The ETC includes template spectra of the Sun and giant planets, and can be used to approximate reflected sunlight and thermal emission from satellites and minor bodies.

See also: JWST Exposure Time Calculator Overview

The JWST ETC can be used to model the spectra of moving targets, but is limited to doing so for a single target brightness. For distant targets (those at least as far from the Sun as Jupiter) on nearly circular orbits, this isn't a major problem because their brightness is fairly constant during the period when the the target is within the JWST field of regard. For closer targets the brightness can change by much more than 50%, so observers must account for those variations manually when creating ETC sources and scenes to represent the target.

Template spectra are available in the ETC for the giant planets. For satellites and minor bodies, the reflected light component can be approximated using the Phoenix stellar model G2V template spectrum or the solar template spectrum, and thermal emission can be approximated using the blackbody template. The user must determine the correct normalizations to apply to those template spectra in order to accurately represent the emission from their target on a given date. Targets expected to have both reflected light and thermal emission components within the wavelength range of interest can be specified as two sources that coincide in the ETC scene.

 

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Normalizing target spectra

The emission from a target has to be normalized in a way to represent the physics controlling the flux density of the spectrum as received at JWST. These factors include:

• Observing circumstances such as heliocentric and observatory-centric distances
• Phase angle
• Size and albedo
• Thermal properties

Observing circumstances can be retrieved from the JPL Horizons web service by entering the string "@jwst" in the observatory search field (see Moving Target Ephemerides). 

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Point and extended sources

For targets too small to be resolved by JWST, the spectrum can be modeled using the ETC point source target type.

Extended targets can also be specified in the ETC as elliptical shapes with brightness distributions that are flat, r^-k power law, Gaussian, or Sersic profiles (the latter is typically used for galaxies).

For observers interested in Jupiter, Saturn, Mars, and highly extended comets, capabilities of the ETC web interface limit the size of the scene to a few arcseconds across. This doesn't prevent estimates of SNR for a given observation, but does require observers to properly normalize the surface brightnesses of these sources. Additionally, see the Solar System Sample Workbook for a workaround for dealing with large, extended sources in the ETC.

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User supplied spectra

Video Tutorial:  Uploading Spectra to the ETC

See also: JWST ETC User Supplied Spectra

The ETC allows users to upload their own spectra for sources. ASCII and FITS format are supported, and the spectrum in either case consists of 2 vectors containing wavelength and flux density (or surface brightness). Format and other requirements are described in the ETC documentation and help (see JWST ETC User Supplied Spectra).

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Example workbooks

See also: JWST ETC Using the Sample Workbooks

See JWST Moving Target Observations for instructions on how to access the Solar System Sample Workbook and the Example Science Program Workbooks. These workbooks focus on providing examples of how to construct an ETC scene useful for Solar System observers and give details on how to set up ETC calculations.

The ETC contains a Solar System Sample Workbook with 3 scenes:

1. An asteroid modeled as a point source using the superposition of a reflected light and thermal component.
   
   
2. A comet modeled as a point source nucleus and 2 extended sources representing the coma. Reflected light and thermal emission components are included for nucleus and coma.
   
   
3. A giant planet modeled as a smaller extended source with a total area of 1 square arcsecond. This demonstrates the workaround for determining the SNR for large, extended sources.

The moving target example science program, NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids, provides an end-to-end walkthrough of how to create a moving target proposal. An example ETC workbook and example APT file are available for download so you can follow along with the associated pages.

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Limitations

The ETC does not currently have:

• A method for using an albedo spectrum to modify the predicted reflected light spectrum.
  
  
• More realistic models for thermal emission, such as the standard thermal model (STM) or near-Earth asteroid thermal model (NEATM).
  
  
• A way to compute a target spectrum based on basic inputs such as the size of and distance to the target, and an albedo.
  
  
• A shortcut to use typical background values near the ecliptic plane. Instead, users must specify an RA and DEC corresponding to a position near the ecliptic plane. 

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Template spectra

Template spectra of the Sun, Jupiter, Saturn, Uranus, and Neptune, covering the full JWST wavelength range, are available in the ETC Source Editor: Continuum tab → Spectral Energy Distribution → Select → Sun & Giant Planets. More details on these spectra can be found on the JWST ETC Source Spectral Energy Distributions page.

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References

JWST Exposure Time Calculator Tool

Pontoppidan, K. M., 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

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