Defining spectral information

Defining spectral information#

Any target that is not a full datacube needs some kind of spectral information. This can range in complexity from a simple stellar spectral type, for which we have basic template spectra, all the way to a user-supplied spectrum. This page gives a quick overview on how spectral information is handled in ScopeSim-Targets.

Nomenclature#

The name spectrum was chosen over SED for the following reasons, in no particular order:

  • SED is an acronym, whereas spectrum is more explicit and easier understood.

  • SED (being an acronym) is usually capitalized, which goes against the common Python conventions of naming function arguments or class attributes (PEP 8). However, calling it sed instead seems even less clear, especially because it can be pronounced in English like a word.

  • In most contexts here, we’re talking about the spectrum of a star, planet or galaxy. SED is more often used for the “larger scale shape” of the spectral energy distribution, which is often only coarsely sampled.

Side note: We consider spectrum to be the singular case, and spectra the plural form. Thus, there is no such thing as “a spectra”, because spectra always refers to multiple. If there is a case where more than one spectrum is meant (such as multiple point sources in a cluster), the word spectra shall be used.

Possible way to specify spectral information#

In short:

  • synphot.SourceSpectrum instance

  • astar_utils.SpectralType instance, in which case the closest available template will be used.

  • A string able to be parsed into a astar_utils.SpectralType instance, see above.

  • “spex:spextra_name”, where “spextra_name” resolves to a spextra.Spextrum database entry.

  • “file:file_name”, where “file_name” points to a local file containing spectral information, see below.

Spectra from file#

Currently, a “file:file_name” identifier will be forwarded as-is to synphot.SourceSpectrum.from_file(), meaning any format supported by that constructor is supported here. Support for more formats is planned down the road.

Wavelengths#

Any spectra should be given in rest-frame and vacuum wavelengths. All shifts are done by the tools (e.g. ScopeSim) using the target definitions. Line-of-sight velocity or cosmological redshift (“z”) can be supplied as part of the positional information, see defining positions.