spectroscopy

Spectroscopy (or spectrometry or spectrography) is the study of electromagnetic radiation (EMR) intensity as a function of wavelength. It is the method for determining the chemical composition of astronomical bodies as well as temperature and radial velocity and other characteristics. The overall shape of the spectrum is of interest, such as its resemblance to a black-body spectrum, and spectral lines and their spectral line shapes are also revealing.

Instruments operating at or near visible light typically use prisms or gratings that angle light according to wavelength (dispersion). Dispersion with prisms is small so often multiple prisms are used in tandem, e.g., a triple prism spectrograph. Photometry is something like an extremely-low-resolution spectroscopy, studying stars and astronomical bodies based upon just a few passbands. Its advantage is that it requires much less EMR, thus can be used for more distant objects, and is also multi-object by default. Some spectroscopy instrument terms/classes:

• monochromator - isolates and measures intensity of a very narrow band.
• spectrometer - measures intensity of the spectrum of incoming EMR by wavelength.
• spectroscope - enables observation of the spectrum.
• spectrograph - creates a graphical representation of the spectrum. (Note that modern research instruments typically could fall under any of these three and the term used is basically arbitrary).
• cross dispersion spectrograph - uses specific method to give it a high resolution.
• multi-object spectrograph - graphs multiple objects in one shot.
• field spectrometer or imaging spectrometer - operates on a whole image, dispersing the spectrum of each pixel.
• integral field spectrograph - an imaging spectrometer, probably implying the use of an integral field unit.
• imaging Fourier transform spectrometer - uses fast Fourier transform with to process multiple images producing limited spectra for each pixel.
• slitless spectrograph - without the usual slit aperture that other spectrographs use to prevent overlapping of the spectra of different points in the sky.
• heterodyne spectrometer - type of submillimeter spectrometer optically generating a beat frequency that can be sensed, then analyzed with electronics.
• spectral correlator - kind of correlator (as used in radio astronomy) that yields spectral information.

Intensity at each wavelength is typically the quality of interest but there are also cases when polarization at each wavelength is the quality of interest (spectropolarimetry). Other terms for various branches of spectroscopy:

• cryogenic spectroscopy lab-based method for chemical research, useful, for example, in planetary science.
• differential spectroscopy - the study of the difference of two spectra.
• transit spectroscopy - differential spectroscopy comparing spectrum during versus not during a transit.
• transmission spectroscopy - the study of the spectrum of EMR passing through some material (such as interstellar medium, or an extra-solar planet's atmosphere).
• occultation spectrography - differential spectroscopy comparing various stages of an occultation observations.

Note that the same terms are also sometimes used for analogous analysis of quantities other than EMR-wavelength, perhaps most often using the word spectrography. Examples are the mass or kinetic energy of some source of particles (e.g., the term mass spectrometer).