JWST Observation Types
There are various types of observations that JWST proposers can request.
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Primary observations are classically targeted observations, which determine the telescope pointing, orientation, and scheduling. There other types of observations that require additional considerations in planning and scheduling. These include parallel observations (coordinated or as pure parallel), time constrained or critical observations, observations of Solar System targets, target-of-opportunity observations (disruptive or non-disruptive), follow-up of targets from JWST pre-imaging, and observations of targets that have not yet been discovered or identified.
Many of these observation types are described with roadmaps and examples on Methods and Roadmaps.
In general, observing programs that require the use of special requirements, especially those that affect timing and schedulability of observations, must include a scientific justification of why the Special Requirements are necessary.
Target-of-Opportunity (ToO) Observations
A target for JWST observation is deemed a Target of Opportunity (ToO) if it is associated with an event that may occur at an unknown time, and in this way ToOs are distinct from time constrained observations.
ToO targets include objects that can be identified in advance, but which undergo unpredictable changes (e.g., some dwarf novae), as well as objects that can only be identified in advance by class (e.g., novae, supernovae, gamma ray bursts, newly discovered comets, etc.). ToOs are generally not suitable for observations of periodic phenomena (e.g., eclipsing binary stars, transiting planets, etc.). ToO proposals must provide a clear definition of the trigger criteria and present a detailed plan for the observations to be performed in the technical justification of the PDF submission if the triggering event occurs. A ToO activation may consist of a single observation or of a set of observations executed with a pre-specified cadence.
ToO response times are specified in the APT Special Requirements. The minimum turn-around time for Non-disruptive ToO activation, without significant impact to the schedule, is 14 days. The minimum turn-around time for Disruptive ToO activation is 48 hours, measured from the time when the activation request is submitted to start of the first observation. Disruptive ToOs can be triggered with turn-around times less than 14 days, provided all of the proposal details (except possibly the precise target position) are available in advance. However, since disruptive ToO observations have a significant impact on the JWST schedule, each cycle (including Cycle 2) will be restricted to a total of 8 disruptive activations. Moreover, due to their scheduling impact, Disruptive ToOs required to be triggered within 3 days will incur an additional overhead of 30 minutes per activation. There is no limit on the number of Non-distruptive ToOs per cycle.
Information on activating an approved target of opportunity program is in JWST Target of Opportunity Program Activation.
Proposers may apply for Long-term status for only Non-disruptive ToO Programs (for up to three cycles) only if the target phenomena have a low probability of occurrence during one cycle. The request must be justified in the APT Special Requirements.
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 2 for moving targets, the rate of motion may not exceed 30 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.
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. 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.
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 (see Follow-up Observations of JWST 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 or 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, and 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.
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.
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.
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 that require a particular telescope orientation (or position angle) are implicitly time constrained; annual visibilities at a specific orientation are typically 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.
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.
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.
Parallel observing refers to simultaneously operating more than a single science instrument (limited to two instruments for Cycle 2). For JWST proposals, there will be two basic modes of parallel operations: coordinated parallels and pure parallels. Further information with examples, roadmaps, and templates are provided on JWST Parallel Observations. Related policy can be found on JWST Science Parallel Observation Policies and Guidelines
Coordinated Parallel Observations
Coordinated science parallel observations are those in which simultaneous observations may be made with instruments other than the primary instrument. Coordinated science parallel observations must have science goals that support or complement the prime science programs, and must be explicitly justified in the proposal. In Cycle 2, the following coordinated parallel modes will be supported:
NIRCam Imaging and MIRI Imaging,
NIRCam imaging and NIRISS Wide-Field Slitless Spectroscopy (WFSS),
NIRCam imaging and NIRISS Imaging (NIRCam must be the prime instrument),
NIRCam imaging and NIRSpec MOS (NIRSpec must be the prime instrument),
MIRI imaging and NIRISS WFSS,
- NIRCam WFSS and MIRI Imaging (NIRCam must be the prime instrument),
- NIRCam WFSS and NIRISS Imaging (NIRCam must be the prime instrument),
- NIRSpec MOS and MIRI Imaging (NIRSpec must be prime instrument).
Only direct imaging with standard narrow, medium, or broad band filters is allowed for NIRCam and MIRI observations in these coordinated parallel modes. Additional instrument combinations may be available in future cycles.
Pure Parallel Observations
Pure-parallel observations utilize instruments other than the primary instrument on observations from unrelated proposals. Unlike coordinated parallels, pure parallel observations are proposed as entirely separate programs of investigation. Pure parallels use parallel observing slots created by observations of programs that do not use coordinated parallels. Pure parallel observations will not be allowed to influence the dither patterns or other aspects of the observing strategy of the primary observations to which they are attached, since the primary observations will belong to entirely separate science proposals.
Pure-parallel programs may propose for observations with NIRCam imaging, NIRISS imaging and WFSS, and MIRI imaging (NIRSpec is not allowed as the parallel instrument). For accepted programs, the observations will be paired with suitable prime observations by GO, GTO and ERS programs. A number of prime templates cannot have pure parallels attached to them, including MIRI Coronagraphic Imaging, NIRCam Coronagraphic Imaging, NIRCam Time Series, NIRCam Grism Time Series, NIRISS Single Object Slitless Spectroscopy, NIRISS Aperture Masking Interferometry, and NIRSpec Bright Object Time Series. Please check JWST Parallel Observations for the full list of template combinations that are allowed and expected to be available for use in Cycle 2. We anticipate that pure parallel opportunities with durations from ~100 seconds to several thousand seconds will be available; the number of such opportunities will not be known until the Cycle 2 GO program is selected. Note that observatory activities such as calibration observations for the instruments will take priority in the assignment of available pure parallel slots.
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