The JWST Exposure Time Calculator initializes a calculation once the user chooses an instrument and mode. Multiple calculations can be compared, copied and modified in the ETC workbook.
The "Calculations" page in the JWST ETC is where a user specifies an instrument and mode, the background, instrument and detector configuration, and observing setup and extraction strategy for a given calculation. This is typically done after scene(s) and source(s) of interest have been defined on the "Scene and Sources" page, although a default scene containing a central point source is always available. The "Calculations" page offers users the ability to do a comparative analysis of multiple sources or locations to determine which calculation offers the best signal-to-noise ratio (SNR).
Calculations page layout
Figure 1 shows the layout of the "Calculations" page, labeled in blue to identify its panes, tabs, and sub-tabs in the graphical user interface.
Initialize a calculation
To begin a calculation, an instrument and mode must be initialized by selecting the mode from the instrument buttons in the calculation table. Selecting an instrument and mode initializes a calculation, which is run with default values and appears in the calculations table with the following columns: ID, Plot, Mode, Scene, (s), SNR and status column. After the calculation is complete, selecting the row will highlight it in yellow and the ETC outputs for that calculation will be shown in the "Images" and "Reports" panes.
Edit a calculation
To edit or modify a calculation, the user may identify a scene from the Scene tab, in the configuration pane, and change default parameters listed under the following tabs: Backgrounds, Instrument Setup, Detector Setup and Strategy.
Scene and background
The Scene tab contains information for both source and scene choices that can be selected for a given calculation. Primarily the Scene tab should only be used for selecting the scene to use in the calculation. The Sources in that Scene and other subtabs are for modifying the source itself, and its location in the scene.
The Scene tab does not inform users about what other calculations will be affected by changing the parameters of the source and scene, while the Scene and Sources page does. Changing the parameters of a source on the configuration pane will cause all calculations using that source to be automatically updated with the modified source properties.
The JWST background model includes zodiacal light, interstellar medium, and cosmic infrared background, as well as telescope self-emission and scattered light (see JWST Backgrounds). In the Backgrounds tab, the user can specify the RA and DEC to be used to calculate the background. The user can specify a date to calculate the background on that date. Alternatively, the user can choose to assume a low, medium, or high background, from the ranges expected for that RA and DEC.
Instrument and detector setup
The Instrument Setup tab allows the user to specify parameters for the desired configuration of the selected instrument mode. A plot of the full system throughput for a given mode and chosen optical element is shown as a function of wavelength (in μm).
The Detector Setup tab allows the user to enter the desired exposure (photon-collecting) time by specifying the readout pattern and subarray to be used, and the number of groups, integrations, and exposures to simulate.
- Groups = number of groups in a ramp (each group can have a number of reads, depending on readout pattern)
- Integrations = number of ramps in a single exposure
- Exposures = number of exposures
Total integration time is: Frame time (depending on the subarray) × number of groups × number of integrations × number of exposures.
Extraction strategies for sources and background
The Strategy tab is where the extraction and observation parameters can be specified. Imaging and spectroscopic modes support aperture extraction, in which the user can choose to specify the extraction aperture centered on a source, or at an arbitrary offset in the scene. This allows for exploration of observation setup and extraction strategies. IFU, MSA, and coronagraphy modes have more complex strategies available. For spectroscopic modes, a wavelength of interest is chosen, which is the wavelength used to calculate various scalar values, including the SNR shown in the calculations list. A user is also allowed to determine whether a background subtraction should be performed. If the background subtraction is performed, a sky annulus for the inner and outer radius will need to be defined.
If the user inputs are not valid, then the affected tab names will turn italicized red, and explanatory red text will appear near the incorrect field.
Performing the calculation
Lastly, the user can now click the Calculate button at the bottom right of the configuration pane. To abandon your changes and restore the last-applied values, click reset. The bottom of the configuration pane lists the calculation id, instrument, and mode that is presently selected. When calculations are deleted by selecting the row of the calculation then selecting Delete Calculation from the Edit menu at the top left of the "Calculations" page, other calculations will retain the original identification number, resulting in non-sequential Calculation ID numbers.
The Calculate button will change to What's wrong? if the calculation cannot be performed. Just click the button to see the problem and suggested solution.
Exploring parameter space
The ‘copy and modify’ workflow allows users to copy and modify their calculations by copying or requesting a batch expansion of a specific calculation. Selecting a single calculation and then selecting Copy Calculation from the Edit menu at the top left of the page enables the copy feature.
Batch expansions are performed based on varying filters and exposure specifications such as number of groups or integrations. Selecting a single calculation and then selecting Expand Groups, Integrations or Filters from the Expand menu at the top left of the page, will duplicate the selected calculation, varying only the expansion parameter, and the resulting set of calculations will appear in the calculations table. Batch expansions are for automating the copying calculations process and varying exposure specifications are described in the Edit a Calculation section above.
Analyze the results
The bottom panes of the "Calculations" page are where users can view the “Reports” pane that displays the scalar results, warnings, errors and a download link for the defined calculation, which allows a user to download all 2D and 1D products, the 3D data cube for IFU calculations and a FITS table of the calculated background spectrum. One may view the 2D images of per-pixel SNR, countrate at the detector and saturation. Furthermore, the SNR vs wavelength, SNR vs time, and contrast vs separation can be compared for multiple calculations. Visit the Outputs Overview page for more information on the ETC results.
JWST User Documentation Home
JWST Exposure Time Calculator, ETC
JWST Exposure Time Calculator Overview
JWST ETC Creating a New Calculation
JWST ETC Scenes and Sources Page Overview
JWST ETC Outputs Overview
JWST ETC User Supplied Spectra
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