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The JWST Exposure Time Calculator (ETC) Strategy tab for the NIRSpec IFU and MIRI MRS modes offers two options: IFU Nod In Scene and IFU Nod Off Scene. These options allow users to specify how and from where the background flux should be extracted for the signal-to-noise calculation.


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Note: In the case of IFU Nod In Scene, the SNR is higher compared to IFU Nod Off Scene because the source flux is being accumulated both at the initial position and at the nod position. For IFU Nod Off Scene, there is no source flux in the off-source nod position because it is completely off the scene and only contains the sky background. Only half of the exposure time required for the IFU Nod Off Scene strategy is on-source and hence the SNR is lower.

IFU Nod In Scene

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Figure 1. Layout for IFU Nod In Scene under ETC strategy

Layout for IFU Nod In Scene under ETC strategy


Selecting the IFU Nod In Scene option allows the user to set the strategy for the sources in the IFU scene. The following are sub-options within this strategy:

  • The Aperture location can be specified to be centered on a source which can be selected from the drop-down menu, or can be specified as an X- and Y-offset from the scene center.
  • The Shape of the aperture for this strategy is circular with the radius measured in arcseconds. A default value is provided but can be changed by the user. Note: For point sources, the SNR can be highly dependent on the choice of aperture radius. It is recommended that users test different values for the aperture radius to optimize the SNR in the calculation.
  • The Nod position in scene is used for background determination from within the scene and is specified by providing the X- and Y-position in arcseconds. The nods for the IFU Nod in Scene strategy are implemented in the ETC as 2-point dithers so the number of integrations, which is specified under the Detector Setup tab, is multiplied by a factor of 2 to account for the dithers.
  • The Wavelength of Slice corresponds to the wavelength at which the 2D images are displayed in the output and the scalar values are reported. A default value is provided but can be changed by the user. The allowed wavelength range depends on the choices made in the Instrument Setup tab (i.e., depending on the channel and disperser combination for the MIRI MRS and the Grating/Filter Pair for the NIRSpec IFU). If the Wavelength of Slice provided is invalid, the affected tab names will turn italicized in red and an explanatory red text will appear to indicate the incorrect parameter.
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Figure 2. Schematic diagram showing the IFU Nod In Scene strategy

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Position 1: Source and scene specified by user. The source may be offset from the center of the scene, but here we present a source centered in the middle of the scene.
Position 2: Nod position specified by the user under the Strategy tab. Default values are provided but can be changed.
Difference image: Position 2 subtracted from Position 1. Red in this image indicates a positive value, while blue indicates a negative value. By default, the SNR is calculated for the positive source.




IFU Nod Off Scene

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Figure 3. Layout for IFU Nod Off Scene under ETC strategy


For the IFU modes, the background can also be estimated by using the IFU Nod off Scene option to mimic a dedicated sky observation. The following are sub-options within this strategy:

  • The Aperture location can be specified to be centered on a source which can be selected from the drop-down menu, or can be specified as an X- and Y-offset from the scene center.
  • The Shape of the aperture for this strategy is circular with the radius measured in arcseconds. A default value is provided but can be changed by the user. Note: For point sources, the SNR can be highly dependent on the choice of aperture radius. It is recommended that users test different values for the aperture radius to optimize the SNR in the calculation.
  • The Wavelength of Slice corresponds to the wavelength at which the 2D images are displayed in the output and the scalar values are reported. A default value is provided but can be changed by the user. The allowed wavelength range depends on the choices made in the Instrument Setup tab (i.e., depending on the channel and disperser combination for the MIRI MRS and the Grating/Filter Pair for the NIRSpec IFU). If the Wavelength of Slice provided is invalid, the affected tab names will turn italicized in red and an explanatory red text will appear to indicate the incorrect parameter.




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References

JWST technical documents

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Last updated


Updated November 5, 2018

  • Updated for ETC v1.3.

Updated May 24, 2017

  • updates to text

Published May 5, 2017



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Published March 02, 2017