Pure Parallel Observations

JWST allows for separate proposals of only pure parallel observations, which are  specified via select observing templates available in the Astronomer’s Proposal Tool (APT).


Purple text indicates the parameter is Limited Access.

Boldface italics type indicates the name of an APT parameter or a value for a parameter.

(warning)Red text indicates restrictions on a parameter.

(red star) Black text indicates an important note.

Brown text indicates notes for the developers.

Green text indicates the name of the parameter used by Commanding.

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.

Science pure parallel proposals do not follow JWST's standard single stream process. In the initial proposal, only example observations are specified in APT. After the proposal is approved, Pure parallel observations are crafted by the observer based on their example observations and sized to fit the pure parallel slots assigned to them.

Calibration pure parallel observations are crafted by the scheduling system to meet the requirements of their example observations.

Pure parallel slots are defined by STScI at the beginning of the observing cycle based on the contents of the approved pool of proposals whose exposures will serve as the primary observations. Each slot corresponds to a single exposure (i.e. a single dither position). Approved pure parallel observations are paired with these pure parallel slots and are then scheduled simultaneously with and according to the plan windows of the original primary visits. The pure parallel observing mode is used for science as well as calibration observing.

Note that currently the pool of pure parallel slots are only created from observations using the following templates:

  • MIRI Imaging
  • MIRI LRS (with FULL subarray)
  • NIRCam Imaging
  • NIRISS Imaging
  • NIRSpec Fixed Slit
  • NIRSpec IFU
  • NIRSpec MOS

 Science Pure Parallel Proposals

To propose for science pure parallel observing, check the PURE PARALLEL PROPOSAL box on the Proposal Information form. Because pure parallel observations do not drive the pointing of the telescope, this will modify APT forms in the following ways:

  • The target section is removed from the proposal.
  • All templates selected will have the target automatically set to PARALLEL.
  • There are no options for dithering in the observing templates.

 Science Pure Parallel Observation Limitations

  • Currently only five templates can be proposed for science pure parallel observing: MIRI Imaging, NIRCam Imaging, NIRCam WFSS, NIRISS Imaging and NIRISS WFSS.
  • For NIRCam WFSS pure parallel proposals, direct image exposures are needed.  At least one direct imaging observation must be associated with each long filter used in the proposal. When DIRECT MODE is selected, a list of all grism observations is shown. Select all the grism observations that should be associated with that direct observation. For more information, see Filling out APT forms for pure parallel proposals.
  • As with NIRCam coordinated parallel observing, certain readout modes are restricted for pure parallel observing because of data rates. 
  • There can only be one exposure specification in a pure parallel observation. 
  • EXPOSURES/DITHER must be set to 1.
  • No special requirements may be specified for a science pure parallel observation.
  • Some NIRCam readout patterns cannot be used with parallel observations because of data rate limitations. 
    • RAPID and BRIGHT2 cannot be used at any time (NIRCam prime or parallel)
    • BRIGHT1 can only be used if MODULE is not ALL (NIRCam prime or parallel)

Approved Science Pure Parallel Programs

Once the TAC has approved one or more science pure parallel programs, there will be a process to divide up the available pure parallel slots between the approved science pure parallel programs and the expected needs of the calibration pure parallel suite of proposals.  Once a science pure parallel program is  approved and marked as such in the database the following changes will occur in the APT forms:

  • A new section called "Pure Parallel Slots" will be added to the proposal. This allows the user to review the slots assigned to their proposal, sort them in various ways and create parallel slot groups. The slots in a group are similar enough that a single observation definition can be used for all of them.
  • A new field called "Parallel Slot Group" will be added to the observations. This allows the user to select a parallel group. For each slot in the selected group a visit will be created using those observation parameters.
  • PARALLEL DURATION and SLOT DURATION are calculated by APT and displayed on the observation form. SLOT DURATION is the length of the shortest slot in the selected group, and PARALLEL DURATION is the exposure duration (including overheads) which must fit in the selected slots.

 Calibration Pure Parallels

Calibration Pure Parallels are calibration observations with the PARALLEL special requirement. Templates that allow (or require) the PARALLEL special requirement are listed in Parallel and No Parallel Special Requirement Rules.

Change log


  1. PROPINSTJWST-91641 Added PARALLEL DURATION and SLOT DURATION to Approved Science Pure Parallel Programs section.


  1. PROPINSTJWST-89956 Added point about NIRCAM WFSS Direct images to Science Pure Parallel Observation Limitations section.

November 16, 2022

  1. PROPINSTJWST-91525 Added link to Par/NoPar rules in Calibration Pure Parallels section.

Version 5 (July 13, 2021)

  1. PROPINSTJWST-89960 Updated list of allowed parallels and pure parallels in the Science Pure Parallel Observation Limitations section.

Version 4 (June 24, 2019)

PROPINSTJWST-91352 - Added BRIGHT2 limitation to Science Pure Parallel Observation Limitations section.

Version 3 (April 16, 2019)

  1. Editorial change (moved change log to main article).

Version 2 (June 19, 2018)

  1. PR 89938 - added NIRCam readout patterns to Limitations section

Version 1 (February 8, 2018)

This is the initial version.