JWST NIRSpec
Dithering Strategies
(and a Cross-SI
View)
Jason Tumlinson
JIM / TIPS
Oct 15, 2009
NIRSpec Refresher
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JWST SI Dithering Principles
Dithering is needed generically for:
1.
2.
3.
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In keeping with the JWST project “template” approach to ops, we are
trying to “pre-define” the SI dither patterns, which will:
1.
2.
3.
4.
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mitigating detector flaws and pixel-to-pixel response variations,
and for
improving the pixel-space sampling of the point- or line-spread
function.
increasing coverage of the sky.
provide the user with a simple set of optimal choices that are still
flexible enough to adapt to a range of science needs.
simplify the on-board scripts that implement dithers
limit the possibilities for user or software errors (patterns are less
handmade than for HST).
simplify things still further by sharing and reusing components
(planning tools, on-board script code) that applies to multiple SIs.
Generally in JWST-land, a visit = one guide star = maximum angular
extent of pointings in a visit limited to ~20” by availability of GS within
2.2’ FGS field.
Primary vs. Secondary Dithers
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“Primary Pattern” : generally large in scale, >> pixel,
designed to mitigate pixel-to-pixel response variations
and/or improve spatial coverage by moving to different
parts of the detector.
Can also apply to large-scale “tiling” of the FOV for large
fields.
“Secondary Pattern” : generally small scale, ~ pixel,
designed to improve pixel-space sampling of the PSF or
LSF.
Generally allowed to nest, so execute a secondary
pattern at each of the primary positions.
NIRSpec Fixed Slits
• Primary Pattern: Step down the slit in 1,
2, 3, or 5 positions separated by ~ 0.5”,
user choice.
• Secondary pattern: 0.5 pixel offsets in
spatial, spectral dimensions, or both as
a user choice.
NIRSpec IFU Primary Pattern: “Tiling”
•
4
2
0
1
3
5
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Primary Pattern: Define a set of “tiles” on
the sky to cover an extended source with
the 3”x3” FOV. (Using a NIRSpec-specific
tool resembling the MIRI mosaic tool in
APT).
Secondary pattern: slitlets (next page)
NIRSpec IFU: Secondary Patterns
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Simultaneous with these moves, 1.5
pixels shifts in the spatial, spectral
dimensions can resample the PSF and
LSF in pixel space.
So at each primary position (“tile”),
there are two positions observed with
small offsets in the dispersion direction,
cross-dispersion, or both, depending
on user choice.
The 30 “slitlets” are interlaced
such that adjoining slitlets fall
on very different detector
regions; moving over 100 - 500
mas (1 - 5 slitlets) can move the
spectrum vertically by > 1000
pixels. Very good for avoiding
detector artifacts.
NIRSpec MSA Strategy 1: “Slitlets”
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Place target in each of 3 shutters in a 3x1 slitlet, use the other two for measuring sky
background.
Places each target in the “sweet spot” within each microshutter where slit transmission is
maximized (60%) and relatively flat.
This strategy works well if 3x1 shutter slitlets are pre-assigned to each target during the
planning of the MSA configuration.
This strategy (and its close relatives) is the ONLY “dither pattern” for the MSA that can be
done deterministically - because we will fly a non-ideal MSA.
NIRSpec MSA Strategy 1I: “Subshutter”
This strategy does not force every
target into the shutter “sweet spot”.
Rather, it allows them to fall where
they may and averages out
uncertainties in the shutter
transmission curve by observing
at, e.g. 16 positions distributed
symmetrically around the shutter.
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However, this strategy requires multiple configurations that can only be derived at planning
time - there is no “canned” dither pattern that can accomplish this pattern in the general
case of any astronomical scene and a non-ideal MSA.
This strategy can be implemented in the tool, with the offsets and the corresponding MSA
configurations passed downstream to a script that doesn’t know about the strategy itself.
NIRSpec MSA Strategy III: “Generic”
The great appeal of the MSA, and
therefore of NIRSpec, lies in its
ability to observe an arbitrary
pattern of sources on the sky.
This large degree of complexity,
coupled to a non-ideal MSA,
means that “canned” dithering
patterns cannot exist outside the
narrow range of strategies that
can be fully planned in advance.
Therefore: MSA patterns will be defined generically at planning time in the MSA
planning tool (already well advanced).
We envision that the user will be provided with planning “shortcuts” that help
implement simple patterns like the slitlet or subshutter strategies and iteratively
arrive at optimal coverage of a given target set. Operationally, MSA dithers are
just a list of MSA configurations and center positions on the sky.
JWST Science Instrument Dithering Strategies Summary
Mode
Instrument
(Report #)
Fixed Slits
(1678)
IFU
NIRSpec
(1749)
MSA
(1769)
LRS
(1634)
MIRI
MRS
(TBD)
Imaging
(1657)
Primary
Secondary
Special Purpose
Comments
Downslit
[N = 1,2,3,5 positions]
Subpixel
(1) spectral [yes/no]
AND/OR
(2) spatial [yes/no]
A_200_1 to A_200_2
“l-gap-filling”
[yes/no]
(1) no dithers for “large
square”
(2) A_200_2 repeats the
pattern in A_200_1
regular grid for >3” field
[Nx, Ny, Dx, Dy intervals,
rotation, etc.]
(1) Slitlet [N = 0,1,3,5
steps] AND/OR Subpixel
(2) spatial [yes/no]
(3) spectral [yes/no]
...
“tiling” uses tool like “MIRI
Mosaic” to construct the
pattern
all, defined by MSA planning
tool
...
...
shortcuts in tool will
create simple patterns
deterministically
Extended Source / Mapping
[Nx, Ny, Dx, Dy intervals,
etc.]
OR
Point Source/Staring
(2 positions within slit)
...
...
...
regular grid for >3” field
[Nx, Ny, Dx, Dy intervals,
rotation, etc.]
2 pt or 4 pt (details TBD)
[channel, N = 2 or 4 pt]
...
report pending
12-pt Reuleaux [S/M/L]
OR
311-pt Cycling
[S/M/L, N + start]
4 pt box
(subpixel)
...
MIRI imaging mosaics are
not dithers
Full-field
[3 tight, 3, 6, or 9 tile]
OR
NIRCam
Imaging
Can nest or combine
N-point intra-module
[N = 3 - 16]
NIRSpec Pre-imaging
N-position subpixel
[N = 1 - 9]
NIRSpec Pre-imaging can
also combine with
Use [3-tight or 3,6,9-tile]
standard secondary
(4 GS acqs)
patterns
OR
OR
“General” 64-pt subpixel
[N = 1 - 64]
“NIRSpec Minimal”
fixed pattern
(1738)
[user options]
OR
large-scale tiling in
separate visits has been
omitted
Regular, Deterministic (“Canned”) with simple parameters
YES
NIRSpec FS
Downslit
NO
NIRSpec FS l-gap
MIRI LRS Point
Source/Staring
YES
MIRI Imaging 12-pt
Reuleaux
MIRI Imaging 311-pt
Cycling
NIRCam N-point intra-module
ALL SECONDARY
PATTERNS
NIRCam Imaging Fullfield
NO
NIRCam N-point intra-SCA
NIRSpec IFU Extended Source
MIRI LRS Extended Source /
Mapping
MIRI MRS Extended Source / Mapping
(TBD)
NIRSpec MSA generic