MIRI Dither Patterns

Christine H Chen

Dithering Goals

1. Mitigate the effect of bad pixels2. Obtain sub-pixel sampling3. 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 ImagingLow 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: 75x112

(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 self-calibration

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 well-suited to the Fixsen least-squares 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

• 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

A. Fruchter

JWST Observatory Offsetting Accuracy

• 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

Anandakrishnan et al. 2006

Proposed Direct Imaging Dither Patterns

Pattern Scale Max Offset Median Offset Sub-Pixel 4-Pt Box N/A 3.5 pix 2.5 pix pix

Small 11 pix 10.5 pix pix Medium 119 pix 53 pix pix

Cycling

Large 161 pix 97 pix pix Small 13 pix 15 pix pix Medium 27 pix 30 pix pix

12-Pt Reuleaux

Large 55 pix 59 pix pix

MIRI LRS Specifications

• Wavelength range: 5-10 m nominal (2-14 m expected)

• Slit Dimensions: 0.65.5 (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

• 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

Proposed LRS Observing Modes

JWST Observatory Offsetting Accuracy

• 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

Anandakrishnan et al. 2006

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

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

10-1

100

101

102

Time To Complete Slew, min

Slew Performance: [Max Accel, Max Rate, T1] = 0.0001453 8.19e-005, 0.0539 0.036, 60

slew capability (6 rwas)slew capability (4 rwas)slew requirement

10-6

10-5

10-4

10-3

10-2

10-1

10-6

10-5

10-4

10-3

10-2

10-1

100

101

102

103

10-1

100

101

100

101

102

103

10-1

100

101

102

Time To Complete Slew, min

Slew Performance: [Max Accel, Max Rate, T1] = 0.0001453 8.19e-005, 0.0539 0.036, 60

slew capability (6 rwas)slew capability (4 rwas)slew requirement

• 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)

Observatory Pointing Efficiency

Mitchell 2008

Angle (degrees)