Herschel/SPIRE Infrared Astronomy Schools Ask an Astronomer Media
Two types of instruments are commonly used by infrared astronomers are photometers (cameras), and spectrometers. Infrared cameras, like standard cameras, take a picture of a certain region in the sky. The difference between the two is that infrared cameras are sensitive to the infrared portion of the electromagnetic spectrum, AND visible light (light with wavelengths between about 1µm and 1000 µm). Unlike standard cameras, more pixels do not always result in a more sensitive infrared camera. Near-infrared cameras can have thousands of pixels, while cameras for the far-infrared in general have much fewer pixels. Most infrared observatories contain an infrared camera. For some, such as the instruments of the JCMT or IRAS missions, this is all they do.
Spectrometers do more than merely measure the overall brightness of a certain position in the sky. Rather, they determine how much light there is of a certain ‘color’, analyzing whether the brightness originates primarily from radiation with longer or shorter wavelengths. In the visible, spectrometers reduce white light into the colors of the rainbow. A prism is a simple example of a spectrometer. Today’s Astronomers utilize more complex instruments such as grating, Fabry-Perot, acoustic-optical, or Fourier Transform spectrometers. Spectrometers differ in sensitivity and resolving power.

A Prism: simple example
of a spectromete
©Gallaudet University
The latest developments in infrared instrumentation are combining photometry and spectrometry in a single instrument, and building imaging spectrometers, which measure a spectrum for each pixel in the infrared camera. Herschel will feature three instruments, all of which have spectrometric capabilities. HIFI is an acoustic-optical spectrometer, PACS is a grating spectrometer and camera, and SPIRE features a camera and an imaging Fourier Transform spectrometer.
Spectroscopy of the Orion nebula:
Spectrum from a glass prism (1931).
Spectrum from a Fourier Transform
Spectrometer (2003).

Infrared Astronomy

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