Coordinated Parallel Observations
JWST allows coordinated parallel observations, which are specified via select observing templates available in the Astronomer’s Proposal Tool (APT).
Introduction
Purple text indicates the parameter is Limited Access.
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Items in brackets - <value> - are required values.
Items in square brackets - [<value>] - are optional.
A coordinated parallel observation consists of a Prime observation (which controls the pointing of the telescope) and a Parallel observation (which will be obtained at the same time as the prime). There are only certain combinations of Prime and Parallel observations supported, and these are listed in Table 1.
Table 1. Allowed combinations of Prime and Parallel Observations.
| Prime Template | Coordinated Parallel Template |
|---|---|
| MIRI Imaging | NIRCam Imaging NIRISS Wide Field Slitless Spectroscopy * FGS External Calibration |
| MIRI MRS | * FGS External Calibration |
| NIRCam Imaging | MIRI Imaging NIRISS Imaging eNIRISS Wide Field Slitless Spectroscopy |
| NIRCam Wide Field Slitless Spectroscopy | NIRISS Imaging MIRI Imaging |
| NIRCam Engineering Imaging | * FGS External Calibration |
| NIRSpec Imaging | * FGS External Calibration |
| NIRSpec Multi-Object Spectroscopy | MIRI Imaging NIRCam Imaging |
| NIRISS Wide Field Slitless Spectroscopy | MIRI Imaging NIRCam Imaging |
| NIRISS External Calibration | * FGS External Calibration |
| * available only when CATEGORY= CAL, COM, or ENG | |
To specify a coordinated parallel observation, check the Coordinated Parallel box and then select the template for the Parallel observation. This will add a second template tab so you can fill in the parameters for the coordinated parallel observation. The Prime observation template will offer additional dithers (see Tables 2a, 2b, 2c, and 2d for MIRI, NIRCam, NIRSpec, and NIRISS prime, respectively) optimized for that coordinated parallel template pairing, while the coordinated parallel template will NOT include dithering parameters, since the Prime observation controls the pointing of the telescope.
Coordinated Parallel Observation Limitations
- The Parallel observation must have the same number of total exposures as the prime observation.
- The first parallel exposure will be done in parallel with the first prime exposure, the second parallel exposure will be done in parallel with the second prime exposure and so forth.
- For each prime/parallel exposure pair, the duration of the parallel exposure cannot exceed that of the prime exposure.
- An error is generated for a coordinated parallel visit if any of the visits would result in too many SAMs (Small Angle Maneuvers). Reduce the number of filters and/or mosaic tiles in the visit, or force visit splitting via special requirement..
- When MIRI Imaging is the prime or parallel observation, the NUMBER OF EXPOSURES/DITHER must be 1.
- Some NIRCam readout patterns cannot be used with parallel observations because of data rate limitations.
- RAPID cannot be used at any time (NIRCam prime or parallel)
- BRIGHT1 can only be used if MODULE is not ALL (NIRCam prime or parallel)
- BRIGHT2 cannot be used when NIRSpec Multi-Object Spectroscopy is prime
A typical NIRISS WFSS observation (not involved in a coordinated observation) consists of a non-dithered Direct image, a dithered Science image, followed by another non-dithered Direct image. The following techniques will help you maintain this structure when using NIRISS WFSS as part of a coordinated parallel observation.
NIRISS WFSS Template as the Prime Observation
Each parallel MIRI or NIRCam observation must consist of 3 exposure specifications for each WFSS science exposure specification. The first and third exposure specifications must have only 1 total dithers (i.e. be undithered) to match the WFSS direct images that surround each WFSS science exposure specification. If you complete the WFSS exposure specification first, APT will automatically give you the correct number of dithers for the parallels as you create them.
NIRISS WFSS Template as the Parallel Observation
MIRI Imaging
When using MIRI Imaging as the prime observation, a special parameter is available to make the exposure count match.
DIRECT IMAGE PRIME = NO_DITHERING (default), DITHER_DIRECT_IMAGES
If you select NO_DITHERING, then APT will direct you to select DITHER = NONE for those MIRI images that match the WFSS Direct exposures. If you select DITHER_DIRECT_IMAGES, then the parallel NIRISS WFSS Direct images will be dithered to match the dithering you have chosen for your MIRI prime exposures.
NIRCam Imaging
When using NIRCam Imaging as the prime observation, a special parameter is available to make the exposure count match.
DIRECT IMAGE EXPOSURES = NO_DITHERING (default), DITHER_DIRECT_IMAGES
If you select NO_DITHERING, then the NIRCam images that match the WFSS Direct exposures will not be dithered. If you select DITHER_DIRECT_IMAGES, then all NIRCam images will be dithered as specified, and the NIRISS WFSS Direct images will also be dithered to match.