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

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Introduction

The JWST MIRI imaging pixel scale, 0.1"/pixel, provides Nyquist sampling at 7 μm. At longer wavelengths, the point spread function (PSF) is oversampled; at shorter wavelengths, it is undersampled.

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 sunshield, dominate the total backgroundSince the telescope thermal emission is not expected to be constant, self-calibration may be needed for observations at these wavelengths in order to self-consistently solve for the background and flat field (Meixner, 2006). Therefore, some dither patterns for longer wavelengths will both optimize self-calibration and remove bad pixels.

 


Available dither patterns

JWST dithering allows for moves specific to MIRI imaging.  Dither patterns for observation can be implemented in the Astronomer's Proposal Tool (APT) with the JWST APT MIRI imaging template.  

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The following types of dither patterns will be offered in this mode:1)

  1. 2-point
2)
  1. 4-point
3) 
  1. Reuleaux
4)
  1. Cycling

Additionally, there are two limited access options:1)

  1. Sparse
2)
  1. No Dithering

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

The list Lists of points for all MIRI imaging dither patterns are available as a pattern points are compiled in this .csv format file: MIRI_Imaging_Dithers.csv. Each  Each list of points is the contains a set of offset positions from a fiducial point that satisfy various sampling requirementssampling requirements.   These fiducials are typically the center of the array or subarray or subarray as specified in the in the Science Instrument Aperture File (SIAF). Note on Subarrays and FIlters: Different

Note

For subarrays and fIlters, different dither patterns may be necessary depending on the

selected

selected subarray

and

 and filter

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2point
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2-point dither

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. These 2 points are recorded in the MIRI_Imaging_Dithers.csv file.

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Figure 1. Two types of 2-point dither patterns for the SUB64 arrays

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Top: 2-point pattern on the SUB64 array showing PSF diameters for 6λ/D at 21 μm.
Bottom: 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.

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4-point patterns

4-point point source patterns (short and long wavelength, with multi-exposure option) 

There are 5 different "4-point point source" patterns listed in the MIRI_Imaging_Dithers.csv file:

1) 4-point point-short
2) 4-point point-short-64-128-256
3) 4-point point-long
4) 4-point point-long-64
5) 4-point point-long-128-256

The use matrix for these 5 patterns is found available in Table 1.

The 4-point point source pattern comes in a short - wavelength version and a long - wavelength version:

  • the "4-point point-short-64-128-256" pattern (used on SUB64, SUB128, and SUB256 arrays) and,
  • the "4-point point-long-128-256" pattern (used on SUB128 and SUB256 arrays)

The long - wavelength pattern is simply a scaled-up version of the short - wavelength pattern—at longer wavelengths, the PSFs are larger and more susceptible to overlap (from one point to the next) on the detector. The long - wavelength pattern is meant to be used for all observations where λ ≥ 12.8 μm. For λ < 12.8 μm, the short - wavelength pattern is used. These patterns subsample the PSF, allow for simple background subtraction, and minimize the effects of bad rows or columns.  

The “multi-exposure” button moves these 2 patterns from the bottom left to the upper right and is available only for the BRIGHTSKY and FULL arrays: these are the "4-point point-short" and the "4-point point-long" patterns, each with 10 sets of 4 points, for 40 points total. The user can request a starting set and an ending set. Additionally, if more than 10 sets are desired, then the pattern will start over at the first set of 4 points and continue on from there.

There is also a mirror parity option that moves the pattern from the bottom right to upper left.

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Figure 2. Four-point point source dither pattern

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Left: four-point point source dither pattern. Note that the long-wavelength pattern is the same as the one shown, just scaled up in size.

Right: This same pattern with offsets to accommodate multiple exposures - it can be repeated 10 times (shown) giving a total of 40 dither points.


Figure 3 shows what the long- and short-wavelength four-point point source patterns look like. Note that the "4-point point-short-64-128-256" pattern is set number 6 (points 21-24) of the "4-point point short" pattern and "4-point point-long-128-256" pattern is set number 6  (points 21-24) of the "4-point  point-long" pattern - where set number 6 (for both patterns) is centered on FULL and BRIGHTSKY.

Top Left: 4-Point Point-Long pattern on BRIGHTSKY showing PSF diameters for 6λ/D at 15 μm.**Top Right: 4-Point Point-Short pattern on BRIGHTSKY showing PSF diameters for 6λ/D at 5.6 μm.**
Bottom Left: 4-Point Point-Long-128-256 pattern on SUB128 showing PSF diameters for 6λ/D at 25.5 μm.**Bottom Right: 4-Point Point-Short-64-128-256 pattern on SUB64 showing PSF diameters for 6λ/D at 11.3 μm.**

**Sizes of subarrays are not to scale relative to each other.

 

Due to the small size of the SUB64 array, a modified four-point point-source pattern for long wavelengths (λ ≥ 12.8 μm) must be used: the "4-Point  Point-Long-64". 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 figures below). This dither pattern is listed in the MIRI_Imaging_Dithers.csv file and is illustrated in the figure below.

Four-Point Point-Long-64 pattern

4-Point  Point-Long-64 pattern on the SUB64 array showing PSF diameters for 6λ/D at 21 μm. 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.


These five 4-point point-source patterns are defined in the MIRI_Imaging_Dithers.csv file.

Four-point extended source patterns (with multi-exposure option)

There are 3 different "4-point Extended-Source" patterns listed in the MIRI_Imaging_Dithers.csv file:

1) 4-Point  Extended
2) 4-Point  Extended-128
3) 4-Point  Extended-256

The use matrix for these 3 patterns is found in the table at the end of this article.

The 4-point Extended-Source patterns are optimized for moderately extended objects by maximizing the dither distance and minimizing slew time (keeping dithers to <20”).  Similar to the four-point point-source pattern, the “multi-exposure” button moves the pattern from the bottom left to the upper right, and is available only in the FULL and BRIGHTSKY arrays: this pattern is listed as the "4-Point  Extended" pattern in the MIRI_Imaging_Dithers.csv file. The "4-Point  Extended" pattern has 10 sets of 4 points, for 40 points total, where sets number 5 and 6 are centered on FULL and BRIGHTSKY. The user can request a starting set and an ending set. Additionally, if more than 10 sets are desired, then the pattern will start over at the first set of four points and continue from there.  A mirror parity option moves the pattern instead from the bottom right to upper left. Note that the "4-Point  Extended-256" pattern is set number 6 (points 21-24) of the "4-Point  Extended" pattern, and it is used for the SUB256 array. The "4-Point  Extended-256" pattern is too large to fit on the SUB128 array, so pattern "4-Point  Extended-128" was created - a scaled down version of the "4-Point  Extended-256" pattern that fits on the SUB128 subarray. Note that there is no 4-point extended source pattern available for the SUB64 subarray (as the subarray is too small). The images below show what the four-point extended-source pattern looks like. 

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Left: The Four-Point Extended-Source dither pattern.

Right: This same pattern with offsets to accommodate multiple exposures.

4-Point Extended pattern on BRIGHTSKY showing PSF diameters for 6λ/D at 25.5 μm.

4-Point Extended-256 pattern on SUB256 showing PSF diameters for 6λ/D at 25.5 μm.
Note this pattern is set number 6 of the 4-Point Extended pattern (points 21-24).**

4-Point Extended-128 pattern on SUB128 showing PSF diameters for 6λ/D at 25.5 μm.**
**Sizes of subarrays are not to scale relative to each other.

These patterns are defined in the MIRI_Imaging_Dithers.csv file. 

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Reuleaux

The Reuleaux pattern consists of 12 points and comes in 3 different sizes (or "scales" as indicated in the MIRI_Imaging_Dithers.csv file):

1) Reuleaux Large
2) Reuleaux Medium
3) Reuleaux Small

The 12-point Reuleaux triangle dither pattern is suitable for observing unresolved (or barely resolved) sources.  The Reuleaux 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.  

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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 arcsec (1σ, each axis).

Reccommended Wavelength Use Matrix for Reuleaux pattern for SUB128 and SUB64:
The only restriction on the Reuleaux pattern in APT is that the Large Reuleaux 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 (e.g. see the image below showing the medium size Reuleaux pattern on the SUB64 array showing PSF diameters for 6λ/D at 25.5 μm). The table below 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 LARGE and MEDIUM 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 the table contains columns that account for a 1σ and 3σ pointing error.

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



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.Same figure as the the figure to the right, but showing PSF diameters for 6λ/D at 7.7 μm - the longest wavelength that fits entirely on the SUB64 at 6λ/D. 

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Cycling

The Cycling pattern consists of 311 points and comes in 3 different sizes (or "scales" as indicated in the MIRI_Imaging_Dithers.csv file):

1) Cycling Large
2) Cycling Medium
3) Cycling Small

The use matrix for these 3 patterns is found in the table at the end of this article.

The random Cycling pattern is 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 four consecutive dithers provides complete ½ pixel sampling. Like the 12-point Reuleaux pattern, there are 3 different size cycling patterns: SMALL, MEDIUM, and LARGE. These patterns are defined in the MIRI_Imaging_Dithers.csv file.

Points for the Cycling Large pattern on the SUB256.
Points for the Cycling Medium pattern on the SUB256.
Points for the Cycling Small pattern on the SUB64.Cycling Small pattern on SUB64 showing PSF diameters for 6λ/D at 25.5 μm

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sparse

Sparse

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.

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nodither

No dither

This option has limited access for engineering purposes only.

 


MIRI imaging dithers .csv file and table overview of all available dither patterns

File: MIRI_Imaging_Dithers.csv
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).  

Table of All Available Dither Patterns:
The column labeled "Subsets of Pattern Available?" indicates weather the user has to use all the points in that pattern, or if the user has the option to use subsets of that pattern. 

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 Table 1. All available dither patterns

PatternScaleNumber of PointsAvailable SubarraysSubsets of Pattern Available?Allowable Wavelengths (μm)Comments
CyclingLarge311

FULL
BRIGHTSKY
SUB256

YesAllNone
CyclingMedium311FULL
BRIGHTSKY
SUB256
YesAllNone
CyclingSmall311FULL
BRIGHTSKY
SUB256
SUB128
SUB64
YesAllNone
ReuleauxLarge12FULL
BRIGHTSKY
SUB256
SUB128
NoAllNone
ReuleauxMedium12FULL
BRIGHTSKY
SUB256
SUB128
SUB64
NoAllNone
ReuleauxSmall12FULL
BRIGHTSKY
SUB256
SUB128
SUB64
NoAllNone
2-Pointn/a2FULL
BRIGHTSKY
SUB256
SUB128
SUB64
NoAllNone
4-PointPoint-Short40FULL
BRIGHTSKY
Yes5.6 - 11.3This pattern consists of 10 sets of 4 points.
4-PointPoint-Short-64-128-2564SUB256
SUB128
SUB64
No5.6 - 11.3This pattern is set number 6 (points 21-24) in the "4-Point Point-Short" pattern.
4-PointPoint-Long40FULL
BRIGHTSKY
Yes12.8 - 25.5This pattern consists of 10 sets of 4 points.
4-PointPoint-Long-644SUB64No12.8 - 25.5None
4-PointPoint-Long-128-2564SUB256
SUB128
No12.8 - 25.5This pattern is set number 6 (points 21-24) in the "4-Point Point-Long" pattern.
4-PointExtended40FULL
BRIGHTSKY
YesAllThis pattern consists of 10 sets of 4 points.
4-PointExtended-1284SUB128NoAllNone
4-PointExtended-2564SUB256NoAllThis pattern is set number 6 (points 21-24) in the "4-Point Extended" pattern.
Sparsen/an/aLimited Accessn/aAllNone
No Dithern/a0Limited Accessn/an/aNone

 



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JWST User Documentation Home
MIRI Imaging Dithering
MIRI Coronagraph Imaging Dithering
MIRI LRS Dithering
MIRI MRS Dithering
MIRI Imaging Mosaics
MIRI MRS Mosaics
MIRI Imaging
MIRI Dithering Overview
MIRI Medium Resolution Spectroscopy
MIRI Detector Subarrays

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APT Dither Implementation
APT Mosaic Implementation
JWST Dithering Overview
JWST Mosaic Overview

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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

JWST technical documents

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Last updated

Published January 13, 2017


 

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