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The JWST Exposure Time Calculator (ETC) uses a library of sources to be included in the astronomical scene for the ETC calculations. The sources are described by parameters that characterize their brightness, spectral properties, and shape. 

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Introduction

The JWST Exposure Time Calculator (ETC) allows a user to define sources that are persistent and reuseable. A source can be newly created, or copied from the available library of sources on the Scenes and Sources page and modified. A Source editor allows to assign the source properties such as; shape, brightness, and spectrum. The scene sketch provides a graphical representation of the source in the scene, and the source spectrum plots can also be viewed. An existing sample workbook may have one or more sources provided as examples, while a new workbook will have one default point source to serve as an example. This page describes how to define a new source, edit the source properties, and associate the source with a scene.

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Figure 1: The Scenes and Sources page layout.

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Create a new source

A new source can be created by clicking the 

Status
colourBlue
titleNew
button within the "Select a Source" pane. This creates a new row in the source table with a default name, and shows the number of calculations equals 0 implying that this source is initially not associated with any scene or calculation. The default new source is a point source with a flat continuum. A new source may also be created by using the "Copy Source" option available under the drop-down menu of the "Edit" button at the top of the Scenes and Sources page. This will create a new row in the source table with the same source ID, but is not associated with any scene or calculation. Once the source is added to a scene and used in a calculation the information gets updated in the "Select a Source" pane. The scenes and sources are interactively linked.

 


Defining the properties of the new source

To modify the properties of the new source, first select it by clicking the row to highlight the source.  The source editor has various tabs that can be used are to change the source properties as described below. Note that the changes made using the source editor under each tab are applied to the selected source only when the 

Status
colourBlue
titleSAVE
button is activated.

ID 

The ID allows the user to change the default name to a new desired name for the source.

Continuum

The "Continuum" tab allows to assign a spectrum to the new source by choosing a user-supplied spectrum, an analytic spectrum (such as, flat continuum, power-law continuum, blackbody spectrum) or one of the template spectra (stellar spectra, extragalactic spectra) available in the spectral templates library.  The menu under "uploaded spectra" is populated with the list of user-supplied spectra. The stellar spectral templates include the Phoenix models and HST standard stars, while the extragalactic spectral templates include an ensemble from Brown et al. (2014). There is an option for "no continuum" which is useful for specific cases, such as when only spectral lines are being used. 

The continuum tab also allows to specify the redshift of the source and the extinction to be applied. The extinction laws available are from Weingartner & Draine (2001), Flaherty et al. (2007), and Chapman et al. (2009). The extinction in magnitudes is specified in a chosen bandpass (H, J, K, or V).

Renormalization

The user can choose to normalize the spectral energy distribution for the source by providing the flux or magnitude of the source at a particular wavelength, or in a specified bandpass. The selection for JWST bandpass offers all the imaging filters on the JWST instruments. Alternatively, the user can also renormalize to the magnitude in an HST bandpass or other standard photometry filters. 

Lines

The user can add emission lines to the source continuum using the options available under the "Lines" tab. An emission-line is defined by the following parameters: a user-defined line name, the line center in microns, a line width in km/s, and the line strength in erg/cm2/s. The line center should correspond to the redshifted wavelength because the lines are applied after redshift and renormalization. Once the line is added using the 

Status
colourGreen
titleAdd
button, the table of lines is populated with the input parameters. Multiple lines may be added to this list of lines to be applied to the source spectrum. To change the input parameters, select the emission line by clicking the row on the table, modify the parameters and click the 
Status
colourBlue
titleUpdate
button. Lines may be deleted from the list by using the 
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titleRemove
button. Remember that the
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titleAdd
,
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titleUpdate
, and 
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titleRemove
buttons will only modify the properties of the line that is being edited. For the emission lines to be applied to the source continuum, use the 
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colourBlue
titleSave
button.

Shape

The source morphology can be specified as a point source or an extended source. In the case of an extended source, the semi-major and semi-minor axes in arc seconds must be provided. The flux distribution may be selected to be flat, 2D Gaussian, or a Sersic profile with a specified Sersic index (ranging from 1 to 4). To assign the shape parameters to the source, use the "Save" button.

Offset

The offset has options to position the source anywhere in a scene. The spatial offsets are defined with respect to the center of the scene, and using large offsets will make the scene accordingly bigger.  The default and maximum scene size depends on the instrument and mode as given in the table on the Scenes and Sources Overview page. The offset parameters link the source and the scene, so it is important that the source is added to a scene for the offset parameters and orientation to take effect. 

 


Examining the Source

The sources that are created may be examined in the scene sketch and the source spectrum plots to ensure that the desired parameters for spectral characteristics, morphology, and location in the scene have been applied. The Source spectrum plots display the spectrum for all the sources that have been selected using the checkboxes in the "Select a Source" pane. Use these plots to make sure that the source spectrum is correct (such as, the emission lines have been applied with the correct wavelength, and line strength). 

The scene sketch requires that the sources to be examined are already placed in a scene. This sketch can be used to examine the source location, and orientation. The scene sketch is also useful to examine complex 2D source geometries that can be created by adding multiple sources to describe different components of an astronomical source or scene.

 


Deleting a Source

A source can be removed from the current source library by using the 

Status
colourRed
titleDelete
button in the "Select a Source" pane. When the source to be removed is selected by highlighting the row in the source table, all the scenes that use the source will be indicated by the rows that are highlighted in green on the "Select a Scene" pane. Similarly, all the calculations which use the scenes containing the selected source will also be highlighted green in the "Used in Calculations" pane. The green highlight is intended to make the user aware of the scenes and calculations that are affected by deleting or modifying the selected source. This feature is particularly important because the Scenes, Sources, and Calculations are interlinked in the ETC.

 


 

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Related links

JWST ETC Overview

JWST ETC Scenes and Sources Overview

JWST ETC Defining a New Scene

JWST ETC Calculations Page Overview

JWST ETC User Supplied Spectra

JWST ETC Source Spectral Energy Distributions

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References

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references

go to the on-line JWST Exposure Time Calculator Tool

Brown, M. J. I. et al. 2014, ApJS, 212, 18 "An Atlas of Galaxy Spectral Energy Distributions from the Ultraviolet to the Mid-Infrared"

Chapman, N. L. et al. 2009, ApJ, 690, 496 "The Mid-Infrared Extinction Law in the Ophiuchus, Perseus, and Serpens Molecular Clouds"

Flaherty, K. M., et al. 2007, ApJ, 663, 1069 "Infrared Extinction Toward Nearby Star-forming Regions"

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"

Weingartner, J. C. and Draine, B. T. 2001, ApJ, 548, 296 "Dust Grain-Size Distributions and Extinction in the Milky Way, Large Magellanic Cloud , and Small Magellanic Cloud"

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JWST Exposure Time Calculator

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