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This page contains information needed to design dither and nod patterns using the NIRSpec Fixed Slit observing mode.

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Introduction

The NIRSpec fixed slits (FS) spectroscopy mode has several dither and nod options available. In FS mode, the dithers are small-scale sub-pixel (<0.1") offsets of the target position, which will be used to mitigate detector effects, help remove cosmic rays, and particularly improve spectral and/or spatial sampling.  The nods in FS mode are larger offsets (>0.5") used to subtract nearby background flux. As a result, the nod options are best used for targets that are not significantly spatially extended on the scale of the offset. The FS mode nods provide the primary offset positions along the spectral slits, and the dither options provide optional subpixel offsets to improve sampling. The nod positions and sub-pixel dither offsets can be used separately or together. Table 1 and Figure 1 show a summary view of these nod and dither options and the ways they can be combined.

 

Table 1. Options for the NIRSpec FS offsets

Primary nods

2-, 3-, or 5-position nods are available.

The PRIMARY NOD POSITIONS option input may take the following values:

NONE

There will be one primary position along the slit.

TWOThere will be 2 primary positions along the slit.
THREEThere will be 3 primary positions along the slit.
FIVEThere will be 5 primary positions along the slit.

The primary nod positions will be defined to optimize throughput based on the FS hardware characteristics.

Subpixel dithers

Small-scale offsets in the spectral (dispersion) direction, spatial (cross-dispersion) direction, or both the spectral and spatial directions are available.

The SUBPIXEL DITHER option input may take the following values:

NONE

No sub-pixel dithering is performed.
SPECTRALThree spectral dither positions are defined for each primary position along the slit.
SPATIALTwo spatial dither positions are defined for each primary position along the slit.
BOTHFour dither positions are defined for each primary position along the slit.

Note that spectral offsets will decenter the source in the slit in the dispersion direction and therefore incur a change in slit losses, which are wavelength dependent. All offered dither options will be calibrated, but this calibration will be more challenging for SPECTRAL and BOTH dither patterns. 

Options selected for dithers and nods determine the number of exposures that are acquired, as shown in Figure 1. Any combination of primary nod positions and sub-pixel dithers can be selected. Table 2 lists the defined dither positions for each of these combinations.

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Figure title

Figure 1. FS dither and nod combinations

FS dither and nod combinations

Figure caption

The approximate locations of the primary nod and dither positions along the specified fixed slit, as indicated in the expandable Table 2 (below). The SUBPIXEL DITHER PATTERN options of NONE, SPECTRAL, SPATIAL and BOTH are shown at the top. Under each sub-pixel dither option, the PRIMARY NOD POSITIONS are shown as four panels: NONE23, or 5.  The number of exposures that result from each selection is shown at the bottom of the figure.

The "A" FSs on the left side of detector NRS1 can have wavelengths lost to the detector gap in the NIRSpec high spectral resolution mode (R ~ 2700). However, the wavelength gap can be filled and the full range of high resolution data can be acquired if the S200A1 and S200A2 slits are used together. When the S200A1 and S200A2 option is specified, the nod/dither pattern executes twice. The target is placed in the S200A1 slit and the requested exposures are executed at that position. Next, a move is made to the S200A2 slit and an identical set of exposures are executed.


Table 2.  NIRSpec fixed slit dither and nod positions

 
Expand
titleClick here to expand Table 2

 

Primary nod positions


Position along slit

Sub-pixel dither pattern

X,Y positions are expressed in arcsec (NIRSpec ideal) relative to the center of the fixed slit.

NONE

SPECTRAL

SPATIAL

BOTH

X

Y

X

Y

X

Y

X

Y

NONE

 

A

0.0

0.0

-0.033

 0.000

+0.033

 0.0

 0.0

 0.0

 0.0

0.0

-0.025

+0.025

-0.025

 0.000

+0.025

 0.000

 0.000

+0.025

 0.000

-0.025

 





2

 

A

0.0

+0.9

-0.033

 0.000

+0.033

+0.9

+0.9

+0.9

 0.0

 0.0

+0.875

+0.925

-0.025

 0.000

+0.025

 0.000

+0.900

+0.925

+0.900

+0.875

 

B

0.0

-0.9

-0.033

 0.000

+0.033

-0.9

-0.9

-0.9

 0.0

 0.0

-0.925

-0.875

-0.025

 0.000

+0.025

 0.000

-0.900

-0.875

-0.900

-0.925

 







3

 

A

0.0

+0.9

-0.033

 0.000

+0.033

+0.9

+0.9

+0.9

 0.0

 0.0

+0.875

+0.925

-0.025

 0.000

+0.025

 0.000

+0.900

+0.925

+0.900

+0.875

 

B

0.0

0.0

-0.033

 0.000

+0.033

 0.0

 0.0

 0.0

 0.0

 0.0

-0.025

+0.025

-0.025

 0.000

+0.025

 0.000

 0.000

+0.025

 0.000

-0.025

 

C

0.0

-0.9

-0.033

 0.000

+0.033

-0.9

-0.9

-0.9

 0.0

 0.0

-0.925

-0.875

-0.025

 0.000

+0.025

 0.000

-0.900

-0.875

-0.900

-0.925

 











5

 

A

0.0

+1.2

-0.033

 0.000

+0.033

+1.2

+1.2

+1.2

 0.0

 0.0

+1.175

+1.225

-0.025

 0.000

+0.025

 0.000

+1.200

+1.225

+1.200

+1.175

 

B

0.0

+0.6

-0.033

 0.000

+0.033

+0.6

+0.6

+0.6

 0.0

 0.0

+0.575

+0.625

-0.025

 0.000

+0.025

 0.000

+0.600

+0.625

+0.600

+0.575

 

C

0.0

0.0

-0.033

 0.000

+0.033

 0.0

 0.0

 0.0

 0.0

 0.0

-0.025

+0.025

-0.025

 0.000

+0.025

 0.000

 0.000

+0.025

 0.000

-0.025

 

D

0.0

-0.6

-0.033

 0.000

+0.033

-0.6

-0.6

-0.6

 0.0

 0.0

-0.625

-0.575

-0.025

 0.000

+0.025

 0.000

-0.600

-0.575

-0.600

-0.625

 

E

0.0

-1.2

-0.033

 0.000

+0.033

-1.2

-1.2

-1.2

 0.0

 0.0

-1.225

-1.175

-0.025

 0.000

+0.025

 0.000

-1.200

-1.175

-1.200

-1.225

 

In addition to using dither patterns to improve sampling and remove detector effects, FS users can also tile multiple FS positions to create "imaging spectroscopy" of larger spatial regions using the mosaic tools in APT. 
 

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Related links

Near Infrared Spectrograph, NIRSpec
NIRSpec Overview
NIRSpec Fixed Slits Spectroscopy

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Last updated

Published March 29, 2017


 

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