Introduction Unlike other exposure time calculators, there is no option for reverse calculation of exposure duration parameters starting from a given signaltonoise ratio (S/N). Furthermore, there is often no unique solution for different instrument modes. Instead, the Exposure Time Calculator (ETC) provides a batch expansion capability to test a number of different detector parameters (groups or integrations) to determine which set of parameters results in the desired S/N. Users desiring a quick check of the sensitivity in different filters may also find useful the batch expansion over filters. On the ETC calculations page, there is an "Expand" tab with a menu of possible selections that includes Groups, Integrations, or Filters. Starting from an existing selected calculation in the Calculations table, these options can be used to expand the calculation for several iterative values of either the number of groups in the integration, the number of integrations in the exposure, or to include other available instrument filters while holding all other parameters constant as in the original calculation. Batch expansion will generate a number of new entries in the calculations table and it is possible to check the checkbox under "Plot" in that table to compare the results of all calculations.
Expansion typesBatch expansion can currently be performed by iterating over a set of groups, integrations and filters. The user must first select or highlight a calculation in the table which will be expanded. Then the user should select one of the items in the "Expand" tab near the top of the ETC pane. For the imaging modes, if "Filters" is selected, a number of new calculations will be initiated simultaneously, resulting in a S/N calculated for all available filters in the instrument mode of the parent (initial or selected) calculation. If either "Groups" or "Integrations" are selected, a new window will pop up and the user then selects a starting value, a step size, and a number of iterations for the expansion. Clicking will initiate the new simultaneous batch calculations. This will generate the same number of new entries or rows in the Calculations table as the number of iterations selected. It make take a few seconds or more to complete the calculations.Note that in each case, all other parameters of the (parent) calculation are held fixed. Same source parameters, instrument parameters, exposure setup parameters, etc. In particular, the extraction strategy remains the same; the new calculations utilize the same source radius and annulus or extraction box height and width. GroupsA batch expansion on the number of groups is initiated by selecting the "Expand" tab at the top of the ETC window, then choosing the "Expand Groups" option. A popup window will appear to prompt the user for the Start Value, Step Size, and Iterations. The start value is the starting number of groups (for the first calculation of the expansion). The Step Size is the increase or decrease in the number of groups to be tested at each iteration. Iterations is the number of new calculations to perform. IntegrationsA batch expansion on the number of integrations is initiated by selecting the "Expand" tab at the top of the ETC window, then choosing the "Expand Integrations" option. A popup window will appear to prompt the user for the Start Value, Step Size, and Iterations. The start value is the starting number of integrations (for the first calculation of the expansion). The Step Size is the increase or decrease in the number of integrations to be tested at each iteration. Iterations is the number of new calculations to perform. FiltersA batch expansion on the filters is initiated by selecting the "Expand" tab at the top of the ETC window, then choosing the "Expand Filters" option. If this option is chosen, a set of calculations will be initiated immediately, by simultaneously expanding upon all filters available for that instrument mode. Only the imaging modes of NIRCam, MIRI, and NIRISS allow expansions over filters. Note, users may want to adjust the aperture for PSFs in different filters, they will need to choose the aperture radius accordingly.
ExamplesNIRSpec group expansion Create a calculation using NIRSpec fixed slit mode by selecting the "Fixed Slit" option in the NIRSpec tab of the Calculations page.
 Select the calculation in the Calculations table by clicking on it.
 Click the "Expand" button at the top of the ETC window, and select "Expand Groups".
 Select the "Start" number of groups, the "Step" size in groups, and the "Iterations" (or number of calculations for the expansion).
 When you click "Submit", the new calculations will appear in the calculations table. It may take a few seconds to complete.
 Check the boxes labelled "Plot" next to each new expanded calculation in the Calculation table. If the "SNR" tab is clicked on the Plot pane at the bottom of the window, the S/N results for all new calculations will appear on the plot together.
NIRISS integrations expansion Create a calculation using NIRISS wide field slitless spectroscopy mode by selecting the "WFSS" option in the NIRISS tab of the Calculations page.
 Select the calculation in the Calculations table by clicking on it.
 Click the "Expand" button at the top of the ETC window, and select "Expand Integrations".
 Select the "Start" number of integrations, the "Step" size in Integrations, and the "Iterations" (or number of calculations for the expansion).
 When you click "Submit", the new calculations will appear in the calculations table. It may take a few seconds to complete.
 Check the boxes labelled "Plot" next to each new expanded calculation in the Calculation table. If the "SNR" tab is clicked on the Plot pane at the bottom of the window, the S/N results for all new calculations will appear on the plot together.
NIRCam filter expansion Create a calculation using NIRCam imaging mode by selecting the "Imaging" option in the NIRCam tab of the Calculations page.
 Select the calculation in the Calculations table by clicking on it.
 Click the "Expand" button at the top of the ETC window, and select "Expand Filters".
 The new calculations will appear in the calculations table. It may take a few seconds to complete.
 Check the boxes labelled "Plot" next to each new expanded calculation in the Calculation table. If the "SNR" tab is clicked on the Plot pane at the bottom of the window, the S/N results for all new calculations will be plotted simultaneously.
