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.
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).
