JWST ETC Batch Expansions

Batch expansion with the JWST Exposure Time Calculator (ETC) makes copies of a calculation to show corresponding results for a number of different groups, integrations, filters, coronagraphic masks, or dispersive elements.  

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Unlike other exposure time calculators, the JWST Exposure Time Calculator (ETC) does not provide an option for reverse calculation of exposure duration parameters starting from a given signal-to-noise ratio (S/N) because there is often no unique solution for a given subarray/readout combination in terms of groups, integrations, and exposures. Instead, a batch expansion capability is available 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 imaging filters may also find useful the batch expansion over filters; coronagraphic imaging may additionally be expanded over masks. The MIRI Medium-Resolution Spectroscopy mode may be expanded over channels, wavelength ranges, or both.

At the top of the ETC calculations page there is an Expand menu of possible selections that, depending on the observing mode, may include Groups, Integrations, Filters, Masks, Channels, or Wavelength Ranges (Figure 1). Starting from an existing selected calculation in the Calculations table, these options can be used to expand the calculation over several iterative values found under the Detector Setup tab or different instrument components found under the Instrument Setup tab while holding all other parameters constant. Batch expansion will generate a number of new entries in the Calculations table and it is possible to select the corresponding checkboxes to compare the results of multiple calculations.

Figure 1. Location of the Expand menu in the ETC web interface


Expansion types

Batch expansion can currently be performed by iterating over a set of groups, integrations, imaging filters, coronagraphic masks, channels (MIRI MRS only), and wavelength ranges (MIRI MRS only). The user must first select or highlight a calculation in the table to be expanded. Then the user should select one of the items in the Expand menu near the top of the ETC pane. After determining parameter values, if applicable, clicking SUBMIT will initiate the simultaneous batch expansion. This will generate the same number of new entries or rows in the Calculations table as the number of iterations selected. 

Note that only the parameter selected for batch expansion will vary in the newly created calculations; all other parameters of the original calculation are held fixed. This includes the source parameters, instrument parameters, exposure setup parameters, etc. In particular, the extraction strategy remains the same; the new calculations utilize the same extraction aperture radius, wavelength of slice, and annulus or extraction box height and width.

The Expand Groups option is available for every instrument mode. The Expand Integrations option is available for every instrument mode except target acquisition (TA). The presence of the other potential options (Expand over Filters, Expand over Masks, Expand Channels, Expand Wavelength Ranges, Expand Channels and Wavelength Ranges) in the Expand menu depends on the instrument mode of the given calculation.

If changes are made to the settings on the Instrument Setup tab, the CALCULATE button should be clicked to "save" the new settings and re-run the calculation with the new settings before using options in the Expand menu. Failure to do so will result in expansion over the original settings.

Example: In a NIRCam SW Imaging calculation, the F070W filter is selected on the Instrument Setup tab and the user changes the filter to F090W, but does not click the CALCULATE button. The user then chooses to Expand over Filters. In this case, batch expansion will create new calculations for every NIRCam SW filter except the F070W filter.

Groups

A batch expansion over the number of groups is initiated by selecting the Expand menu at the top of the ETC window, then choosing the Expand Groups option. A pop-up window (Figure 2) 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 and must be larger than 2 (this is the minimum number of groups allowed in the ETC). The Step Size is the increase in the number of groups to be tested at each iteration. Iterations is the number of new calculations to perform and must be less than 10 to keep the number of simultaneous calculations manageable.

Example: Specify Start Value=10, Step Size=2, and Iterations=10. This will create 10 new calculations starting with groups=10 and ending with groups=28.

Figure 2. Pop-up window for batch expansion over groups

Integrations

A batch expansion on the number of integrations is initiated by selecting the Expand menu at the top of the ETC window, then choosing the Expand Integrations option. A pop-up window (Figure 3) 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 in the number of integrations to be tested at each iteration. Iterations is the number of new calculations to perform and must be less than 10 to keep the number of simultaneous calculations manageable.

Example: Specify Start Value=1, Step Size=1, and Iterations=10. This will create 10 new calculations starting with integrations=1 and ending with integrations=10.

Figure 3. Pop-up window for batch expansion over integrations

Filters

A batch expansion over imaging filters is initiated by selecting the Expand menu at the top of the ETC window, then choosing the Expand over 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. Batch expansion over filters is available for the following modes: MIRI Imaging, MIRI Target Acquisition, NIRCam SW Imaging, NIRCam LW Imaging, NIRCam Coronagraphic Imaging, NIRISS AMI, NIRISS Imaging, NIRISS WFSS, NIRSpec Target Acquisition.

Example: In a NIRCam Coronagraphic Imaging calculation, if the MASK201R mask and F182M filter are selected on the Instrument Setup tab and the Expand over Filters option is chosen, five new calculations will be created for each of the other unused filters associated with this mask. All other parameters will remain unchanged.

Note that if a user wants to adjust the extraction aperture in different filters to account for changing PSF size, this will need to be done by-hand for each individual calculation; there is currently no batch expansion for Aperture radius.

Masks

A batch expansion over coronagraphic masks is initiated by selecting the Expand menu at the top of the ETC window, then choosing the Expand over Masks option. If this option is chosen, a set of calculations will be initiated immediately by simultaneously expanding upon all masks associated with the selected coronagraphic imaging filter. Batch expansion over masks is only currently available for NIRCam Coronagraphic Imaging.

Example: In a NIRCam Coronagraphic Imaging calculation, if the MASK201R mask and F182M filter are selected on the Instrument Setup tab and the Expand over Masks option is chosen, one new calculation will be created for the MASKSWB mask. This is because the F182M filter is only used in conjunction with the MASK201R and MASKSWB masks. See a full list of mask/filter combinations on the NIRCam Coronagraphic Imaging page.

Channels (MIRI MRS only)

A batch expansion over MIRI Medium Resolution Spectroscopy (MRS) channels is initiated by selecting the Expand menu at the top of the ETC window, then choosing the Expand Channels option. If this option is chosen, a set of calculations will be initiated immediately by simultaneously expanding upon all channels for the selected wavelength range

Example: Channel 2 (MRS_Short) and Long (C) are selected for Channel and Wavelength Range, respectively. The user selects Expand Channels. Three new calculations are created so that all 4 choices of Channel are represented for the Wavelength Range selection of Long (C).

Wavelength Ranges (MIRI MRS only)

A batch expansion over MIRI Medium Resolution Spectroscopy (MRS) wavelength ranges is initiated by selecting the Expand menu at the top of the ETC window, then choosing the Expand Wavelength Ranges option. If this option is chosen, a set of calculations will be initiated immediately by simultaneously expanding upon all wavelength ranges for the selected channel

ExampleChannel 2 (MRS_Short) and Long (C) are selected for Channel and Wavelength Range, respectively. The user selects Expand Wavelength Ranges. Two new calculations are created so that all 3 choices of Wavelength Range are represented for the Channel selection of Channel 2 (MRS_Short).

Selecting the option to Expand Channels and Wavelength Ranges will result in a total of 12 MIRI MRS calculations covering all the possible combinations of Channel and Wavelength Range.



References

JWST Exposure Time Calculator Tool

Pontoppidan, K. M., Pickering, T. E.,  Laidler, V. G.  et al., 2016, Proc. SPIE 9910, Observatory Operations: Strategies, Processes, and Systems VI, 991016
Pandeia: a multi-mission exposure time calculator for JWST and WFIRST




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Latest updates

  • Information added for the new batch expansions available in ETC v1.3