead, plus all the additional overheads for mechanism movements, pointing changes (SAMs), and exposure overhead from setup, cleanup, and frame resets noted above.


Table 3. Overhead duration

ActivityTime (seconds)Comment
Disable fine guide30 if visit has fine guiding


  1. This is also called Instrument Overheads in the APT GUI and Timing report.
  2. #Pointings may be 1 more than the number of Dithers if the Template has a Target Acquisition and the final pointing of the TA is not the same as the first science pointing.
  3. If two pointings in succession are equal then there will be no SAM for that pointing change.
  4. Some templates have multiple filters or filter/grating combinations and will get 2*FILTER_MOVE_TIME. This includes NIRCAM, where it is possible for the wheels to move simultaneously, and therefore not actually require that time.

Guide star acquisition time

= 0 if an Internal template is used
= 284 otherwise

Guide star acquisition time is calculated using three different values (see Table 4).

GS_ACQ_TIME = AD + (AD + SD) * FP * (1 + FP) = 240 + 300 * 0.13 * 1.13 = 284.07 (rounded down to 284s in APT)

Table 4. Components of guide star acquisition time

attempt duration (AD)[2]240s
slew duration (SD)[3]60sInitial visit in a scheduling set
failure probability (FP)0.13Failure probability = 1 - guide_star_acquisition_probability[4]

Slew duration

(also see Slew times)

APT uses a conservative model for the slew duration of a visit when it cannot determine how far it is likely a given visit will need to slew to start. This comes from the PRD, timing_model_overheads table. However, additional visits within an observation, and some observations which can be scheduled in near proximity can have their expected slew times calculated more accurately based on the known position of other visits. APT performs an analysis of an entire program to determine the expected slew time of the visits within it. This process is called Smart Accounting. Smart Accounting seeks to identify the likelihood that any particular observations in a program can be scheduled together. APT will then assign either the default initial slew time for an independent visit, or will calculate the slew time by looking up values from the PRD visit_slew_table.

= 0 for unpointed visits
= initial slew duration[5] from PRD for initial visit in a scheduling set
= derived from PRD visit_slew table[6] otherwise


1. base_target_acq_duration: PRD timing_model_parameters.xml (link to PRD)

2. guide_star_acquisition_attempt_duration: PRD spacecraft_parameters.xml (link to PRD)

3. guide_star_acquisition_slew_duration: PRD spacecraft_parameters.xml (link to PRD)

4.  guide_star_acquisition_probability: PRD spacecraft_parameters.xml (link to PRD)

5. initial_slew_duration: PRD spacecraft_parameters.xml (link to PRD)

6. visit_slew.xml: PRD (link to PRD)

Change log

Version 4 (December 4, 2018)

  1. Editorial - fixed some bad links and references.

Version 3 (July 13, 2018)

  1. PR 90792 - updated value of failure probability and GS acquisition time

Version 2 (May 2, 2018)

  1. PR 90157 - update overheads for guide star acquisition. Added footnotes to document values from PRD.

Version 1 (June 6, 2017)

  1. Initial version.