MIRI Imaging Dithering

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

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See also: MIRI Imaging Recommended Strategies: DitheringJWST Dithering Overview

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 0.1" 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 backgroundSince 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.  



Types of dither patterns

The following types of dither patterns will be offered in APT for MIRI imaging: 

  1. 4-Point-Sets 1

  2. CYCLING

  3. 2-Point

  4. REULEAUX

The footprints of the exposure coverage for each of these dither patterns are shown in Figure 1 below.

Additionally, there are two limited access options:

  1. Sparse
  2. No dithering

A list of all dither options and details about them is 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). 

Different dither patterns may be necessary depending on the selected subarray and filter  

Bold italics style indicates words that are also parameters or buttons in software tools like the APT and ETC. Similarly, a bold style represents menu items and panels.

Figure 1. Exposure coverage of the MIRI imaging dither patterns

Exposure coverage footprints for the MIRI imaging dither patterns offered in APT. The colors indicate the number of overlapping frames.


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 the subarray; this set is the default Starting Set in APT.  For all other subarrays, only one 4-point set is allowed. 

Centering of the 4-point dither patterns on the BRIGHTSKY and FULL arrays.

The user must select a Starting Set and Number of Sets in APT for the FULL and BRIGHTSKY arrays. It is important to note that for these dither patterns, the single set of 4-point dithers that is centered on the subarray of choice is going to be Starting Set = 5 or Starting Set = 6. A Starting Set = 1 will start the 4-point pattern towards the bottom of the subarray

4-point point source patterns

Depending on the wavelength of the observation, APT will automatically select a 4-point pattern optimized for shorter (5.611.3 μm) or longer wavelengths (12.825.5 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. 

Figure 2. 4-point point source dither pattern


Top: 4-point point source dither pattern for short wavelengths (5.6–11.3 μm) 
Bottom: 4-point point source dither pattern for long wavelengths (12.8–25.5 μm)
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 2 below).
Figure 3. 4-point point source dither pattern for subarrays


Top: 4-point point-long-64 pattern
Bottom: 4-point point-long-64 pattern on the SUB64 array showing PSF diameters for 6λ/D at 25.5 μm. Note that the PSFs do not fit on the SUB64 at this wavelength.
Four-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.  

Figure 4. 4-point extended-source dither pattern

The 4-point extended-source dither pattern.


CYCLING

The CYCLING pattern consists of 311 points. The CYCLING 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 312th dither position is the same as the 1st position. Each set of 4 consecutive dithers provides complete ½ pixel sampling.



2-Point

The 2-Point dither option provides two 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 two points for this dither are recorded in the MIRI_Imaging_Dithers.csv file.

Figure 5. 2-Point dither pattern for the SUB64 arrays

2-point pattern on the SUB64 array showing PSF diameters for 6λ/D at 25.5 μm. Note that the PSFs do not fit on the SUB64 at this wavelength.


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 peak of the point spread function (PSF) along same row or column of the detector and minimizes the effects of persistence from bright sources.  

Figure 6. 12-point REULEAUX pattern showing the order of the offsets


Each REULEAUX pattern is constrained by competing requirements to:

1. Fit the pattern on the subarray

2. Ensure that the distance between successive pointings is ≥6λ/D

3. 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

λ Central (μm)

PSF Diameter @ 6λ/D (pixels)

Sub 128

Sub 128 with 1σ Error

Sub 128 with 3σ Error

Sub 64

Sub 64 with 1σ ErrorSub 64 with 3σ Error
5.69.693

Large

Large

Medium

Medium

Small

Small

7.713.328

Large

Large

Medium

Medium

Small

Small

1017.309

Large

Large

Medium

Small

Small

None

11.319.559

Large

Large

Medium

Small

Small

None

12.822.155

Large

Large

Medium

Small

Small

None

1525.963

Large

Medium

Medium

Small

Small

None

1831.156

Large

Medium

Medium

Small

Small

None

2136.349

Medium

Medium

Medium

Small

None

None

25.544.138

Medium

Medium

Medium

None

None

None

Figure 7. MEDIUM size REULEAUX pattern on the SUB64 array showing PSF diameters

MEDIUM size REULEAUX pattern on the SUB64 array showing PSF diameters for 6λ/D at 25.5 μm. Note that the PSFs do not fit on the SUB64 at this wavelength.


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”

This limited access pattern requires strong justification in the proposal and pre-approval prior to use in APT.



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.



Coordinated Parallels



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). 



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





Published

 

Latest updates

  • Added Figure 1 to show coverage for the different imaging dither patterns.