JWST ETC Source Spectral Energy Distributions

The JWST Exposure Time Calculator (ETC) offers a series of spectral energy distributions (SEDs) that may be used when building a source. Templates include many flavors of stellar and extragalactic spectra, and analytic spectral distributions. A user-supplied spectrum may also be uploaded to the ETC for use in calculations. 

On this page

When creating a new source in the Exposure Time Calculator (ETC), the user may choose to apply a template spectrum for the source's continuum. Many of the commonly used stellar and extragalactic templates have been provided, which cover a wide range of observed spectral energy distributions, along with several analytic functions. The Continuum tab is also where the redshift and extinction parameters may be entered. The available choices for source spectral energy distributions are described below. There are two ways to access the Continuum tab of the Source Editor, as shown in Figures 1 & 2 below.

Figure 1. Calculations → Select calculation → Scene → Continuum

Figure 2. Scenes and Sources → Select scene → Select source → Continuum

Analytic spectra

Analytic spectral energy distributions include a Flat Continuum, Power-law Continuum, and a Blackbody Spectrum.

Power-law Continuum

The power-law continuum is defined as F(λ)~λn, where n is specified by the user in the exp box. Units of flam (wavelength units of erg/s/cm2/Å) or fnu (frequency units of erg/s/cm2/Hz) can be chosen from a drop-down menu. The Power-law Continuum is converted to fnu units (if necessary) and normalized to 1 mJy at 1 μm before being re-normalized to the value input by the user under the Renorm tab.

Flat Continuum

A flat continuum is a special case of the power law spectrum, where n=0. This distribution is so-named because the spectrum has constant energy per unit wavelength (flam) or unit frequency (fnu). The source spectrum plot will show a straight line profile for a Flat Continuum in fnu units but a curved profile when flam is selected due to the conversion factor between fnu and flam.

Blackbody Spectrum

The Blackbody Spectrum is computed at the temperature of the blackbody specified by the user using the equation:

{B_{\nu}(\nu, T)=\frac{2h\nu^3}{c^2}\frac{1}{e^{\frac{h\nu}{kT}}-1}.}

The units are ignored and the blackbody profile is normalized such that the value under the curve is the flux density (in mJy) at each wavelength of the input spectrum.

Stellar spectra

Stellar template spectral energy distributions include a library of Phoenix Stellar Models and spectra of HST Standard Stars

Phoenix Stellar Models 

The pull-down menu of Phoenix stellar models contains synthetic spectra spanning spectral types from O3 to M5, obtained using the Star, Brown Dwarf, and Planet Simulator. They use static, spherically symmetric, 1D simulations to completely describe the atmospheric emission spectrum. The models account for the formation of molecular bands, such as those of water vapor, methane, or titanium dioxide, solving for the transfer equation over more than 20,000 wavelength points on average, resulting in synthetic spectra with 2 Angstrom resolution. The line selection is repeated for each iteration of the model until it has converged and the thermal structure obtained. The models here are calculated with a cloud model, valid across the entire parameter range. Each model's name contains a concatenation of the spectral type, effective temperature, and gravity (log g value).

Known issue: The effective temperature and gravity values for the G2V and G5V spectral types are identical, resulting in identical source spectra. A solution to this issue is currently being investigated.

HST Standard Stars

Several HST calibration standard star spectra are available. These spectra are stored in the Calibration Database System (CDBS) and were originally chosen from the paper Spectrophotometric Standards from the Far-UV to the Near-IR on the White Dwarf Flux Scale by Bohlin (1996) and later updated as new data became available. See also Comparison of White Dwarf Models with ACS Spectrophotometry by Bohlin et al. (2001).

More information, along with a list of the complete set of files, including older versions, can be found in the CALSPEC Calibration Database. This page provides a table with the available Flux Standards and their CDBS name. In this table the order of preference for the choice of a standard flux distribution is from left to right in the table, i.e., from the best in column 6 to the last choice with the lowest quality in column 9. In this case, models have higher fidelity and extend to longer wavelength ranges while the more outdated are those derived applying corrections to the original IUE and optical fluxes. Note that for the cases when the CALSPEC data are updated after the ETC software is released, the ETC will not be able to access the most recent files, but only those that were available at the time of the build. If the ETC produces an error when trying to access an HST Standard Star spectrum, review the update history at the bottom of the CALSPEC page to determine when the spectrum was updated. If it was updated after the current ETC version, you may want to use the previous version of the model, or download the most recent spectrum, and apply it as a user-supplied spectrum.

Each stellar model's spectral type and related identifying information is shown in the pull-down menu listing the star by name. The related information includes the spectral type and V magnitude in parentheses, and wavelength range in Angstroms in brackets.

The use of spectra that do not fully overlap with the wavelength range corresponding to the selections under the Instrument Setup tab will cause warnings or errors in the calculation. Typically, HST calibration spectra will be useful only for near-IR (λ<5 μm) calculations.

Extragalactic spectra

Extragalactic template spectral energy distributions include models from Brown et al. (2014). Wavelength coverage for these models spans UV to mid-infrared wavelengths. The atlas includes a broad range of galaxy types, including ellipticals, spirals, merging galaxies, blue compact dwarfs and luminous infrared galaxies.


Go to the online JWST Exposure Time Calculator Tool

Brown et al. 2014, ApJS, 212,18
An Atlas of Galaxy Spectral Energy Distributions from the Ultraviolet to the Mid-infrared

Bohlin et al. 1996, AJ, 111, 1743
Spectrophotometric Standards From the Far-UV to the Near-IR on the White Dwarf Flux Scale

Bohlin et al. 2001, AJ, 122, 2118
Spectrophotometric Standards from the Far-Ultraviolet to the Near-Infrared: STIS and NICMOS Fluxes 

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



Latest updates
    Updated for ETC v1.5.

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