MIRI Dither Patterns
Christine H Chen
Dithering Goals
1. Mitigate the effect of bad pixels
2. Obtain sub-pixel sampling
3. Self-calibrate data if changing scattered
light and/or thermal emission background is
significant
 It is anticipated that dithering will enhance
the majority of science observations
(although some programs will require no
dithering)
MIRI Observing Modes
• Direct Imaging
Full array
– Subarray
 Coronagraphic Imaging
Low Resolution Spectrograph (LRS)
• Medium Resolution Spectrograph
(MRS)
MIRI Direct Imaging Specifications
• Available Filters: 5.6, 7.7,
10.0, 11.3, 12.8 15, 18, 21,
and 25.5 m
• Plate Scale: 0.11/pixel
• Critically sampled at 7 m
• Field of View: 75x112
(680x1024 pixels)
• Geometric Distortion: <0.9%
at array corners
Gordon & Meixner 2008
Time-Variable Thermal Background
• Telescope thermal emission is
expected to dominate the
background for  >15 m
• Thermal background is expected
to change due to variable
telescope illumination as
telescope is slewed
• Self-calibration of deep fields
with time-variable pedestals has
been demonstrated using
NICMOS HDF-N and NDF-S
data (Arendt, Fixsen, & Mosley
2002)
• Propose using 12-point
Reuleaux and 311-point random
cycling patterns to optimize selfcalibration
Reuleaux Triangle
• Reuleaux polygon is a curve of
constant width; the distance
between two opposite, parallel,
tangent lines to its boundary is
constant
• The Reuleaux triangle optimizes
the figure of merit (Arendt
Fixsen, & Mosley 2000),
samples a wide range of spatial
frequencies in a uniform
manner, and is therefore wellsuited to the Fixsen leastsquares flat field technique
• The 36-point Reuleaux triangle
has been use in detailed
characterization of the IRAC
PSF (Marengo et al. 2008)
The Random Cycling Pattern
• Predetermined table of
311 dither positions
• The x- and y- offsets
from the array center
are randomly drawn
from a Gaussian
distribution with a
specified FWHM
• Observer specifies
beginning position and
end position in dither
pattern
• Every contiguous 4
offset positions contain
1/2 pixel offsets in each
direction
Subpixel Sampling
A. Fruchter
• Since MIRI is not badly
undersampled, 0.5 pixel
subsampling should be adequate
for the majority of science
observations
• Reuleaux and Cycling patterns
have 0.5 pixel offsets built-in to
provide some subpixel sampling
• The measured geometric distortion
(<0.9% in the corners) implies that
10 pixel offsets in the center of the
array will correspond to 10.1 pixel
offsets in the corners of the array
• A 4-point box pattern
(0,0),(0,2.5),(2.5,0),(2.5,2.5) will be
offered that can be used alone or
in conjunction with either the
Reuleaux or Cycling Patterns
JWST Observatory Offsetting Accuracy
•
•
•
Anandakrishnan et al. 2006
Offsets smaller than 0.5 (270
pixels) do not require use of
new guide stars
Commanded offsets <10 pixels
will have adequate source
placement precision (11 mas)
for interlacing from 1/2 pixel
sub-sampled images at the
center of the array
Observatory will possess 7 mas
jitter while pointed at a fixed
position
Proposed Direct Imaging Dither Patterns
MIRI LRS Specifications
• Wavelength range: 5-10 m
nominal (2-14 m expected)
• Slit Dimensions: 0.65.5
(5x45 pixels)
• Spectral Resolution: R=100
at 7.5 m
• Spatial Plate Scale:
0.11/pixel
• Spectral Plate Scale: 2
pixels/resolution element
• Critically sampled (spatially)
at 7 m
Gordon & Meixner 2008
Background Subtraction
• Simultaneous
measurements of the sky
are needed to perform
background subtraction
• PSF size: (1.22/D=) 0.54
at 14 m, ~1/10th slit
length, suggesting that 2
dither positions separated
by 1/3 of the slit length
should be adequate for
background subtraction
Proposed LRS Observing Modes
• Point Source/Staring Mode
• Two dither positions with source near the center of the slit
• Extended Source/Mapping Mode
• Observer specified dither pattern
• Number of slit positions parallel and perpendicular to the slit
• The size of the offset in each direction
JWST Observatory Offsetting Accuracy
•
•
•
Anandakrishnan et al. 2006
Offsets smaller than 0.5
(270 pixels) do not require
use of new guide stars
Observatory will possess 7
mas jitter while pointed at a
fixed position
Commanded dither offsets
of 1/3 slit length will place
the source onto the detector
with 17.1 mas precision
(20% precision) adequate
for 1/2 pixel subsampling
Summary
• Direct Imaging (full array)
– Subpixel sampling: 4 point box
– Self-Calibration: 12 point Reuleaux triangle
and random cycling
• LRS
– Extended Source/Mapping mode
– Point Source/Staring Mode
Observatory Pointing Efficiency
Slew Performance: [Max Accel, Max Rate, T1] =
0.0001453 8.19e-005, 0.0539
2
0.036, 60
10
slew capability (6 rwas)
slew capability (4 rwas)
slew requirement
1
Time To Complete Slew, min
10
0
10
Mitchell 2008
-1
10 -6
10
-5
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
2
10
3
10
Angle (degrees)
•
•
The slew time for offsets up to 3.6 (33 pixels) will be 10 sec independent of
slew size (4-point box, 12-point Reuleaux, and small Cycling patterns)
Larger slews will take exponentially longer times (medium and large Cycling
patterns)