This page describes the types of JWST observations and science cases that will likely have "Special Requirements" that must be justified in the proposal.

Observing programs that require the use of special requirements, especially those that affect timing and schedulability of observations, must include a justification of why the Special Requirements are necessary.

Common types of observations needing special requirements are described below. This list may not be exhaustive. Additional information regarding specifying special requirements in APT can be found on the APT Special Requirements page.

Time constrained observations

Time constrained observations with JWST are observations required to begin within a specified date and time interval, or specified phase for sources with known periods. They constrain the JWST schedule to an extent dependent on the length of the window for the start time.

Aperture position angle (orientation) constraints are functionally identical to time constraints, as they limit the possible observing windows and thereby affect the JWST schedule. Annual visibilities at a specific orientation can be limited to 10 days or less. The JWST Target Visibility Tools and/or JWST Moving Target Visibility Tools may be useful in determining these time constraints on a fixed orientation at a given date of observation.

Common types of time-constrained observations may include (but are not limited to):

• transits;
• Solar System targets;
• transient phenomena;
• mapping extended objects, where the orientation of the mapped area relative to the object matters;
• time-series observations;
• MIRI MRS dedicated sky background observations;
• background-limited observations;
• coordinated observations with other facilities (i.e., Joint Observing proposals).

Time critical observations

Time critical observations are those required to start within a constrained window that is less than 1 hour. 

Due to their impact on the schedule, time critical observations will incur an additional overhead of 1 hour per visit. (Observations with execution windows greater than or equal to 1 hour are not considered to have a significant impact on the scheduling, and therefore do not incur any additional overheads.) See JWST Observing Overheads and Time Accounting Overview for a description of accounting, including Smart Accounting, and overhead terms.

There are several kinds of time constrained observations that could be considered time critical in some way. Some scientific examples might include observations of specific phases of variable stars, many transiting exoplanet observations, and some Solar System observations.  

Observations of targets that have not yet been discovered or identified

Investigators may wish to propose for JWST observations of targets that have not yet been discovered or identified. With the exception of NIRSpec MOS observations that require pre-imaging, such proposals are generally allowed only if there is a certain time-criticality to the observations, where proposing for the same observations in the next regular review cycle (after the target has been discovered) would be impossible, would make the observations more difficult (e.g., the object fades rapidly, or its temporal behavior is important), or would lead to diminished scientific returns. Those criteria are generally satisfied for GO observations of ToO targets, but there may also be other circumstances in which proposals for such targets are justified. However, in the absence of demonstrated time-criticality, observations will generally not be approved for targets that have not yet been discovered or identified. Examples of targets that are not suitable for this type of proposal include color-selected galaxies, transiting exoplanets, or stars newly discovered in the course of an ongoing survey.

Solar System targets

JWST can observe most targets within our Solar System, although there are a few exceptions. The Sun, Earth, Mercury, Venus, and the Moon cannot be observed due to the orientation of JWST's sunshade. As moving targets, Solar System targets may have reduced periods of visibility as compared to fixed targets. In Cycle 6, the rate of motion for moving targets may not exceed 75 milliarcseconds per second. Proposers should consult the JDox Moving Target Field of Regard, JWST Moving Target Observations, and the JWST Moving Target Visibility Tool pages for additional information in planning this type of observation. Proposers must ensure that an ephemeris of sufficient accuracy is available for the appropriate epoch; observations that prove infeasible for technical reasons will be disallowed.

NIRSpec MSA-based observations of moving targets may only be proposed using the MOS Longslit observing method with the Wide Aperture Target Acquisition or Verify Only TA options. MSA-based Target Acquisition (MSATA) is not possible on moving targets. 

It is now possible to execute "shadow" observations for moving targets (i.e., observations of the background field at a later time after the science target has moved out of the field). See JWST Moving Target Acquisition and Tracking for more information.

Users will be able to visualize the track of a moving target over short periods of time against a sky image using the Aladin visualizer tool in APT.

Coordinated observations with other facilities 

Coordinated JWST observations with other observatories are by definition time constrained observations, which may or may not be time critical.

Linked subsequent observations (specified using the SEQUENCE OBSERVATIONS NON-INTERRUPTIBLE timing special requirement in APT) do not necessarily incur additional overheads, unless they are also specified as time critical visits with critical scheduling windows. Linked observations that are scheduled to occur within 4 hours of a previous observation will be considered time critical observations, incurring the additional overhead. Proposers can only request SEQ NON-INT with a clear scientific justification. Unjustified requests will not be permitted. 

Time-series observations

Proposals may request time constrained observations for a specific date or range of specific dates, when scientifically justified, and can be specified in APT with Timing Special Requirements. See JWST Time-Series Observations for planning monitoring sequences.

Target of Opportunity (ToO) observations

ToO observations are distinct from time constrained observations, but they still affect schedulability and hence almost always have special requirements. Proposers should carefully consult the information given in the ToO section of JWST Additional Flags for GO Proposals and JWST Target of Opportunity Observations.

Follow-up observations of JWST pre-imaging

Same-cycle follow-up spectroscopic observations of sources identified through JWST NIRCam imaging programs are permitted. For example, a proposal may request imaging with NIRCam as a means of identifying a specific type of target (e.g., high redshift galaxies) for subsequent spectroscopy with NIRSpec. The proposal must include the imaging observation defined in APT, and specify the expected number density and magnitude distribution in the anticipated discovery of new targets. 

Proposers should be aware of the minimum timeline for pre-imaging, and other restrictions, detailed in NIRSpec MOS and MSATA Observing Process and NIRSpec MOS Operations - Pre-Imaging Using NIRCam.

Next: JWST Filling Out the APT Proposal Form