JWST ETC IFU Nod in Scene and IFU Nod off Scene Strategy

The JWST Exposure Time Calculator (ETC) Strategy tab for the NIRSpec IFU, MIRI MRS, and MIRI MRS time-series modes offers 2 options: IFU Nod In Scene and IFU Nod Off Scene.

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IFU Nod In Scene and IFU Nod Off Scene allows users to specify how and from where the background flux should be extracted for the signal-to-noise calculation.

IFU Nod In Scene will produce a higher SNR for the same Detector Setup parameters compared with 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 requested for the IFU Nod Off Scene strategy is on-source and hence the SNR is lower.


IFU Nod In Scene

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:

  • 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.

  • Shape of the aperture for this strategy is circular with the radius measured in units of arcseconds or pixels. A default value is provided but can be changed by the user.

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.

  • 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 second nod (the offset nod) is subtracted from the first nod (see Figure 2), then flux is extracted within the specified aperture at each position and co-added. The number of integrations, which is specified under the Detector Setup tab, is multiplied by a factor of 2 to account for both nods.

  • Wavelength of Slice corresponds to the wavelength at which the 2D images are displayed in the output and the scalar values are reported in the Reports pane. 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 or MIRI MRS time series 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.

  • Angular units can be either arcseconds or pixels. The choice made here determines the units for the Aperture location, Aperture radius, and Nod position in scene.

Figure 2. Schematic diagram showing the IFU Nod In Scene strategy

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. The SNR is the co-addition of both positions.

IFU Nod Off Scene

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. For this strategy, the aperture radius applies to two different scenes: one with the source and one without the source (just the background). The flux from all pixels is summed within each aperture and then the total background flux is subtracted from the total flux (source + background). The following are sub-options within this strategy:

  • 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.

  • Shape of the aperture for this strategy is circular with the radius measured in units of arcseconds or pixels. A default value is provided but can be changed by the user.

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.

  • Wavelength of Slice corresponds to the wavelength at which the 2D images are displayed in the output and the scalar values are reported in the Reports pane. 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.

  • Angular units can be either arcseconds or pixels. The choice made here determines the units for the Aperture location and Aperture radius.



Latest updates
  •  
    Updated for ETC 2.0.

  •  
    Updated for ETC 1.6.


  • Wording changes and clarification.


  • Updated for ETC 1.3.
Originally published