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.
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" (no quotes) in the observatory search field. Note: It is critical to include solar elongation constraints of 85° - 135° when using Horizons to generate target ephemerides for JWST observations.
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, Gaussian, or sercic profiles (the last is typically used for galaxies). For observers interested in Jupiter, Saturn, Mars, and highly-extended comets, capabilities of the web interface of the ETC limit the size of the scene to a few arc seconds across. This doesn't prevent estimates of SNR for a given observation, but does require observers to correctly specify the brightness of the source in the units the ETC currently supports (i.e NOT surface brightness units). The basic procedure is:
- Compute the spectrum of the target integrated over the disk or emission region.
- Convert the integrated emission to surface brightness by dividing by the area of the target (e.g., in arcsec2).
- Specify an extended source in the ETC small enough to fit within the small dimensions of the available scene (e.g., 1 arcsec across).
- Normalize the target brightness to a level determined by multiplying the area of the ETC extended source by the surface brightness computed in step 2) above.
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 two vectors containing wavelength and flux density. Format and other requirements are described in the ETC documentation and help (see JWST ETC User Supplied Spectra).
The ETC contains an example workbook with two scenes specified.
- An asteroid modeled as a point source using the superposition of a reflected-light and thermal component.
- A comet modeled as a point-source nucleus and two extended sources representing the coma. Reflected-light and thermal emission components are included for nucleus and coma.
These workbooks are primarily focused on providing examples of how to construct an ETC scene useful for solar system observers. Details of how to specify ETC calculations (which equate to observations in APT) are given in detail in other workbooks that are specific to the instruments.
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 way to allow users to specify a spectrum in surface brightness units (e.g., mJy / arcsec2).
- A 1/r brightness distribution, such as is typically used for cometary comae.
- A short-cut to use typical background values near the ecliptic plane. Instead, users must specify an RA,DEC corresponding to a position near the Ecliptic plane.
While adding such features has been discussed, there is currently no schedule for adding them to the ETC.
- 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 iconic 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 become completed observers can find additional information here, and should look for announcements on solar system community forums such as the DPS and PEN newsletters.
The engine driving ETC calculations, along with necessary throughput curves for imaging and spectral performance data, and a library of monochromatic PSF models, are available to the community as the Pandeia Python package, which can be installed from here (Pontoppidan et al., 2016). As with template spectra, a community-based effort to develop an interface to Pandeia that can serve the needs of the solar system science community will be discussed at various community forums.
Moving target articles
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"