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.

        Video Tutorial:  ETC General Overview  

Images

Types of Images

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 Saturation. The images contain the entire scene used in the calculation as well as drawings representing the location of the extraction aperture and background region(s). 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 group and cannot be recovered. More details can be found in 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 or 1 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.

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.
• Toggle Spike Lines: Turn guiding lines on or off in the image.

All images also present drawings of the extraction aperture and background region(s) (if applicable). The extraction aperture is denoted by the solid yellow line and the background regions are denoted by the dashed light green lines. These aperture drawings can be toggled on or off by clicking on the associated items in the legend below each image. Clicking on "background" will toggle all background regions in the image. Note that aperture drawings are supported for all modes except NIRISS SOSS.

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Plots

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 modes. This 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.
  
  

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.
• Box Select: Draw a re-sizable, re-positionable box in the image. Double-clicking on the box removes it from the plot.
• Lasso Select: Draw a shape in the image. Double-clicking on the shape removes it from the plot.
• Zoom in
• Zoom out
• Reset axes: Return the image to its original state.
• Toggle Spike Lines: Turn guiding lines on or off in the image.
• 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 time 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). 

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