JWST NIRSpec Dithering Strategies

(and a Cross-SI View)

Jason TumlinsonJIM / TIPS

Oct 15, 2009

NIRSpec Refresher

JWST SI Dithering Principles• Dithering is needed generically for:

1. mitigating detector flaws and pixel-to-pixel response variations, and for

2. improving the pixel-space sampling of the point- or line-spread function.

3. increasing coverage of the sky.

• In keeping with the JWST project “template” approach to ops, we are trying to “pre-define” the SI dither patterns, which will:

1. provide the user with a simple set of optimal choices that are still flexible enough to adapt to a range of science needs.

2. simplify the on-board scripts that implement dithers3. limit the possibilities for user or software errors (patterns are

less handmade than for HST). 4. simplify things still further by sharing and reusing

components (planning tools, on-board script code) that applies to multiple SIs.

1. 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 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.

Primary vs. Secondary Dithers

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.

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NIRSpec IFU Primary Pattern: “Tiling”

• 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)

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1

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NIRSpec IFU: Secondary Patterns

• 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.

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.

NIRSpec MSA Strategy 1: “Slitlets”

• 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”

• 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.

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.

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.

Instrument Mode(Report #)

Primary SecondarySpecial Purpose

Comments

NIRSpec

Fixed Slits(1678)

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 “-gap-filling”

[yes/no]

(1) no dithers for “large square”

(2) A_200_2 repeats the pattern in A_200_1

IFU(1749)

regular grid for >3” field[Nx, Ny, x, y 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

MSA(1769)

all, defined by MSA planning tool . . . . . .

shortcuts in tool will create simple patterns

deterministically

MIRI

LRS(1634)

Extended Source / Mapping

[Nx, Ny, x, y intervals, etc.]OR

Point Source/Staring(2 positions within slit)

. . . . . . . . .

MRS(TBD)

regular grid for >3” field[Nx, Ny, x, y intervals,

rotation, etc.]

2 pt or 4 pt (details TBD)

[channel, N = 2 or 4 pt]. . . report pending

Imaging(1657)

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

NIRCam Imaging(1738)

Full-field [3 tight, 3, 6, or 9 tile]

OR

N-point intra-module [N = 3 - 16]

OR

N-point intra-SCA [LW vs. SW, N = 1 - 25,

S, M, L]

N-position subpixel [N = 1 - 9]

OR

“General” 64-pt subpixel [N = 1 - 64]

NIRSpec Pre-imaging

Use [3-tight or 3,6,9-tile]

(4 GS acqs)

OR

“NIRSpec Minimal” fixed pattern

(2 GS acqs, does not cover SCA gaps)

NIRSpec Pre-imaging can also combine with standard secondary

patterns

large-scale tiling in separate visits has

been omitted

report is still under review

JWST Science Instrument Dithering Strategies Summary

Can nest or combine [user options]

Execu

tab

le w

ithi n

a s

ing

le v

isit

Regular, Deterministic (“Canned”) with simple parameters

YES

NO

YES

NOYESNIRSpec FS

Downslit

ALL SECONDARY PATTERNS

NIRSpec FS -gap

NIRSpec IFU Extended Source NIRSpec MSA generic

MIRI LRS Extended Source / Mapping

MIRI LRS Point Source/Staring

MIRI Imaging 311-pt Cycling

MIRI Imaging 12-pt Reuleaux

NIRCam Imaging Full-field

NIRCam N-point intra-module

NIRCam N-point intra-SCA

MIRI MRS Extended Source / Mapping (TBD)