The JWST Exposure Time Calculator (ETC) uses a scene for the calculations. A scene may contain one or multiple sources; it is an idealized representation of the spatial distribution of a target and nearby sources with defined intrinsic spectral properties.

Video Tutorial:  ETC Scenes and Sources

The JWST Exposure Time Calculator (ETC) uses a scene for performing its calculations, and a scene can include one or more sources. Scenes are idealized representations in two spatial dimensions (angular coordinates) with source properties determining the brightness distribution and shapes. The scene sizes are roughly 10–20 square arcseconds depending on the instrument and mode. The user can create many scenes and have a scene library with unique scene IDs or names. The scenes and sources defined in a workbook are persistent and can be reused in various calculations. 

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Creating a new scene 

The default workbook that is created when a user creates a new workbook contains a default scene with one default source (a point source with flat continuum) on the Scenes and Sources page (Figure 1). The user can create a new scene by clicking on the NEW button in the Select a Scene pane. The new row in the scene list will show that there are no sources and that the scene has not yet been used in any calculations. When the scene is populated with sources, and then used in the ETC calculations, the information about sources and number of calculations in which the scene is used gets updated. An empty scene with no sources can be useful for examining the sky background by using it in a calculation and defining the sky background parameters under Backgrounds in the Calculations page. To add sources to the new scene, there must be an existing list of one or more sources in the Select a Source pane.

Adding sources

The source to be added to a scene is first selected by clicking the row containing the source in the Select a Source pane. The scene into which the source has to be placed is selected by clicking the row in the Select a Scene pane. The selected rows are indicated by yellow highlighting. From the Select a Scene pane click on the ADD SOURCE button and the scene is populated with the currently selected source. The same procedure can be repeated by selecting the desired sources from the source list one at a time. At every step make sure that the correct source and scene are highlighted in yellow. 

Source offsets

The Offset tab under source editor (see Figure 1) is directly linked to the scene. The offsets and orientation define the placement or location of the sources in the scene and their orientation. To change the offsets of a source in a scene, both the source and the scene for which the change is to be applied have to be highlighted in yellow. This is particularly important because the same source may be used in multiple scenes, and when the source is selected, all the scenes that contain the source will be highlighted in green. Clicking on a particular scene will activate the yellow highlighting to indicate this is now the active scene. On the Source Editor pane, edit the X- and Y-offsets and orientation and use the save button to apply the changes. The offsets are defined in arcseconds with respect to the center of the scene, and orientation in degrees.

The use of larger offsets will make the scene bigger. While there is no fundamental limitation to how big a scene can be, the restrictions offered by the maximum scene size conserves computational resources and reduces the time for the calculations.

Removing sources

A source that has already been placed in a scene can be removed by using the REMOVE SOURCE button in the Select a Scene pane. Before this task is performed, make sure that the source to be removed is highlighted yellow by selecting it from the source list, and the scene from which it needs to be removed is also highlighted. 

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Viewing the scene

The newly created scene can be selected and viewed in the Scene Sketch pane. The Scene Sketch is interactively linked to the source table, such that a source selected in the source table will appear yellow in the sketch. Similarly if the source is selected in the sketch then it is also selected in the table. The scene sketch has a size that shows all the assigned sources, and does not always correspond to the maximum scene size allowed for ETC calculations for a given instrument mode. To enlarge the scene sketch that is displayed, add a faint source at a large offset not exceeding the maximum scene size limit.

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Default scene

The default scene in a workbook is used for any default calculation that is triggered when an observing mode is selected from the instrument tabs on the Calculations pane. The default scene provided with a new workbook has a single default point source with a flat continuum, located at the center of the scene. However, the user has the option to change the default scene to any of the new scenes that they have created in the Select a Scene table. Clicking on the purple star in the row containing a scene will make that the default scene. Users are advised to use a simple scene with simple source properties as the default scene. A more complex scene or scene containing complex sources that is assigned to be the default scene can slow down the default calculations considerably.

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Deleting a scene

To delete a scene, select the row corresponding to that scene by clicking it and highlighting it in yellow. If the scene has been used in any ETC calculation after it was created, this information will be displayed in the "Used in Calculations" pane, with all the calculations using the selected scene shown highlighted in green. Users should be aware of the calculations that will be affected before performing the scene deletion. The scene may be deleted by using the DELETE button on the Select a Scene pane.

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References

Go to the on-line JWST Exposure Time Calculator Tool

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

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