Detectors

Generally the same detectors can be used for astronomical spectroscopy as those used for direct astronomical imaging. The photographic emulsion is still to be found, especially in older instruments and for objective prism spectroscopy. However it is rapidly being replaced over the visible, near infrared and near ultraviolet by CCD detectors. These have a much higher quantum efficiency, even at the shorter wavelengths where fluorescent coatings have to be used. The image is fed directly into a computer where it may be easily processed (see later discussions). CCDs are, however, still relatively small, and so inappropriate for physically large spectra. Image Photon Counting Systems (IPCS) are also to be found and have the advantage of a continuous read-out of the image during exposure (see below). At wavelengths longer than about 1 /¿m, various array detectors based upon cooled semiconductor bolometers, some with charge coupling read-out, are now in widespread use and have similar advantages to CCDs. In the ultraviolet, observations at wavelengths less than about 350 nm require the instrument to be lifted above the Earth's atmosphere. The main ultraviolet spectra to date have been obtained by the International Ultraviolet Explorer (IUE) spacecraft and the Hubble space telescope. The former uses an SEC vidicon TV camera as the detector within an echelle spectroscope operating down to 100 nm. The latter is discussed further in chapter 10.

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