Chapter 10
MIRI Imaging
In this chapter…
10.1 Introduction
The MIRI Imaging template/mode is to be used for all observations using the direct imaging
mode of MIRI. These observations may be to obtain photometry of a single point source or
mapping/mosaicking of a large area in any of the 8 broad band filters. The parameters described
in this section are used to specify the Observations for MIRI Imaging. More complete
descriptions of the detector readout parameters, filters, detector characteristics, etc. are available
in the MIRI Instrument Handbook.
The following fields are defined in Chapter 5: Observation Number (5.1), Observation Label
(5.2), Observation Comments (5.3), Target Name (5.4), Observation-Level Special Requirements
(5.5), and On Hold Comments (5.6), and will not be discussed in this Chapter.
The MIRI Imaging template [MIR0001] consists of the following parameters:
Field
Pattern Type
[MIR0345]
Starting Point
[MIR0347]
Details
select Dither pattern
Values
choose from list
[MIR0346]
1, 2, 3, …, 310, 311
[MIR0348]
Number of Points
[MIR0349]
select number of points
for CYCLING pattern
3, 4, 5, … [MIR0350]
Pattern Size
[MIR0351]
select pattern size
choose from list
[MIR0352]
Subpixel Sampling
[MIR0353]
select subpixel sampling
YES, NO [MIR0354]
Subarray [MIR0212]
select subarray
Filter(s) [MIR0213]
select filter name(s)
Readout Pattern
select readout pattern
choose from list
[MIR0004]
choose from list
[MIR0005]
choose from list
select starting point for
CYCLING pattern
Notes
see Table 10-1
see Table 10-2
see Table 10-3
see Table 10-4
Only valid if "Override
1
[MIR0410]
Number of Groups
[MIR0412]
Number of Integrations
[MIR0414]
[MIR0411]
specify number of
groups
number [MIR0413]
specify number of
integrations
number [MIR0415]
Exposure time
Parameters" flag is set
[MIR0416]
Note that ALL parameters listed below are required as part of your
Phase I submission.
.
10.2 MIRI Imaging
The following parameters are used to define MIRI Imaging observations.
10.2.1
Dither Pattern
Most observations with JWST will require dithering, although there are some types of
observations (e.g. planetary transits) for which dithering is not desired. The following
parameters define the dither patterns for MIRI Imaging observations.
10.2.1.1
Pattern Type
PATTERN TYPE [PATTERN_TYPE] = NONE [MIR0355], CYCLING
[MIR0356], REULEAUX [MIR0357] ], GAUSSIAN [MIR0500]
Note for developers: PATTERN TYPE should be a required field, and there should be no default
value (i.e. do not make NONE the default). [MIR0003]
This parameter specifies the primary pattern to be used for dithering (see Table 10-1).
Table 10-1 Dither Patterns Available for MIRI Imaging
Dither Pattern
NONE
CYCLING
Description
No dithering
A table of 311 positions in which x- and y-offsets
are randomly drawn from a Gaussian distribution,
from which users can select their beginning
position and the number of dithers they would like
to use (see Figure 10.1)
A table of 12 positions that define a Reuleaux
REULEAUX
2
triangle (see Figure 10.3)
A table of 5 positions in a Gaussian distribution.
(see Figure 10.3)
GAUSSIAN
10.2.1.1.1 None
This value indicates no dithering will be performed.
10.2.1.1.2 Cycling
The CYCLING pattern is shown in Figure 10.1, and can be used to obtain data optimized for
self-calibration at arbitrary sky depths. The pattern consists of a list of 311 pointing positions
where the x- and y-offset distributions (from the central position) are well-described by a
Gaussian; the pattern is drawn from the Spitzer IRAC Cycling pattern. Observations may begin
at any position in the cycling pattern and include an arbitrary number of dithers. Each contiguous
block of four pointing positions provides some 0.5 pixel sub-sampling in both the x- and ydirections. However, to provide optimized sub-pixel sampling, this pattern should be used in
conjunction with the sub-pixel sampling option (a Parallelogram pattern).
Figure 10.1 – The MIRI CYCLING Pattern
The origin is the aperture center.
If the CYCLING pattern has been selected the following inputs are also required (along with
PATTERN SIZE [MIR0065]).
.
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10.2.1.1.2.1
Starting Point
STARTING POINT [STARTING_POINT] = 1, 2, 3, …, 310, 311 [MIR0358]
This parameter is used to determine the offset of the first dither position.
10.2.1.1.2.2
Number of Points
NUMBER OF POINTS [NUMBER_OF_POINTS] = 3, 4, 5, … [MIR0359]
This parameter indicates the total number of dither positions in the pattern.
If the combination of STARTING POINT and NUMBER OF POINTS exceeds 311, the
pattern cycles back to use points 1, 2, 3, etc [MIR0389].
No explicit maximum is set on the number of points [MIR0360]. The number of points,
however, is effectively limited by the maximum allowed visit duration.
10.2.1.1.3 Gaussian
The GAUSSIAN pattern is shown in Figure 10.2, and is designed for observing bright objects
with the SUB128 and SUB64 subarrays. To provide optimized sub-pixel sampling, this pattern
should be used in conjunction with the sub-pixel sampling option (a Parallelogram pattern).
Figure 10.2 – The GAUSSIAN Pattern
The origin is the aperture center.
4
If the GAUSSIAN pattern has been selected, you must also select a PATTERN SIZE
[MIR0501].
10.2.1.1.4 Reuleaux
The REULEAUX pattern is shown in Figure 10.3, and is optimized to provide to obtain
observations of compact point sources that are suitable for self-calibration. The pattern is a set of
12 dither positions that defines a Reuleaux triangle; the pattern is drawn from the Spitzer IRAC
12-point Reuleaux pattern. The Reuleaux pattern includes some 0.5 pixel sub-sampling in both
the x- and y-directions. However, to provide optimized sub-pixel sampling, this pattern should be
used in conjunction with the sub-pixel sampling option (a Parallelogram pattern).
Figure 10.3 – The REULEAUX Pattern
The origin is the aperture center.
If the REULEAUX pattern has been selected, you must also select a PATTERN SIZE
[MIR0361].
10.2.1.2
Pattern Size
PATTERN SIZE [PATTERN_SIZE] = DEFAULT [MIR0365], SMALL
[MIR0362], MEDIUM [MIR0363], or LARGE [MIR0364]
5
This parameter specifies either that all exposures are to be done with the same dithering pattern
(SMALL, MEDIUM, or LARGE) as indicated in Table 10-2. The choice of subarray restricts
the available options for pattern and pattern size as well, as indicated in Table 10-3. The
subarrays SUB128 [MIR0502] and SUB64 [MIR0503] may only use the Gaussian pattern and
are limited in what pattern size can be selected.
Table 10-2 Allowed dithering patterns and sizes by subarray
Subarray
FULL
Pattern
Reuleaux, cycling
BRIGHTSKY Reuleaux, cycling
SUB256
Reuleaux, cycling
SUB128
SUB64
Gaussian
Gaussian
Allowed Pattern Size
SMALL [MIR0504], MEDIUM [MIR0505]
or LARGE [MIR0506]
SMALL [MIR0507], MEDIUM [MIR0508]
or LARGE [MIR0509]
SMALL [MIR0510], MEDIUM [MIR0511]
or LARGE [MIR0512]
SMALL [MIR0513] or MEDIUM [MIR0514]
SMALL [MIR0515]
When the pattern size is not specified by the user the DEFAULT pattern will be used based upon
the choice of filter, pattern and subarray as indicated in Table 10-3.
Table 10-3 Default Dithering Patterns
Filter
Subarray
F560W
F770W
F1000W
F1130W
F1280W
F1500W
F1800W
F2100W
F2550W
F560W
F770W
F1000W
F1130W
F1280W
12 pt. Reuleaux
FULL,
BRIGHTSKY,
or SUB256
SMALL
SMALL
MEDIUM
MEDIUM
MEDIUM
MEDIUM
LARGE
LARGE
LARGE
5-pt Gaussian
SUB128
SMALL
SMALL
MEDIUM
MEDIUM
MEDIUM
[MIR0366]
[MIR0367]
[MIR0368]
[MIR0369]
[MIR0370]
[MIR0371]
[MIR0372]
[MIR0373]
[MIR0374]
[MIR0516]
[MIR0517]
[MIR0518]
[MIR0519]
[MIR0520]
311 pt. Cycling
FULL,
BRIGHTSKY,
or SUB256
SMALL
MEDIUM
MEDIUM
MEDIUM
MEDIUM
MEDIUM
MEDIUM
MEDIUM
MEDIUM
5-pt Gaussian
SUB64
SMALL
SMALL
SMALL
SMALL
SMALL
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[MIR0375]
[MIR0376]
[MIR0377]
[MIR0378]
[MIR0379]
[MIR0380]
[MIR0381]
[MIR0382]
[MIR0383]
[MIR0525]
[MIR0526]
[MIR0527]
[MIR0528]
[MIR0529]
F1500W
F1800W
F2100W
F2550W
MEDIUM
MEDIUM
MEDIUM
MEDIUM
[MIR0521]
[MIR0522]
[MIR0523]
[MIR0524]
SMALL
SMALL
SMALL
SMALL
[MIR0530]
[MIR0531]
[MIR0532]
[MIR0533]
When multiple filters are selected and PATTERN SIZE = DEFAULT, you should list the
filters in order by default pattern size to avoid unnecessary additional visits [MIR0384].
Note for developers: If multiple filters are selected in the template and PATTERN SIZE =
DEFAULT and the default sizes are different for the selected filters then APT will break up the
visit into multiple visits (one for each size) [MIR0385]. APT should also warn the user to list
their filters in order by default pattern size [MIR0386].
10.2.1.3
Subpixel Sampling
SUBPIXEL SAMPLING [SUBPIXEL_SAMPLING] = YES [MIR0387],
NO [MIR0388]
This parameter specifies whether or not subpixel sampling will be performed.
Note for developers: see Appendix F1 for dithering details.
10.2.2
Subarray
SUBARRAY [SUBARRAY] = FULL [MIR0014], BRIGHTSKY [MIR0015],
SUB256 [MIR0534], SUB128 [MIR0535], SUB64 [MIR0536]
This parameter specifies the region of the detector that is to be read out. See Table 10-4 for
details.
Table 10-4 Subarrays for MIRI Imaging
Subarray
Rows
Columns
FULL
BRIGHTSKY
SUB256
SUB128
SUB64
1024
512
256
128
64
1290
512
608
132
68
Light Sensitive
Columns
1024
512
604
128
64
7
10.2.3
Filters
For each filter that you use, specify the name of the filter and the requested exposure time.
10.2.3.1
Filter Name
Select the name of each FILTER [FILTER] (see Table 9-5) you wish to use..
Table 10-5 Filters Available for MIRI Imaging
Filter Bandpass () Comment
F560W
Center Wavelength
0 ()
5.6
1.2
broad band
[MIR0016]
F770W
F1000W
F1130W
F1280W
F1500W
F1800W
F2100W
F2550W
7.7
10.0
11.3
12.8
15.0
18.0
21.0
25.5
2.2
2.0
0.7
2.4
3.0
3.0
5.0
4.0
PAH, broad band
Silicate, broad band
PAH, broad band
broad band
broad band
Silicate, broad band
broad band
broad band
[MIR0017]
[MIR0018]
Filter
[MIR0019]
[MIR0020]
[MIR0021]
[MIR0022]
[MIR0023]
[MIR0024]
10.2.3.2 Readout Pattern
READOUT PATTERN [READOUT PATTERN] = FAST [MIR0417],
SLOW [MIR0418], FASTGRPAVG [MIR0419], FASTINTAVG [MIR0420]
This field specifies the readout pattern to be used to obtain the data. . FAST is used for bright
targets and long wavelength imaging, SLOW is used for faint targets, deep imaging, and MRS
spectroscopy, FASTGRPAVG and FASTINTAVG are used for bright targets and to decrease
the data rate.
10.2.3.3 Number of Groups
NUMBER OF GROUPS [NGROUPS] specifies the number of groups in an integration
[MIR0421]. If READOUT PATTERN = FASTGRPAVG, then the number of groups should
be greater than 16 [MIR0416] and a multiple of 4 [MIR0214]. If READOUT PATTERN =
FASTINTAVG, then the number of groups should be 1 [MIR0006].
10.2.3.4
Number of Integrations
8
NUMBER OF INTEGRATIONS [NINTS] field specifies the number of times the integration
is repeated [MIR0422]. . If READOUT PATTERN = FASTINTAVG, then the number of
integrations should be a multiple of 4 [MIR0385].
9