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James Webb Space Telescope User Documentation

 

 

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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 reusable. A source can be newly created, or copied from the available library of sources on the Scenes and Sources page and modified. The Source Editor allows the user 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 with a flat continuum 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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The functionality of the different panes, tabs, and sub-tabs are described in the Scenes and Sources overview.

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The video tutorial 

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 provides a walk-through of creating sources with a ranges of properties and assigning them to scenes within ETC.



Create a new source

A new source can be created by clicking the 

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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.



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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 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 

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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.

When an analytic spectrum is selected the user will be presented with options to specify the parameters relevant to the chosen analytic spectrum, such as, the flux units, the exponent for the power-law, or temperature for the black-body spectrum.

The stellar spectral templates include the Phoenix models (specified by their spectral type, effective temperature, and log g values), and HST standard stars.

The extra-galactic spectral templates include an ensemble from Brown et al. (2014) with wavelength coverage spanning from the ultraviolet to the mid-infrared, and are representative of diverse galaxy spectral shapes. 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), and Chapman et al. (2009). The extinction in magnitudes is specified in a chosen bandpass (V, J, H, or K).

Renormalization

The user can choose to normalize the spectral energy distribution for the source by providing the integrated magnitude (in AB magnitude or Vega magnitude units) 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 total magnitude in an HST (ACS, WFC3/UVIS, WFC3/IR, or NICMOS) bandpass or other available standard photometry filters (e.g., Bessell, Cousins, SDSS, Johnson, or Spitzer). 

The normalization may also be specified in flux units either in flam (erg/s/cm2/Å), or fnu (erg/s/cm2/Hz), or in terms of spectral flux density in Jansky units.

For extended sources, if the normalization is selected to be in surface brightness units in the Shape tab, the units presented in the normalization tab will be per arcsec or per steradians.

Lines

The user can add emission lines to the source continuum using the options available under the Lines tab (Figure 2). 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 

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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 
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button. Lines may be deleted from the list by using the 
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button. Remember that the
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, and 
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buttons will only modify the properties of the line that is being edited in the line list. For the emission lines to be applied to the source continuum, use the 
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button.

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In ETC versions earlier than v1.2.2, the line strength for extended sources was not shown per unit area when the normalization was in surface brightness, though the calculations were done using surface brightness units.

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Figure 2. The Lines tab on the Source Editor pane of the ETC

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The layout of the Lines tab in the Source Editor pane shows how the properties of the emission lines can be specified. The lines can be added to, updated, or removed from the lines library using the buttons to perform those functions. The changes are applied to the source spectrum only when the Save button is activated.

Shape

The source morphology can be specified as a point source or an extended source. For extended sources, the flux distribution may be selected to be flat, 2D Gaussian, power-law or two flavors of Sersic profiles (since ETC version 1.3) with a specified Sersic index (ranging from 1 to 4). In the case of an extended source, the parameters that specify the extent of the source in arc seconds must also be provided (such as,  σx and σy for the 2D Gaussian, or semi-major and semi-minor axes for flat source or Sersic profiles). To assign the shape parameters to the source, use the Save button. See JWST ETC Defining an Extended Source for more in-depth descriptions of the different flux distributions.

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 and more computationally intensive. The default and maximum scene size depends on the instrument and mode as given in the table on the Scenes and Sources Page Overview. 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). 

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In ETC versions older than v1.3, the source spectrum plots do not change to surface brightness units when the normalization is selected to be in surface brightness, though the calculations were done with the appropriate surface brightness units.

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 

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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. 

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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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References

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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

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 

JWST technical documents


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Last updated

Updated November 5, 2018

  • Updated for ETC v1.3. Updated information on Sersic profiles.

Updated May 26, 2017

  • Modifications in section "Defining the properties of the new source"

Published December 30, 2016



Comment

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Updated October 08, 2018

  • Sersic and power law profiles for ETC v1.3

Published March 02, 2017