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 sources properties determining the brightness distribution and shapes. The scene sizes are roughly 10-20 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.
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 Figure 1). The user can create a new scene by clicking on the 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.
Figure 1: The Scenes and Sources Page Layout
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
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.
The "Offset" tab under source editor (see figure 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 has 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 arcsecs with respect to the center of the scene.
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.
A source that has already been placed in a scene can be removed by using the
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.
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.
Deleting a Scene
To delete a scene, select the row corresponding to that scene by clicking it and it will be highlighted 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 button on the "Select a Scene" pane.