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.
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; 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.
The expected audiences for this report are the members of the GO and GTO community, the STScI Proposal Planning and Scheduling team and the STScI Webb Instrument Team. Other STScI teams may find this report useful in optimizing the design of their components of the JWST system.
JWST Long Range Plan Scheduling
After launch and following a period of commissioning, JWST will begin the 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, will be 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.
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 10%; 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 observations.
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 may not be 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.
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.
Modifications to visits within an observation generally may not be requested once the observation has started to execute. Under exceptional circumstances, the Principal Investigator may request that unexecuted visits within a given observation are removed from the schedule.
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 may 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 10% 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.