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The JWST Exposure Time Calculator (ETC) Strategy tab for NIRSpec IFU and MIRI MRS modes offers two options: the 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 signal-to-noise calculation for these modes.

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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 for setting strategy for the science sources in the IFU scene. 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. This strategy uses a circular aperture with radius in arcsec that is specified by the user. 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 arcsec. The nods for the IFU Nod in Scene strategy are implemented in the ETC as 2-point dithers and the number of integrations which is specified under the detector setup is multiplied by a factor of 2 to account for the dithers. The Wavelength of Slice should also be provided for this strategy and corresponds to the wavelength at which the 2D images are displayed in the output, and the scalar values are reported.
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Figure 2. Schematic diagram showing the IFU Nod In Scene strategy

Schematic diagram showing the IFU Nod In Scene strategy

 


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 aperture location for the science target can be centered on source or can be specified using offset positions in X and Y directions in the scene. The flux extraction uses a circular aperture, and the aperture radius in arcsec is specified by the user. The Wavelength of Slice should be provided and is the wavelength at which the 2D images are displayed in the ETC output, and the scalar values are reported. The allowed wavelength range will depend on the instrument setup (i.e., depending on the channel and disperser combination for the MIRI MRS and the Grating/Filter Pair for the NIRSpec IFU) previously defined by the user. If the inputs provided are 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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Note: In the case of IFU Nod In Scene, the SNR is higher compared to the IFU Nod Off Scene because the source flux is being accumulated both at the initial position and at the nod position. In the 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.

 


 

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Last updated

Updated May 24, 2017

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Published May 5, 2017


 

 

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UpdatedMay 24, 2017
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TeamETC
AuthorRavindranath
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UpdatedMay 24, 2017
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