Observing with JWST

This page is intended for observers with an accepted observing program. The processes and schedules for updating observing specifications, requesting changes to observations, program technical reviews by STScI staff, and telescope scheduling are outlined. It also describes how to request repeat observations if problems occur.

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Program updates

While there is no official "Phase II" for JWST observing specifications, observers may update their observations and resubmit them in APT for any number of reasons. Here are a few common changes that observers may wish to make:

  • Coordinates, proper motion, and ephemeris updates
  • Changes in target acquisition strategy
  • Changes to filters or dispersers that don't duplicate existing observations
  • Changes in dithers

Certain instrument modes and use cases require special attention because they cannot be completely specified in the original APT submission:

Each new APT submission will be reviewed by your program coordinator (PC) and instrument scientist(s), and when validated, will become the new observing specification.

Any changes should be completed as soon as possible, at least two weeks before the plan window start date, to avoid any delay in scheduling.

Late changes to JWST programs are discouraged, and may cause scheduling delays.


Program change requests

Program change requests (PCRs) should be filed via the "Request an observing change" link at the bottom of the JWST Program Information webpage for your program. The following types of changes generally require approval by the Telescope Time Review Board (TTRB) :

  • Changes that affect the science goals
  • Changes in science target—note that TTRB authorization is not necessary for changes in calibration star or reference star targets
  • Changes that require additional observing time—small requests for additional time (<5%) that are recommended by the PC or instrument scientist(s) for following best practices do not need to go through the TTRB
  • New constraints that affect observation scheduling

More guidelines are given in the JWST Observing Program Modification Policy and TTRB policies articles. Please consult your assigned program coordinator and instrument scientist(s) if you are unsure whether or not your contemplated change requires submission of a PCR.

Technical review

Observation scheduling is contingent on successful technical review by STScI staff, as stipulated in the acceptance letter. An instrument scientist will be assigned to your program soon after acceptance, one for each instrument utilized, to serve both as technical reviewer and as a resource for questions about observing best practices and specifications.

Initial technical reviews are typically provided at latest 4 weeks before your observation plan window start date (exception: 6 weeks for MOS). The instrument scientist will contact you by email with any recommendations or questions about your existing APT specifications. Please reach out to your PC or instrument scientist if you have any questions about your observing specifications. You may then make any program updates you feel are necessary, subject to the rules and procedures referenced above. After the instrument scientist signs off on the observation, the PC will validate the APT submission and the observation will be made ready for scheduling.

Observing best practices checklist

Certain observing specifications are critical for observation success and should always be checked by the observer before they submit their APT file. Improperly specified observing parameters can potentially lead to observation failure. If raised in technical review, these issues must be explained, addressed, or mitigated by the observer in order to pass technical review:

  • Accurate coordinates, coordinate epoch, proper motions, parallax, and ephemerides are necessary to properly point the telescope. 
    • Use values registered to stars measured by the Gaia survey, when possible.
    • Objects with high proper motion require special care in specifying coordinates, coordinate epoch, proper motion, and parallax. 
    • Do not assume that coordinates from a single reference are correct. Coordinates should be cross-checked against other independent references, when possible.
    • Accurate coordinate specification is the responsibility of the observer. Observation repeats will not be given in most cases for observations that fail because of errors in user-supplied coordinates.
  • Assess whether the blind pointing accuracy of the telescope is sufficient or if it is necessary to specify target acquisition to refine the telescope pointing.
  • In use cases where precision pointing matters, such as coronagraphy, slit spectroscopy, or multi-object spectroscopy, accurate coordinate specification may require pre-imaging observations with JWST or HST, depending on science goals. 
  • Target acquisition (TA), when utilized, requires accurate TA reference object coordinates, coordinate epoch, proper motion, and parallax. Appropriate TA subarray, filter, and exposure parameters for the TA target are necessary for successful target acquisition.
  • For science exposures, the correct filter or disperser element should be chosen to match the science goals. Please check that the wavelength of interest is captured and doesn't fall in the detector gaps or off the detector. Wavelength ranges for filters, gratings, and grisms are given in the JDox pages for each instrument.
  • Exposure parameters should be tuned to meet the desired signal to noise and avoid saturation, based on ETC calculations.
  • Choose appropriate constraints to avoid bright objects that may cause persistence, leakage, or other undesirable effects, if possible. Bright objects are generally not a safety concern for the telescope, but may compromise data quality in some cases.
  • Data excess may not exceed the upper limit. If a data excess error is displayed in APT, it must be corrected, for example, by changing the readout pattern to reduce the data volume. Data excess warnings should be mitigated, if possible.
  • Observations are scheduled to avoid the Micrometeroid Avoidance Zone (MAZ), when possible. Constraints that force scheduling in the MAZ must either be specified and explained in the original proposal or authorized by the TTRB.
  • Do not include unnecessary or overly tight constraints in your APT file that may make scheduling difficult.


Scheduling in any particular date range is not guaranteed. Roughly 4–6 weeks before the start of the next observing cycle, observations will be assigned plan windows that indicate the next observing opportunity. These plan windows sometimes change as the observing cycle progresses due to schedule updates. Scheduling may be altered in response to unanticipated events such as an instrument safing, targets of opportunity, etc. In order to facilitate scheduling, observers should be careful to not overly or unneccessarily constrain their observations. Please see the Program Information page for latest plan windows and observation status. You may also subscribe to be notified in MAST when data products are archived.

Observations are generally scheduled according to visibility. The schedule is also arranged in order to optimize observatory efficiency and to respect health and safety concerns. Any science-driven scheduling constraints must be specified in the APT file at proposal submission so they may be approved by the TAC. Additional constraints may be added as recommended by the instrument scientist, or if they do not have a big impact on the visibility window, as shown in the APT visit planner. Adding constraints that require the plan window to move by a lot, or that imply a specific observing window, must be approved by the TTRB via a program change request. 

How to report a problem and request a repeat observation

Scheduled observations that are skipped by the observatory, fail, or do not meet the science needs of observers may be repeated if certain conditions are met. A notification is emailed to the observer whenever an observation is skipped or does not run to completion. In general, visits that fail because of a telescope or instrument malfunction may be repeated on request, whereas visits that fail due to user error in designing the observations will not be repeated. Observations are never automatically repeated. Instead, the observer should submit a Webb Operations Problem Report (WOPR) within 90 days, via the "Report an observing problem" link at the bottom of the Program Information webpage for their program. The Telescope Time Review Board (TTRB) will deliberate on your request and email you a response, typically within 10 working days. Please see TTRB Policies for a list of issues that are eligible for a repeat, and those that are not.

Repeat observations granted by the TTRB must be specified in APT (typically by duplicating and updating the failed observation) and resubmitted. Repeat observations may or may not require additional technical review, depending on whether or not substantive changes are made. When repeat observations are granted, the original observations are made public with the understanding that they were not sufficient for the science of the program but may be useful for archival investigations.  TTRB-approved repeat observations will normally be scheduled according to the standard scheduling process. If there is time criticality to an observation repeat, this must be pointed out by the observer and strong science justification presented for TTRB consideration. 

Notable updates

Originally published