Question

I've heard reference to many telescope and spacecraft that have a device known as a spectrometer, and I'm curious, what is the purpose of these device? What's the working principal behind them and what do we use them for?

Answer 1

EM radiation, including light, is a spectrum of different wavelengths. Spectroscopy is the detailed analysis of a light signal by wavelength. Ordinary color images break up light into 3 channels (red, green, and blue), but spectroscopy is generally concerned with breaking up light into a higher number of bands (e.g. 10, 100, or more), and a spectrometer is the instrument that does just that.

The basic principle of spectrometry is simple, various methods (the most ordinary being the use of a prism) can be used to cause the different wavelengths of light to follow different paths, which can be used in combination with a monochromatic imaging sensor to record the spectrum. Alternately, multiple images of the same scene can be recorded while using different narrow band filters (either separate filters or a device which can be adjusted to pass through different wavelengths such as a fabry-perot filter).

Spectrometry has multiple uses:

Composition

Ions of different elements have different emission spectra due to the differences in electron energy levels. This makes it possible to determine the elemental composition of objects that are significantly ionized such as stars (which are composed of high temperature plasma). Additionally, at lower temperatures molecules have characteristic absorption and emission spectra which can be used to determine the composition of lower temperature objects such as planets and asteroids.

Temperature

The large scale structure of a light spectrum will be dominated by the characteristics of the black body spectrum, making it possible to determine an object's temperature.

Motion

As mentioned above the composition of an object will result in a very characteristic spectrum. However, this spectrum will be shifted a certain amount one way or another depending on whether the object is moving away or towards us, due to doppler shifting. This makes it possible to measure the relative velocity of an object along the line of sight. By studying changes in an object's motion we can infer certain information about the object such as whether or not it is orbited by another otherwise unseen object. To date this is one of the most prolific methods for detecting extrasolar planets.

Since spectroscopy splits up a light signal into many tiny buckets it's very helpful to have as much light to work with as possible, which is why most of the largest telescopes in the world (such as the Keck or VLT telescopes) spend a lot of their time collecting spectra and have very sophisticated spectrometers.

The invention of CCDs and other electronic imagers has been a gigantic boon to spectrometry, since such devices have very high quantum efficiency (meaning the vast majority of photons from the source light are converted into usable signals) and can have fairly flat spectral response curves. Most importantly, they are already finely divided into different bins spatially and they contain a huge number of individual detectors (pixels).

One of the most interesting advances in modern spectroscopy is the increasing predominance of "imaging spectrometers" in interplanetary spacecraft and observatories. Instead of merely collecting multiple color channel data for each pixel in an image these instruments collect entire spectra for every pixel. This dramatically increases the amount of data collected and the speed of data collection by a spacecraft many fold, making it possible to extract a lot more information from a single view of a planet, moon, rock or what-have-you than was possible before. A few examples of imaging spectrometers would be the Mars Reconnaissance Orbiter's CRISM, the JWST's NIRSpec, and Dawn's VIR instrument