Astronomical Spectroscopy - Physics - University of Cincinnati

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Fig. 10.— Optical design of STIS from Woodgate et al. (1998). Reproduced by permission.
(dithering), and subtracting adjacent frames, such that the sky obtained in the first exposure
is subtracted from the source in the second exposure, and the sky in the second exposure
is subtracted from the source in the first exposure. Nearly featureless stars are observed
at identical airmasses to that of the program object in order to remove the strong telluric
absorption bands. These issues will be discussed further in § 3.1.2 and § 3.3.3 below.
The differences in the basics of infrared arrays compared to optical CCDs also affect
how NIR astronomers go about their business. CCDs came into use in optical astronomy in
the 1980s because of their very high efficiency (≥50%, relative to photographic plates of a
few percent) and high linearity (i.e., the counts above bias are proportional to the number of
photons falling on their surface over a large dynamic range). CCDs work by exposing a thin
wafer of silicon to light and to collect the resulting freed charge carriers under electrodes.
By manipulating the voltages on those electrodes, the charge packets can be carried to a
corner of the detector array where a single amplifier can read them out successively. (The
architecture may also be used to feed multiple output amplifiers.) This allows for the creation
of a single, homogenous silicon structure for an optical array (see Mackay 1986 for a review).
For this and other reasons, optical CCDs are easily fabricated to remarkably large formats,
several thousand pixels to a side.
Things are not so easy in the infrared. The band gap (binding energy of the electron)
in silicon is simply too great to be dislodged by an infrared photon. For detection between 1