A checklist of essential elements and special requirements for specifying JWST observations in APT is covered in this page. Observation specifications must be fully specified at the proposal stage, including all special requirements.

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JWST proposals are submitted via a single-stream process through Astronomer's Proposal Tool Observations and should be fully specified in APT at the proposal submission stage, including special requirements. (There are a few exceptions; survey programs, ToOs, and NIRSpec MOS observations requiring pre-imaging must be specified as completely as possible for a generic target or representative set of sources within the survey area.)

This process requires the proposer to carefully plan all observations at the beginning, paying special attention to observing best practices and including any observing special requirements, particularly the ones listed below. Any and all special requirements must be entered in the APT file and described in the "Special Requirements" section of the science PDF.

This article includes a checklist of commonly forgotten observing essentials and missed special requirements to help observers submit complete observations.

Please note that observations that are not completely specified may not pass technical review and may not be allocated any additional time or scheduling resources. Updating observations after program acceptance with additional exposures, target acquisition, dithers, backgrounds, scheduling constraints, or special requirements requires justification and must be approved by the Telescope Time Review Board (TTRB).

An observer is required to file a TTRB change request as soon as possible for any additional resources or special requirements that affect time allocation or scheduling. This requires strong technical and science justification, including a valid explanation of why the resources were not requested at the proposal stage, and may result in the loss of the original scheduling window, leading to rescheduling at a potentially less desirable time.

Specifications required for all observations

See also: Observing with JWST

The following tables give observation essentials that observers sometimes omit at the proposal stage and subsequently have to request from the TTRB. The first table lists elements that are common to all observing modes.

Table 1. Specifications for all observations

Observing element	Science requirement	When
Exposures	Adequate exposure times, avoiding saturation if possible	Always
Target acquisition	Pointing accuracy better than telescope blind pointing is desired	Target must be placed accurately in a spectroscopic aperture Target must be placed accurately with respect to a coronagraphic mask Target must be placed accurately in a time-series aperture Target coverage at field edges is important
Dithers	Dithers and/or nods to sample the PSF Mitigation of bad pixels or detector artifacts	Always for most instrument modes Almost never for time-series observations (except NIRISS TSO)
Backgrounds	In-scene or off-scene background measurements	Sometimes, depending on source/background contrast
PA constraints	Target coverage Maximizing number of targets acquired Mosaic coverage Bright object or diffraction spike avoidance Mitigating crowding or overlapping spectra	As needed
Explicit timing constraints	Account for source or background variability Capture a particular phase or location for a variable phenomenon Simultaneous or sequential observations with multiple instruments or telescopes	As needed for time variable sources or backgrounds Coordinated observations are planned
Solar System special requirements	Constrain object separation, distance, angular rate, radial velocity, apparent diameter, solar phase, or Galactic latitude Observe occultations, transits, or eclipses Constrain central meridian longitude	As needed. See Solar System Special Requirements
Background-limited requirement	Restrict scheduling to achieve low background	If source brightness is comparable to or fainter than the background
Pre-imaging	Ensure sufficient coordinate and proper motion accuracy for placement in a spectroscopic or coronagraphic aperture	Recent space-based imagery or coordinates and proper motion (e.g., JWST, HST, or Gaia ) are not available For high proper motion targets

Specifications by observing template

The following tables list crucial observing elements that are specific to particular observing modes for each Instrument.

MIRI

Table 2. Observing elements for MIRI

Mode	Observing element	Science requirement	When
MIRI imaging	Dithers	Improve PSF sampling Mitigate bad pixels	Always
MIRI imaging	Backgrounds	Measurement and removal of background	The source fills a large portion of the field of view
MIRI MRS	Backgrounds	Measurement of telescope thermal background at long wavelengths Detector characterization at short wavelengths	Almost always, except for bright point sources
MIRI MRS	PA constraints	Measure background away from an extended source Obtain simultaneous imaging at a particular location	Depending on source extent and morphology
MIRI LRS slit	Target acquisition	Target placement in slit	Almost always, except for off-source backgrounds or very extended sources
MIRI LRS slit	Dithers	Improve PSF sampling Mitigation of bad pixels	Always
MIRI LRS slit	Backgrounds	Background measurement and removal	Extended sources
MIRI LRS slitless	Target acquisition	Accurately measure wavelengths	Always, for accurate wavelength calibration
MIRI WFSS	Dithers	Improve PSF sampling Mitigation of bad pixels	Always
MIRI WFSS	PA constraints	Mitigate overlapping spectra	In crowded fields
MIRI coronagraphic imaging
MIRI coronagraphic imaging	Roll dithers	Increase contrast, remove instrumental artifacts and cosmic rays Discriminate between residual PSF artifacts and true signal	High contrast is important
MIRI coronagraphic imaging	Small grid dithers	PSF diversity for reference star	For PSF reference star only (not science target)
MIRI coronagraphic imaging	PA constraints	Avoid occulted or strongly attenuated regions, SCA gap, and/or neutral density squares	MIRI 4QPM is used
MIRI coronagraphic imaging	Sequence-Non-Interruptible special requirement	Optimize reference star PSF subtraction	High contrast is important
MIRI coronagraphic imaging	Backgrounds	Removal of stray scattered light	Always; scattered light is always present in MIRI coronagraphic exposures
MIRI coronagraphic imaging	Offset TA target pre-imaging	Optimize target acquisition centering	When using a TA target separate from the science target, and the TA target has poor positional or MIR photometric constraints

NIRCam

Table 3. Observing elements for NIRCam

Mode	Observing element	Science requirement	When
NIRCam SW (DHS) and LW grism time series	PA constraints	Avoid spectra from contaminating sources	Contaminating sources are present
NIRCam coronagraphic imaging
NIRCam coronagraphic imaging	Roll dithers	Increase contrast, remove instrumental artifacts and cosmic rays Discriminate between residual PSF artifacts and true signal	High contrast is important
NIRCam coronagraphic imaging	Small grid dithers	PSF diversity for reference star	For PSF reference star only (not science target)
NIRCam coronagraphic imaging	PA constraints	Avoid occulted or strongly attenuated regions, SCA gap, and/or neutral density squares	NIRCam bar occulter is used NIRCam MASK430R occulter is used Targets fall in chip gaps or neutral density squares
NIRCam coronagraphic imaging	Sequence-Non-Interruptible special requirement	Optimize reference star PSF subtraction	High contrast is important
NIRCam coronagraphic imaging	Offset special requirement	Optimize target acquisition centering	For all NIRCam occulters except MASK335R

NIRISS

Table 4. Observing elements for NIRISS

Mode	Observing element	Science requirement	When
NIRISS AMI	Dithers	Pixel phase sampling and reject artifacts	Strongly encouraged when high contrast is important
NIRISS imaging	Dithers	Improve PSF sampling and reject artifacts	Always
NIRISS SOSS	PA constraints	Minimize contamination by other sources in the field	Strongly encouraged to consider. See NIRISS SOSS Recommended Strategies
NIRISS WFSS	Dithers	Improve PSF sampling and reject artifacts; optimize field coverage of first-order spectra of sources in the direct images	Always; optimizing field coverage of first-order spectra is done using GRISM = BOTH option. See NIRISS WFSS Recommended Strategies

NIRSpec

Table 5. Observing elements for NIRSpec

Mode	Observing element	Science requirement	When
NIRSpec FS	Target acquisition	Accurately place target in a fixed slit	Always
NIRSpec IFU	PA constraints	Avoid very bright sources in MOS FOV that would generate significant leakage	As needed
NIRSpec IFU or MOS	Leakcals	Remove contamination from sources in the MOS field of view	In fields with many bright stars In fields with extended nebulosity or dust emission For very faint targets where background observations are otherwise impossible See NIRSpec MSA Leakage Subtraction Recommended Strategies
NIRSpec MOS	Target acquisition (MSATA)	Accurately place multiple targets in multiple slits.	Always. See NIRSpec MSA Target Acquisition
NIRSpec MOS	PA constraints	Increase the number of observed primary sources in the field Specify the angle range preferred for MOS observations of primary sources	As needed
NIRSpec MOS + FS	PA constraints	Enable simultaneous acquisition of MOS and FS targets.	Usually necessary for this combined mode

Notable updates
Originally published	16 Jul 2025

JWST User Documentation

JWST Observation Planning Essentials