Spectroscopy with CCDs

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Although imaging and photometry have been and continue to be mainstays of astronomical observations, spectroscopy is indeed the premier method by which we can learn the physics that occurs within or near the object under study. Photographic plates obtained the first astronomical spectra of bright stars in the late nineteenth century, while the early twentieth century saw the hand-in-hand development of astronomical spectroscopy and atomic physics. Astronomical spectroscopy with photographic plates, or with some method of image enhancement placed in front of a photographic plate, has led to numerous discoveries and formed the basis for modern astrophysics. Astronomical spectra have also had a profound influence on the development of the fields of quantum mechanics and the physics of extreme environments.1 The low quantum efficiency and nonlinear response of photographic plates placed the ultimate limiting factors on their use.

During the 1970s and early 1980s, astronomy saw the introduction of numerous electronic imaging devices, most of which were applied as detectors for spectroscopic observations. Television- type devices, diode arrays, and various silicon arrays such as Reticons were called into use. They were a step up from plates in a number of respects, one of which was their ability to image not only a spectrum of an object of interest, but, simultaneously, the nearby sky background spectrum as well - a feat not always possible with photographic plates. Additionally, and just as important, were the advantages of higher obtainable signal-to- noise ratios, higher quantum efficiency, and very good linearity over a large dynamic range. These advances permitted spectral observations of much fainter sources than were previously available. Two-dimensional spectroscopy allowed the large error contribution

1 Extreme physical environments, such as very high temperatures, magnetic fields, and gravitational fields, are not often available within earthly laboratories.

from the often unknown sky background to be removed during data reduction procedures. However, the above electronic devices still had their problems. Use of high voltage, which caused distortions in the image, and the often low dynamic range both limited the ability to numerically resolve strong spectral lines from a weak continuum or to resolve weak lines in general.

The introduction of CCD detectors for use in astronomical spectroscopy was quick to follow their introduction into the field of astronomy itself. One of the first devices put into general use for astronomical spectroscopy was a Fairchild CCD placed into service at the Coudé feed telescope on Kitt Peak circa 1982. This author remembers that particular device and the remarkable advance it made to astronomy at the time. Observing without photographic plates was amazing.1 The introduction of CCDs allowed test observations to be made, no chemical development was needed, you could view your data almost immediately after taking it, and mistakes caused you very little in lost time. In addition, fainter sources and better spectral resolution were easily obtained and caused a renaissance in astronomical spectroscopy. The fact that the Fairchild CCD had a read noise of 350 electrons seemed unimportant at the time.

We will begin our discussion of astronomical spectroscopy with point source observations. The term "point source" is generally taken to mean a star, but under various conditions, other objects can be observed as a point source. For example, an active galaxy does indeed often show extended structure in terms of spiral arms, but short exposures or observations intended to study only the nuclear regions are essentially point source measurements. A more formal definition might be that point sources are objects whose angular size is determined by the seeing disk or instrumental resolution. We will follow point source observations by introducing extended object spectroscopy. The major difference in these two types of spectroscopy is the type of output data product you end up with and the science obtained from the collected data. Our discussion here will concentrate more on the CCD aspects of astronomical spectroscopy with some discussion of the actual observational techniques and data reduction procedures. Various types of spectrographs and other related topics are discussed in detail in the excellent reviews given by Walker (1987), Pogge (1992), and Wagner (1992).

1 For those readers interested in a bit of nostalgia, remember how one needed to cut the photographic plates completely in the dark, attempt to fit them into the plate holder licking one side along the way to find the emulsion, and then suffering the agony of defeat when you discovered that your 1 hour integration was made with the dark slide closed or the plate had been placed in the holder backwards!

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