JWST Observing Program Modification Policy

JWST observing programs may be modified or refined to better achieve approved science goals, or to accommodate changes in instrument performance. All modifications to observing programs must be approved by STScI.

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About modifications

Modifications may be requested to refine programs to better achieve the approved science goals, and modifications may also be required to accommodate changes in instrument performance with respect to that expected at the time of the proposal submission. Minor modifications to observations may be made in consultation with the program coordinator and/or instrument scientist. Major modifications to observations are requested by the Principal Investigator submitting a Program Change Request (PCR). PCRs for GO programs must be submitted to the Telescope Time Review Board (TTRB); PCRs for GTO programs must be submitted to the STScI Director. Modifications may be permitted if they are scientifically justified and have minimal impact on the overall observing schedule. All modifications to observing programs must receive explicit approval by the STScI Director or their official designee.

Telescope time allocations are based on TAC recommendations for specific science programs. If the science goals of those programs can be achieved in a more efficient manner, the residual telescope time is returned to the observatory.

JWST long range plan scheduling

After launch on December 25 2021 and following a period of commissioning, JWST began execution of science observations for GO, GTO, and DD programs. Each observation will be scheduled as one or more visits for a specific astronomical target, which after constraints are set, is laid out on the JWST Long Range Plan (LRP); combining visits to maximize the observing efficiency of the observatory, while minimizing the build-up of angular momentum.  Modifications to the science duration (sum of exposure times) or the overhead duration (sum of acquisitions, motions of components, and detector reads) of a given visit may change its scheduling duration and therefore impact the ability to schedule the visit efficiently on the LRP. 

Modification principles

Principal Investigators of GO, GTO and DD programs may request modifications to their program to maximize the scientific value of an approved observation. Additionally, deviations in on-orbit performance with respect to expectations may require modifications to approved visits.  Such modifications may impact either or both the science case and the ability to schedule observations in the LRP; they are therefore subject to review following the procedures outlined in this document. The guiding principles underlying these procedures are:   

  • All programs executed by JWST are reviewed and approved through an appropriate mechanism.  The process described below is intended to ensure that the modified program, as executed, maintains the original science goals, avoids unnecessary duplicate observations and does not have a disproportionate impact on the LRP.
  • In general, modifications to GO and DD programs requested by the PI for scientific reasons shall be such that the program stays within the originally-allocated total charged duration assigned to the program; modifications to GTO programs are limited by the total time allocated to the appropriate PI.
  • In general, modifications requested to GO and DD programs to mitigate changes in instrument/telescope performance may not increase the charged duration of a program by more than 5%; modifications to GTO programs are limited by the total time allocated to the appropriate PI. 
  • In general, for all types of proposal, changes introduced by differences between the APT timing model used to compile the proposals and the actual execution times will be absorbed by the Observatory.

Modifications requested by the PI for scientific reasons

The Principal Investigator, according to the precepts and schedule outlined below, can request modifications to an approved visit or visits within a program. All requests for modifications of observations will be subject to review by the JWST Telescope Time Review Board (TTRB) and the JWST Science Policies Group. There are two types of modification:

Minor modifications to observations consist of small changes in the observational parameters, such as adjustments of a few arcseconds in celestial coordinates, optical element changes or revisions in the distribution of science exposure times. Minor modifications might also include changes in instrumental parameters (e.g. sampling sequences) provided that those changes do not alter the scientific content of the original program. Any such modification may not result in the duplication of an approved observation.

Requests for minor modifications may be made in consultation with the program coordinator or instrument scientist. In general, these requests will be communicated to the JWST Science Policies Group but will not require review by the TTRB. Minor modifications may neither increase the total duration of the program nor impact the scheduling of observations; if those criteria are not met, the requested changes will be reviewed as major modifications. Minor modifications to observations are generally not permitted within 20 days of the scheduled date of observations.

Major modifications to observations consist of changes that would either substantially alter the scientific content or intent of observations, or have an impact on scheduling within the LRP. Examples of major changes include: 

  • Changing the instrumental mode for an observation (e.g. from NIRCam Imaging to MIRI Imaging);
  • Changing the target coordinates, or boundary area, by an astronomically significant amount;
  • Changing the target to a different astronomical source judged by the investigator to be scientifically equivalent to the original target;
  • Changes to any parameters that may have a major impact on schedulability, including, but not limited to, revised orientations, additional links to other observations and significant re-balancing of time among observations within the same program.
  • Changes from MAZ-compliant to non-MAZ-compliant scheduling.
  • adding a Position Angle (PA) constraint resulting in a change in the scheduling windows. 
  • adding exposure time to compensate for the decline in MIRI MRS sensitivity. 
  • adding exposure time for MIRI background observations to calibrate detector “glowsticks."
  • adding timing constraints such as BETWEEN, BEFORE, or AFTER. 
  • adding NIRSpec MSA targets outside of the approved pre-imaging area.

Common Examples of Minor Changes (not requiring TTRB review):

  • adding a Position Angle (PA) constraint without a change in the scheduling windows. 
  • moving the location of a nearby background observation.
  • changes in target acquisition (TA) strategy without an increase in allocated time.
  • filter change (provided there are no changes to the science goals)
  • switch of PSF reference star (provided there are no changes to the scheduling windows)

The execution of an approved observation may become infeasible or prove to be scientifically of limited value due to unanticipated circumstances.  If such events occur, and if an observer can a priori demonstrate that the approved visit will yield minimally useful data, the Principal Investigator can submit a request to make major modifications to the program.  The proposed modifications must be consistent with the original scientific intent of the approved observation and the observing time granted.  In addition, the revised observation may not duplicate any other approved observation without an appropriate justification, and must be approved by the STScI Director.

Proposers requesting target changes are generally limited to targets that were known at the time of the original proposal submission. Exceptions may be made for NIRSpec MSA observations based on JWST pre-imaging. Other mechanisms will be provided for observations of newly discovered targets. GTO proposers who eliminate observations of a target from their current cycle program after the GO program for that cycle is selected may not add observations of that target to their list of protected observations for future cycles.

Under exceptional circumstances, the Principal Investigator may request that unexecuted visits within a given observation are removed from the schedule. Technical considerations may make their removal infeasible, in which case the observations will be executed and the program charged accordingly.

All requests for major modifications of approved observing programs must be submitted as Program Change Requests for review by the JWST TTRB, and must be accompanied by an adequate justification. Changes in target will require particularly strong justification and may not be permitted during the period when new proposals are being solicited or are under review. The TTRB will make a recommendation to the JWST Science Policies Group lead and/or the STScI Director. Modifications are contingent upon approval by the STScI Director, or designee. Once the request for a modification is approved, the Principal Investigator may modify the visit/program. Major modifications to visits are generally not be permitted within 20 days of the scheduled date of observations.

Modifications due to changes in instrumental performance

Circumstances may arise where the actual performance of a JWST instrumental mode differs significantly from that anticipated at the time of acceptance of a GO, GTO or DD proposal. In such cases, Principal Investigators will have an opportunity to modify program visits to accommodate the performance changes. Proposers may choose to re-balance the time allocated within the proposal to different instrumental modes; however, the revised program must be consistent with the scientific goals approved for the original proposal. Proposers may request increased observing time for specific instrument modes, but the increase in the total time charged to GO and DD proposals must be less than 5% of the total time allocated to that program. GTO proposers have more flexibility in re-allocated time to different observing modes, but may not make changes that exceed the total time allocated to the individual Principal Investigator.

Modifications for instrumental performance should be requested by submitting a Program Change Request. All such requests for GO and DD proposals will be reviewed by the TTRB, whose recommendations will be passed on to the JWST Science Policies group and the STScI Director or designee for final approval. Requests for changes to GTO proposals will be subject to approval by the STScI Director. Modifications to visits may not be permitted within 20 days of the scheduled date of observations.

Observers may also be required to modify programs due to the unavailability of a subset of the instrumental modes requested by that program. Those programs will be dealt with on a case by case basis, but under most circumstances any additional overhead (for example, due to additional target slews) will likely not be charged to the program.

Addition of co-Is not included in the original proposal

STScI understands that there are many reasons why a co-I might not have been included when a proposal is first submitted and permits investigators to add additional collaborators after the proposal is accepted. This addition requires a justification that needs to be reasonable and scientifically motivated (examples: a new graduate student who has been tasked with working on the observations, an admin PI change if the PI moves institutions, etc.) and that will be reviewed by the Science Policies Group for approval.
If you need to add a co-I, please submit a Helpdesk ticket to the JWST Helpdesk (use the one labeled for JWST Science Policies) with the name and contact information of the requested co-I, as well as a justification for the requested addition.

Related links

JWST Cycle 1 Proposal Opportunities

JWST Cycle 1 Guaranteed Time Observations Call for Proposals

Notable updates
    Clarified when TTRB is required with examples. Changed limit from 10% to 5%. Added MAZ-compliance related aspects.

  • clarified policy on GTO changes to observations of targets.
Originally published