JWST NIRSpec has several dither and nod patterns for the MOS observing mode to mitigate detector effects, help remove cosmic rays, improve flux accuracy, and improve spatial and spectral sampling.


Dither and nod options for NIRSpec MOS

Parent articlesNIRSpec Operations → NIRSpec Dithers and Nods 
See also: NIRSpec MOS Recommended StrategiesNIRSpec Dithering Recommended StrategiesNIRSpec MSA Leakage Correction for IFU ObservationsNIRSpec MSA Leakage Subtraction Recommended Strategies

The NIRSpec multi-object spectroscopy (MOS) mode has several dither and nod options available. In MOS mode, these offsets can be used in data processing to mitigate detector effects, help remove cosmic rays, improve spatial or spectral sampling, and improve flux accuracy.  Additionally dithers and nods can help mitigate the effects of light leakage through the MSA.

Micro-shutter assembly (MSA) slitlets are two or more open shutters, adjacent in the cross-dispersion (spatial) direction, that form a longer "slitlet," as described in the NIRSpec Multi-Object Spectroscopy article (see Figure 1).  

In NIRSpec MOS mode, nods are defined as offsets of the telescope that move each science source into a new shutter of the same MSA configuration (the same open shutter pattern of slitlets).  This generates a set of nodded exposures, three in the case of the 3-shutter slitlet.  These images will be subtracted in data processing to remove in-field background flux (see Figure 1 and Table 2). Nods are typically recommended for point-like targets. 

It is possible to apply nods for MOS spectroscopy observations using either the MPT Planner which designs observations automatically, or manually using the Manual Planner.

Figure 1. The NIRSpec MSA nod strategy

NIRSpec MSA nod strategy

An illustration of the philosophy for using MSA shutter slitlet nods to subtract background flux.  A 3 shutter MSA slitlet configuration is opened on each science target (left), and the telescope executes nods to place the targets within each of the three shutters in successive exposures (right). For each shutter, the average of two of the exposures is used to subtract background flux from the third (right). 

The Fixed Dither 1 and Flexible Dither options in the MPT Planner (see Figures 2 and 3can be used to observe science sources of interest at different positions and through different MSA open-shutter configurations. The Fixed Dither option can be used to move source spectra to new locations on the MSA, so that spectral traces are on a different part of the detector. Both dither options can be combined with nods. Nods are selected by clicking the Nod in Slitlet checkbox in the MPT Planner pane.  If dithers and nods are both selected, nods will occur at each of the defined dither positions


Figure 2. NIRSpec MSA fixed dither example

An illustration of a small portion of the MSA during a 2-point Fixed Dither. The MSA is moved approximately three shutters up and to the left on this sketch. Because of optical distortions, slitlets cannot be merely translated from one position in the MSA to the other, so the MPT software locates the sources and vets the new slitlets to make sure the target is completely observed. Fixed dithers require that sources are observed in both pointings, and only sources that avoid failed shutters at both positions are observed. Hence, unlike the flexible dithers below (Figure 3), the slitlets that appear only in pointing one or pointing two are absent from this figure.

Figure 3. NIRSpec MSA flexible dither example

The Flexible Dither methodology is presented here. Unlike fixed dithers, some sources are only observed in one pointing. There are three reasons for this: (1) some sources do not fall in the overlap between the pointings, as is the case for four slitlets outside the overlap; (2) failed shutters interfere with where a slitlet should have gone, as is the case for the upper pair of red and blue slitlets, "A" and "B" in the overlap region; (3) optical distortion may place some of the sources behind the bars of the MSA, as is the case for the second pair of red and blue slitlets, "C" and "D" in the overlap region).

The NIRSpec MOS mode has a specialized planning interface template in the Astronomer's Proposal Tool (APT) software: the MSA Planning Tool (MPT).  The options for creating nods and dithers in MPT are described in Table 1.  It is also possible to design a set of nodded exposures in the Manual Planner, where simple MOS observations can be designed. The Manual Planner article describes that process.


Table 1.  NIRSpec MOS dither and nod options in MPT

Offset option

APT parameter

Description

Nodding

Nod in Slitlet (checkbox)

The telescope is repositioned slightly between exposures to place the targets in different open shutters within their respective slitlets using a single MSA configuration. This is called nodding. 

Table 2 presents the possible configurations for nodding patterns using the NIRSpec MOS observing mode. Figure 1 presents the strategy for MSA nods in a 3-shutter slitlet configuration. These offsets are executed within the same MSA configuration.  Standard pipeline processing subtracts consecutive nodded exposures to remove the background, before coadding the data into a single calibrated spectrum. 

Nods can also be used, as with dithers, to mitigate detector effects and help remove cosmic rays. Some users may wish to only nod their MOS observations, and not include fixed dithers, because observing with fewer configurations improves multiplexing and saves a small amount of overhead. For extended sources, where subtraction of the nod positions in the standard pipeline processing would be inappropriate, users should plan to reprocess their data.   Additionally, in this scenario, the MSA configuration should include background slits to obtain a master background.

Fixed dither option

Fixed Dithers

This option can be used to translate a planned MSA slitlet configuration to a new location within the MSA field of view, so that spectra move to a different part of the detector.

These are larger dithers (outside a slitlet) that require a reconfiguration of the MSA in order to re-observe the same targets. Users can request a dither in units of shutters in X (positive offset moves the MSA to the right along dispersion axis) and Y (positive offset moves the MSA upward in the cross-dispersion (spatial) direction). These fixed dithers can be used in conjunction with slitlet nods to increase the number of exposures acquired with offsets. MSA science sources will only be included and considered complete if they fall into operable and available shutters at all dither positions. Due to distortions, different sources at the MSA plane will move by slightly different distances between dither points. (Hence, "fixed dithers" is a bit of a misnomer.)

Flexible dither option

Flexible Dithers

For this option, the MPT first finds the best pointing to observe the most science sources within one MSA configuration, and then searches for subsequent optimal MSA configurations that maximize the number of observed sources in all offset positions. 

Dither constraints are specified in units of arcseconds in either the X (dispersion) or Y (spatial) direction. 

Users specify constraints on the distances of each new dither point relative to the previous dither point (or the set of all previous dither points). Minimum or maximum separation constraints may be specified. There are four options for constraints on offset distances:

  • separation (x,y) >= from last pointing

  • separation (x,y) <= from last pointing

  • separation (x,y) >= from all pointings

  • separation (x,y) <= from all pointings

X and Y offset values are in units of arcseconds in the MSA aperture plane. Please refer to the APT documentation for the NIRSpec MSA Planning Tool (MPT) for additional details. At each dither point, sources are mapped independently to the MSA aperture plane, so the differential distortion is correctly accounted for. For example, a second configuration can be requested at more than an ~18" distance in the dispersion direction from the initial position in order to cover the wavelength gap caused by the physical distance between the NIRSpec detectors. This issue is described in a summary in the MOS Mode overview page. Because of the computational complexity of this approach, for primary candidate lists with more than a few hundred sources, this option is limited to planning fewer than four or five MSA configurations at one time.


See the MSA Planning Tool Planner description and tutorials for expanded information on MOS mode dither and nod options. There are considerations to take into account when deciding whether to nod or to use Fixed or Flexible dithers. For example, nodding is useful for point-like targets and costs little in overheads.  Dithering provides a means to sample areas of the detectors with different quantum efficiency and to be able to fill in the wavelength gap in MOS spectra, but requires more overheads to re-configure the MSA.  The article NIRSpec Dithering Recommended Strategies will help the user make a more informed decision about when to use each.

  

Table 2.  NIRSpec MSA SLITLET NODS in MPT

MSA SLITLET NODS

Position index

Y (spatial)

arcseconds

OFFSET meaning

NONE

1

0.000

No offset

2-Shutter Nod


1

0.000

Initial position

2

-0.510

Offset from initial position


3-Shutter Nod 


1

 0.000

Initial position

 0.510

Offset from initial position

 -0.510

Offset from initial position

     

5-Shutter Nod
with 5 points


 1

 0.000

Initial position

 0.510

Offset from initial position

1.020 

Offset from initial position

-0.510 

Offset from initial position

-1.020

Offset from initial position

   

5-Shutter Nod 
with 3 points

 0.000

Initial position

1.020 

Offset from initial position

-1.020

Offset from initial position

Nods are offsets along the cross-dispersion (Y) axis only. MSA slitet nods will be generated automatically in MPT when the Nod in slitlet checkbox is checked. The number of nod positions will equal the number of open shutters in the Slitlet that was chosen. When the 5 Shutter Slitlet is selected, an additional option is provided to select either 3 or 5 exposures per configuration.

A 2-Shutter Nod is obtained when the 2 Shutter Slitlet option and the Nod in slitlet checkbox are selected. It starts with science sources in the upper shutter of the 2-shutter slitlet, and then offsets the sources to the lower shutter.

The 3-Shutter Nod and both 5-Shutter Nod patterns start with the target in the central shutter of the 3 Shutter Slitlet, or the 5 Shutter Slitlet.










Last updated

Updated March 23, 2018

  • Corrected wording referring to APT parameters in the text and table.


Updated November 27, 2017

  • Changed flexible dithers figure


Updated July 12, 2017

  • Added fixed and flexible dither figures


Published April 19, 2017


 

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Updated November 27, 2017

  • Updated Flexible Dithers figure

Published March 02, 2017