MIRI Parallel Observations

See also: JWST Parallel Observations

JWST's Mid-Infrared Instrument (MIRI) can be used in parallel with the Near-Infrared Camera (NIRCam). 

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MIRI coordinated parallel observations are planned as part of a primary program (as opposed to pure parallel observations that are part of distinct, separate programs). For cycle 4, MIRI coordinated parallel imaging observations are supported alongside the following observatory modes:

  • NIRCam wide field slitless spectroscopy (WFSS) prime + MIRI imaging in parallel
  • NIRCam imaging prime + MIRI imaging in parallel
  • NIRISS WFSS prime + MIRI imaging in parallel
  • NIRSpec MOS prime + MIRI imaging in parallel

In addition, the observatory supports the following parallel mode with MIRI Imaging in prime:

  • MIRI imaging prime + NIRCam imaging in parallel
  • MIRI imaging prime + NIRISS WFSS in parallel

These coordinated observations are useful for sensitive images that require near-infrared through mid-infrared coverage. The design of coordinated observations has to consider the instruments characteristics, type of science, and operational aspects and limitations. The figures below show a comparison of the MIRI imaging and NIRCam fields of view, and an example of a MIRI + NIRCam coordinated parallel placement for deep imaging in the Hubble UDF field. 

Figure 1. Comparison of the MIRI and NIRCam FOVs


The MIRI imager is depicted in red, including the coronagraphic mask. The blue lines show the FOV covered by the NIRCam short wavelength A and B modules. The characteristic NIRCam 5” inter-SCA gaps are depicted as well.

Figure 2. Possible pointing for proposed deep observations


In this configuration, NIRCam is prime in the HUDF and MIRI is located in the GOODS South. The instrument overlays have been generated with the JWST Astronomers Proposal Tool. The NIRCam and MIRI overlays are highlighted in red and blue, respectively. Green crosses mark viable guide stars. Background image credit: STScI.

Operational aspects

See also: JWST Parallel Observations, JWST Coordinated Parallels Roadmap, JWST Coordinated Parallels Custom Dithers

The operational aspects to be considered are both observatory and instrument driven. From the observatory perspective, there are 2 basic rules: (i) no mechanism movement is allowed, and (ii) no calibration lamp can be switched on while another instrument is acquiring an exposure. These rules prevent both spurious vibrations and thermal instability, and require the 2 instruments in use to coordinate their mechanism changes (e.g., filter wheels) in between exposures. Calibration activities such as darks can be simultaneously carried out in the other instruments.  

From the instruments perspective, it is important to adjust the dwell time per dither position such that it minimizes the detector "dead time" per instrument (i.e., the time that one instrument is waiting for the other instrument to complete its exposure). The length of time that an instrument should acquire data (single or multiple integrations) in a single dither position is dictated by the detector performance. The initial plan for NIRCam deep field programs is to integrate for few thousand seconds. As for MIRI, recent analysis of data from the cryo-vacuum campaigns carried out at NASA Goddard Space Flight Center have shown that these exposure lengths fall into the regime in which the MIRI detectors' performance is optimal at short wavelengths. This guarantees that both instruments can be operated with detector dead time driven to a minimum.

Observing strategies that are designed to minimize the presence of persistence are crucial for deep fields programs. Although there are usually no bright sources in the field of interest, signal can build up making the identification of faint sources challenging. Observations that use a combination of short-to-medium length ramps with multiple integrations, and background matching and self-calibration techniques have been identified as useful strategies to both prevent and mitigate persistence. 


Dither pattern definitions

See also: MIRI Dithering, JWST Coordinated Parallels Custom Dithers

Coordinated parallel templates in APT provide custom dither patterns optimized for the particular combination of instruments and modes. These custom dither patterns involve non-zero steps along both detector axes (x and y) such that none of the dithers fall on the same detector row or column. Each dither constitutes an integer plus a fractional pixel (subpixel) step. The integer pixel component of the dither step mitigates bad pixels and flat field uncertainties, while the fractional pixel component improves PSF sampling and achievable spatial resolution in the combined image. A full description of the custom dither patterns used for coordinated parallel observations is given on the JWST Coordinated Parallels Custom Dithers article. 


MIRI filter choice

See also: MIRI Filters and Dispersers, MIRI Imaging Recommended Strategies

The dither patterns used in coordinated parallel observations take into account the choice of filter for MIRI, to account for the changing PSF size with wavelength. All filters offered for MIRI imaging in prime mode are also available for MIRI imaging as coordinated parallels. The recommendations in the MIRI Imaging Recommended Strategies also apply to coordinated parallel observations; the science goals should drive the choice of filter (e.g. Bisigello et al 2016).

 

References

Bisigello, L., et al. 2016, ApJS, 227, 19 (arXiv:1605.06334)
The impact of JWST broadband filter choice on photometric redshift estimation



Notable updates
  •  
    Added full list of supported coordinated parallel modes
    Removed outdated content and added additional links to complementary pages
Originally published