JWST ETC Wavelength of Interest

For spectroscopic calculations in the JWST Exposure Time Calculator (ETC), users have the option to specify a Wavelength of Interest (or Wavelength of Slice) in the Strategy tab that affects the outputs that are displayed. 

Video Tutorial:  ETC IFU Strategies

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For spectroscopic calculations in the ETC, one of the required Strategy tab parameters is called the Wavelength of Interest or Wavelength of Slice, depending on the observing mode (Figure 1). These 2 parameters are equivalent, so Wavelength of Interest will be used to refer to both parameters for the remainder of this article.

The Wavelength of Interest comes pre-filled with a default value, but the user can change this to any other valid wavelength within the grating's wavelength range (denoted by the values in parentheses above "microns"). This quantity is to determine the wavelength of the images displayed and the scalar values presented in the Reports pane. The Wavelength of Interest is listed as the "Wavelength of Interest Used" in the Reports pane.

The Wavelength of Interest determines which 2-D images are presented upon the completion of a calculation. The images are presented at that wavelength only and are not images of the collapsed data cube. Changing the value of the Wavelength of Interest updates the images to correspond to the desired wavelength.

The user can supply a Wavelength of Interest to arbitrary precision, but the ETC will always use the wavelength value of the nearest pixel. The wavelength of the nearest pixel is reported as the "Wavelength of Interest Used" in the Reports pane.

The Wavelength of Interest is provided for the following calculation types: MIRI low resolution spectroscopy (LRS) slit, MIRI low resolution spectroscopy (LRS) slitless, NIRCam LW grism time series, NIRCam wide field slitless spectroscopy, NIRISS SOSS, NIRISS WFSS, NIRSpec multi-object spectroscopy, NIRSpec fixed slit, and NIRSpec bright object time series. The functionally equivalent Wavelength of Slice is provided for MIRI medium resolution spectroscopy (MRS), MIRI MRS time series, and NIRSpec IFU.

Keep in mind that changing the Wavelength of Interest value requires the calculation to be re-run. Changing this value does not change the calculation results, only the images displayed and the values shown in the Report. Download the tar file or examine the plots to see the results over the full wavelength range.

There is no equivalent parameter to the Wavelength of Interest specifiable by the user for imaging and coronagraphic imaging calculations. In these instances, the "Effective Wavelength" listed in the Report tab is the flux-weighted average of the input spectrum convolved with the filter transmission profile, and cannot be changed by the user.

Figure 1. Location of Wavelength of Interest and Wavelength of Slice in the Strategy tab

Wavelength of Interest error message

Specifying a value for the Wavelength of Interest that is outside the grating's stated wavelength range will result in an error message. Changing the configuration on the Instrument Setup tab will also result in an error message, but only if the chosen Wavelength of Interest is not valid for the new configuration.

The ETC does not automatically update the Wavelength of Interest when the user changes the configuration in the Instrument Setup tab. The only time this value is automatically populated is when a new calculation is created (including batch expansions). This is in order to prevent the ETC from making changes to a workbook that the user may not be aware of.

There are 2 options for removing the error message (Figure 2):

  1. Click on the Update to midpoint button on the Instrument Setup tab. This will update the value in the Wavelength of Interest to the value specified on the button without requiring the user to manually input a valid wavelength on the Strategy tab.
  2. Manually update the value on the Strategy tab.

Figure 2. Two options for updating the Wavelength of Interest

Latest updates
    Updated for ETC 3.0.

    Updated for ETC 2.0.

    Updated for ETC 1.6.

  • Updated for ETC 1.5. Video link added.
Originally published