JWST ETC Images and Plots

The JWST Exposure Time Calculator (ETC) outputs 2-D images and line plots for all the calculations it performs. These outputs enable visual inspection of the results for a chosen set of input parameters.

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        Video Tutorial:  ETC General Overview  


Types of Images

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

The 2-D images shown in the output Images pane are associated with the calculation selected by clicking a row in the Calculations table. The selected calculation is highlighted in yellow in the Calculations pane. The ETC displays various 2-D output images for a calculation and these can be selected using one of the 4 tabs: (1) 2D SNR, (2) Detector, (3) Saturation, and (4) Groups Before SaturationThe images contain the entire scene used in the calculation. For spectroscopic observations, the images shown correspond to one particular wavelength specified by the Wavelength of Interest, not the collapsed data cube.

  • 2D SNR - Image showing the signal-to-noise ratio (SNR) per pixel over the 2-D scene (Figure 1).

    For NIRSpec calculations (all modes), the 2D SNR image does not include contributions from correlated read noise, while the 1-D SNR plots do. This leads to discrepancies between the 2D SNR image and the SNR (λ) and SNR (time) 1-D plots.

  • Detector - Image showing the count rate (in e/sec) for each detector pixel.

  • Saturation - Image showing the 2-D distribution of saturated pixels, allowing the user to examine the exact locations of saturated pixels and decide whether the data would be useful even with saturation in a few pixels. When there are saturated pixels, the status column of the calculations table shows a warning sign (yellow exclamation point). In the Reports pane, the Warnings tab will appear in red indicating that there is a warning message which will inform the user about the number of pixels with partial and full saturation. A pixel value of 0 (blue) indicates no saturation, 1 (grey) indicates partial saturation, and 2 (red) indicates full saturation.

    Saturation Warning

    Partial saturation implies that partial ramps may still be used in some cases. Full saturation implies that some of the pixels are saturated at the end of the first two groups and cannot be recovered. Note that the ETC does not support single group (Groups per integration = 1) integrations. More details can be found on the JWST ETC Saturation article.

  • Groups Before Saturation - Image showing the maximum number of Groups the user can specify before that pixel saturates. The value for the brightest pixel is given in the Reports pane as the "Maximum Number of Groups Before Saturation." A pixel value of 0, 1, or 2 indicates that the flux in that pixel is high enough to cause full saturation; the value can be 0 for extremely bright sources because the fractional number of groups at which saturation occurs is rounded prior to being displayed.

Figure 1. 2D SNR output images

These 2D SNR images are from a NIRCam SW imaging calculation using a scene with multiple sources (left) and from a NIRSpec fixed slit spectroscopy calculation using a single source (right).

Navigating Images

The interactive plot (Figure 2) allows individual pixel values to be displayed by hovering the cursor over each pixel. A pop-up box appears when hovering over pixels. The x and y values correspond to the horizontal and vertical locations in the scene, respectively. The z value corresponds to the value in that pixel, which depends on the image being examined.

From left to right, the different labels and functions are:

  • Download plot as PNG
  • Zoom: A user-specified zoom box. Drag a box in the image to zoom in on that box.
  • Pan: Hover the cursor in the image, click and hold, then move the cursor to examine different regions of the image.
  • Zoom in
  • Zoom out
  • Reset axes: Return the image to its original state.

Figure 2. Examining images


The ETC Plots pane serves 2 purposes: (1) to visualize the output from a single calculation, and (2) to compare the results from multiple calculations within a workbook. 

Visualizing the results from a calculation

The results shown in the Plots pane correspond to the selected calculation that is checked using the checkbox in the calculations table. The color and line style in the plot corresponds to the color and line style of the checkbox. 

The desired output quantity to plot can be selected using one of the tabs within the Plots pane:

  • ApFlux - shows the extracted flux (e/s) from the source in the aperture versus wavelength.
  • ApBackground - shows the extracted sky background flux (e/s) versus wavelength.
  • SNR - shows the signal-to-noise ratio (SNR) versus wavelength. The SNR in the extracted spectrum is per pixel along the wavelength axis, not per resolution element. For imaging calculations, the SNR value is shown at the Wavelength of Interest (the effective wavelength of the filter, which is a flux-weighted average that depends on the filter and the input source spectrum).
  • SNR (time) - shows the SNR versus on-source time. For imaging calculations, the SNR (time) value is shown at the wavelength of interest (the effective wavelength of the filter).
  • Contrast - shows the contrast versus separation, and is only for coronagraphic imaging modesThis shows the limiting contrast, the faintest possible point source that can be detected for the chosen observation strategy, as a function of angular separation from the host source.

The NIRSpec wavelength scale is nonlinear, but the FITS WCS standard used by the ETC only supports linear wavelength scales. This results in discrepancies between the wavelengths reported in the Images and Plots for a given calculation; the 1-D plots use the correct, non-linear wavelength range, whereas the 2-D images use the approximated linear wavelength range.

Navigating Plots

The Plots pane provides options for more in-depth exploration of the plots. Hovering the cursor over the plot brings up a list of options in the top right of the pane. From left to right, the different labels and functions are:

  • Download plot as PNG
  • Zoom: A user-specified zoom box. Drag a box in the image to zoom in on that box.
  • Pan: Hover the cursor in the image, click and hold, then move the cursor to examine different regions of the image.
  • Zoom in
  • Zoom out
  • Autoscale: Automatically set the x- and y-axis to the optimal ranges. Similar to Reset axes.
  • Reset axes: Return the image to its original state.
  • Show closest data on hover: Default. Hover over the plot to get a pop-up showing the value for the nearest data point (imaging or spectroscopy).
  • Compare data on hover: Hover over the plot to get pop-ups showing the values at each wavelength for multiple over-plotted calculations (imaging or spectroscopy).

Comparing results from multiple calculations 

The ETC allows users to easily compare the results from multiple calculations by over-plotting them. The desired calculations can be selected from the Calculations pane by clicking on the checkboxes, and the results from all of them will be displayed in the same plot. The color of the outline for each checkbox corresponds to the color in the plot.

All calculations in a given workbook may be over-plotted simultaneously by using the All option in the drop-down menu that is available on the checkbox pull-down menu above the Calculation pane. This feature is useful for comparison of the calculated parameters for different filters/grisms of the same instrument, or for different instruments. This feature also makes it possible to compare the SNR vs. wavelength for multiple filters (Figure 3) and across instruments covering a broad spectral range, by selecting the calculations to be plotted simultaneously.

The ability to compare multiple calculations using the plot is especially useful to analyze the outputs from batch expansions (over filters, groups, integrations, etc.). The SNR (time) plot can be used to determine the exposure required to achieve a given SNR by performing an expansion over groups or integrations. When expanding over the time parameters (Groups per integration or Integrations per exposure), all the results from the calculations done in batch expansion can be selected by using the checkbox and plotted simultaneously, and the exposure time required to obtain the desired SNR can be directly read off the graph (Figure 4). 

Figure 3. The SNR versus wavelength from multiple imaging calculations

The results shown are from multiple calculations performed using a batch expansion over all the broadband and medium-band filters for the NIRISS imaging mode. Each point corresponds to a different calculation that uses a different imaging filter.
Figure 4. The SNR versus wavelength from multiple spectroscopy calculations

The SNR of extracted spectra from multiple calculations performed using a batch expansion over groups for NIRSpec fixed slit spectroscopy of a single source (left panel). The batch calculation provides SNR for different exposure times resulting from the different number of groups, and the result has been plotted to show the SNR versus on-source time (right panel) for the Wavelength of Interest, which was set to 3 μm.

Latest updates
    Updated for ETC 3.0.

  • Update for ETC 2.0. Added a caveat about the wavelengths reported by the 1-D plots and 2-D images.

    Updated for ETC 1.7.

    Updated for ETC 1.5. Video link added.
Originally published