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MIRI Dither Patterns

MIRI Dither Patterns. Christine H Chen. Dithering Goals. Mitigate the effect of bad pixels Obtain sub-pixel sampling Self-calibrate data if changing scattered light and/or thermal emission background is significant

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MIRI Dither Patterns

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  1. MIRI Dither Patterns Christine H Chen

  2. Dithering Goals • Mitigate the effect of bad pixels • Obtain sub-pixel sampling • 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)

  3. MIRI Observing Modes • Direct Imaging • Full array • Subarray • Coronagraphic Imaging • Low Resolution Spectrograph (LRS) • Medium Resolution Spectrograph (MRS)

  4. 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

  5. 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

  6. 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)

  7. 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

  8. 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

  9. 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

  10. Proposed Direct Imaging Dither Patterns

  11. 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

  12. 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

  13. 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

  14. 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

  15. 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

  16. Observatory Pointing Efficiency • 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) Mitchell 2008 Angle (degrees)

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