MIRI Parallel Observations
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 1, Mid-Infrared Instrument (MIRI) coordinated parallel imaging observations are possible only with the Near-Infrared Camera (NIRCam). 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 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
The JWST ground system will offer specific sets of dither patterns optimized for each instrument/mode. Similarly, new dither patters that effectively work for both instruments taking into account their individual detector characteristics, PSFs, FOV, and relative orientations need to be defined. In all cases, the prime instrument will drive the number of instrument configurations and length of the exposure times.
This section discusses preliminary dither patterns that have been defined using the NIRCam short wavelength channel and the MIRI imager, a configuration expected to be used in deep fields. Specific aspects and assumptions that have been considered are:
- The MIRI PSF is Nyquist-sampled at wavelengths longer than about 6.3 μm. Subpixel sampling may be needed at shorter wavelengths to properly recover the PSF. At longer wavelengths individual dither pointings should ideally be separated by at least 4× FWHM. This will improve both the PSF and the image cosmetics. The final optimal separation between dither positions is still not fully defined; testing with different distances using simulated data in all filters is underway. Aspects like detector effects (cross-like structure on the detector substrate) or the importance of diffraction spikes (also from sources outside the MIRI FOV) that may be relevant for faint sources are also being taken under consideration.
- NIRCam needs subpixel sampling for almost all wavelengths bluer than 2 μm in the short wavelength detector and 4.4 μm in the long wavelength detector.
- The dither pattern design of choice must offer a compromise between spatial coverage and depth, and that should be driven by the particular science objective. In this case, one basic need for MIRI is to have a reasonably large area covered, without having a negative impact on the depth of the NIRCam data.
- We have used 4.772° offset between the MIRI and the JWST V2 axis (as measured in the cryo-vacuum campaigns), whereas NIRCam is assumed to be aligned with the JWST V2 and V3 axis.
The following dither examples target the MIRI filters that are under-sampled or close to Nyquist-sampled (5.6 μm, 7.7 μm, and 10.0 μm) and the NIRCam short wavelength channel. So far. no distortion has been considered, and there is no optimization for the NIRCam long wavelength channel.
Choice of a MIRI filter
See also: MIRI Filters and Dispersers
The filters selected to carry out the observations impact the dither pattern definition. In the case of MIRI, the filter determines the minimum separation between different positions and the need of subpixel sampling.
The example dither patterns defined above favor the NIRCam short wavelength channel and the MIRI short wavelength filters (the most sensitive ones). However, there are other considerations driven by the science case and scientific aim of the proposal.
For example, using more than 5,000 simulated galaxies, Bisigello et al. (2016) studied how the NIRCam broadband and the MIRI 5.6 μm and 7.7 μm filter combinations recover photometric redshifts for galaxies in the z = 0–10 range. They concluded that this combination of NIRCam broadband and MIRI 5.6 μm and 7.7 μm photometry not only lessens the impact of not having ancillary observations at λ < 0.6 μm but also refines the photometric redshift estimation. However, the time availability and desired depth of the exposures may constrain projects to use only a single MIRI filter. In that case, the filter selection needs careful consideration.
The filter of choice for MIRI is still a complex open question, and to make a decision, further data analysis, simulations and on-orbit performance should be considered.
Bisigello, L., et al. 2016, ApJS, 227, 19 (arXiv:1605.06334)
The impact of JWST broadband filter choice on photometric redshift estimation