The JWST MIRI imaging mode provides dither templates for both point and extended sources.

See also: MIRI Imaging Recommended Strategies: Dithering, JWST Dithering Overview, JWST Coordinated Parallels Custom Dithers

For most MIRI imaging science cases dithering is a highly recommended practice. Dithering provides improved sampling, removes bad pixel effects, and optimizes self-calibration.  

The MIRI imaging pixel scale of 0.11" per pixel offers Nyquist sampling at 7 μm. At longer wavelengths, the point spread function (PSF) is oversampled; at shorter wavelengths, it is undersampled. The dithering patterns for shorter wavelengths will both oversample the PSF and remove bad pixels. At wavelengths longer than 15 μm, thermal self-emission, mostly from the primary mirror and sun shield, dominate the total background. Since the telescope thermal emission is not expected to be constant (e.g., Meixner et al. 2006), self-calibration may be needed for observations at these wavelengths in order to self-consistently solve for the background and flat field. Therefore, some dither patterns for longer wavelengths will both optimize self-calibration and remove bad pixels.

Dither patterns for observation can be implemented in the Astronomer's Proposal Tool (APT) with the MIRI imaging APT template. Information on MIRI Imaging dither patterns for coordinated parallel observations can be found in the JWST Coordinated Parallels Custom Dithers article.

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Types of dither patterns

The following types of dither patterns will be offered in APT for MIRI imaging—the footprints of the exposure coverage for each of these dither patterns are shown in Figure 1: 

1. 4-Point-Sets 

2. CYCLING

3. 2-Point

4. REULEAUX

If performing a coordinated parallel observation, see the JWST Coordinated Parallels Custom Dithers article.

Additionally, there are 2 limited access options:

1. Sparse
2. No dithering

A list of all dither options and details about them are available in Table 1 at the end of this article. 

Lists of all MIRI imaging dither pattern points are compiled in this csv format file: MIRI_Imaging_Dithers.csv. Each list contains a set of offset positions from a fiducial point that satisfy various sampling requirements. These fiducial points are typically the center of the array or subarray as specified in the Science Instrument Aperture File (SIAF). 

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4-Point-Sets

The 4-Point Sets dither patterns are the optimized dither solution for most science cases. These patterns subsample the PSF, allow for simple background subtraction, and minimize the effects of bad rows or columns. For observations using BRIGHTSKY and FULL arrays, users can select between 1–10 repeating sets of 4-point patterns with the Number of Sets parameter in APT. There is also a mirror parity option in APT, Direction, that moves the pattern from the bottom right to upper left if the option NEGATIVE is selected; the default option for Direction is POSITIVE which is illustrated in Figure 2. Note the first set (Set 1 in Figures 2 and 4) starts near the bottom of each array; this set is the default Starting Set in APT. For all other subarrays, only one 4-point set is allowed. 

4-point point source patterns

Depending on the wavelength of the observation, APT will automatically select a 4-point pattern optimized for shorter (5.6–11.3 μm) or longer wavelengths (12.8–25.5 μm). Figure 2 illustrates all 10 sets of the 4-point pattern on the BRIGHTSKY subarray for the long and short wavelength 4-point point source patterns. 

Due to the small size of the SUB64 array, a modified 4-point point-source pattern for long wavelengths (λ ≥ 12.8 μm) is used. Note that because the SUB64 is so small, this pattern still does not entirely fit observations at 25.5 μm for PSF diameters at 6λ/D (see Figure 3 below).

4-point extended source pattern

The 4-point extended-source pattern is optimized for moderately extended objects by maximizing the dither distance and minimizing slew time (keeping dithers to <20”). Figure 4 below shows what the 4-point extended-source pattern looks like.

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CYCLING

The CYCLING pattern consists of 311 points. This pattern is a random Gaussian pattern designed to be flexible. Observers will be able to choose (1) the starting position in the dither table and (2) the number of dither positions to maximize observational flexibility. For observers who request more than 311 dither positions, the CYCLING pattern will wrap so that the 312^th dither position is the same as the 1^st position. Each set of 4 consecutive dithers provides complete ½ pixel sampling.

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2-Point

The 2-Point dither option provides 2 exposures separated on the array and avoids placing the peak of the PSF along the same row or column. This pattern (1) allows for simple background subtraction and (2) minimizes the effects of bad pixels, rows, and columns. The 2 points for this dither are recorded in the MIRI_Imaging_Dithers.csv file.

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REULEAUX

The 12-point REULEAUX triangle dither pattern is suitable for observing unresolved (or barely resolved) sources. The triangle is constructed by connecting the vertices of an equilateral triangle with circular arcs, which maximizes the distance and minimizes the slew time between consecutive pointings. This pattern minimizes the number of exposures that place the point spread function (PSF) peak along the same row or column of the detector. And it minimizes the effects of persistence from bright sources.

Each REULEAUX pattern is constrained by competing requirements to:

• Fit the pattern on the subarray
• Ensure that the distance between successive pointings is ≥6λ/D
• Account for the observatory blind pointing uncertainty of 1" (1-σ, each axis)

Recommended wavelength use matrix for REULEAUX pattern for SUB128 and SUB64

The only restriction on the REULEAUX pattern in APT is that the LARGEREULEAUX cannot be performed on the SUB64 array, as the pattern does not physically fit on the subarray. Otherwise, the user can select any size REULEAUX pattern for any subarray at any wavelength.

However, it is important to note that the SUB128 and SUB64 subarrays may not be able to contain observations at longer wavelengths (see Figure 6 showing the MEDIUM size REULEAUX pattern on the SUB64 array in the top graphic).

Table 1 shows the matrix of patterns that fit for a given filter and subarray combination (note: "Large" indicates LARGE, MEDIUM, and SMALL patterns can fit, "Medium" indicates MEDIUM and SMALL patterns can fit, "Small" indicates only SMALL patterns can fit, and "None" indicates no REULEAUX pattern can fit for that given subarray/filter combo). Note that the table also contains columns indicating which pattern sizes account for a 1σ and 3σ pointing error.

Table 1. Matrix of patterns that fit for a given filter and subarray combination

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SPARSE-CYCLING

This limited access pattern, enables the observer to specify (consecutive) positions from the cycling lookup table using a list.

For example, the observer will be able to enter a string like this:

“1,3,6-10,23, 29-55”

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NO DITHER

No dithering is only allowed for time-series observations (TSOs) and in the SUB64 subarray for standard imaging. It otherwise has limited access which will require strong justification in the proposal and pre-approval prior to use in APT.

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Coordinated Parallels

See also: MIRI Parallel Observations

See the JWST Coordinated Parallels Custom Dithers article for information on these dither patterns.

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MIRI imaging dithers .csv file

The file MIRI_Imaging_Dithers.csv is a compilation of all dithers. Each list of points is the set of offset positions from a fiducial point that satisfy various sampling requirements. These fiducials are typically the center of the array or subarray as specified in the Science Instrument Aperture File (SIAF). 

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References

Gordon, K. et al. 2015, PASP, 127, 953
The Mid-Infrared Instrument for the James Webb Space Telescope, X: Operations and Data Reduction

Meixner, M., Gordon, K.D., Indebetouw, R., et al. 2006, AJ, 132, 2268
Spitzer Survey of the Large Magellanic Cloud: Surveying the Agents of a Galaxy's Evolution (SAGE). I. Overview and Initial Results

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