Moving Target ETC Instructions

The ETC has not implemented any specific features for Solar System targets, but can be used to approximate reflected sunlight and thermal emission from them.

On this page

Main article: JWST Exposure Time Calculator Overview

Reflected light can be approximated using the Phoenix stellar model G2V 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.



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). 

It is critical to include solar elongation constraints of 85°–135° when using Horizons to generate target ephemerides within the JWST field of regard.



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.



User supplied spectra

Video Tutorial:  Uploading Spectra to the ETC

Main article: 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. Format and other requirements are described in the ETC documentation and help (see JWST ETC User Supplied Spectra).



Example workbooks

Main article: 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.


*Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.



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. 



Future improvements

Template spectra

  • A template spectrum for the Sun, absolutely calibrated to represent flux density at 1 AU, and at a spectral resolution high enough to support modeling for the high resolution gratings of NIRSpec and for the MIRI MRS is under development.

  • Template spectra for the giant planets (disk-integrated) are also under development.

  • A community-based effort to create template spectra for a range of spectral classes and/or representative examples of asteroids and TNOs will be explored at various community forums.

One or more of these template spectra may be implemented instead as a library of spectra users can share external to the ETC and then upload, rather than residing within the ETC as true template spectra. As these materials are completed, observers can find additional information on this page, and should look for announcements on Solar System community forums such as the DPS and PEN newsletters.



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




Published

 

Latest updates
  •  
    Made minor wording changes. Added video link.


  • Fixed incorrect links.

  •  
    Updated for ETC 1.3, which includes an r-k profile for comets.