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 µm
• Plate Scale: 0.11″/pixel
• Critically sampled at 7 µm
• 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 λ >15 µm
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
Pattern
4-Pt Box
Cycling
12-Pt Reuleaux
Scale
N/A
Small
Medium
Large
Small
Medium
Large
Max Offset
3.5 pix
11 pix
119 pix
161 pix
13 pix
27 pix
55 pix
Median Offset
2.5 pix
10.5 pix
53 pix
97 pix
15 pix
30 pix
59 pix
Sub-Pixel
pix
pix
pix
pix
pix
pix
pix
MIRI LRS Specifications
• Wavelength range: 5-10 µm
nominal (2-14 µm expected)
• Slit Dimensions: 0.6″×5.5″
(5x45 pixels)
• Spectral Resolution: R=100
at 7.5 µm
• Spatial Plate Scale: 0.11″
/pixel
• Spectral Plate Scale: 2
pixels/resolution element
• Critically sampled (spatially)
at 7 µm
Gordon & Meixner 2008
Background Subtraction
• Simultaneous
measurements of the sky
are needed to perform
background subtraction
• PSF size: (1.22λ/D=) 0.54″
at 14 µm, ~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)
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•
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)