Annu. Rev. Astron. Astrophys. 1978. 16: 103-39 Copyright © 1978 by Annual Reviews. All rights reserved

2.1. Methods of Observation

The methods of observation and reduction of long-slit spectra have been discussed by Burbidge (1962), for example, and reviewed by Burbidge & Burbidge (1975). A recent analysis of the reduction procedures and the resulting accuracy has been published by van der Kruit (1976b), with particular attention given to the well-known correction for the curvature of the slit image on the spectrogram (Minkowski 1942) and for other distortions due to the image tube. With dispersions of about 50 Å mm^-1, a precision of 10 km/sec or better is possible. Schweizer (1975) has called attention again to effects of asymmetric illumination of the slit and pointed out that the resulting distortions on the spectra may not always be negligible, especially at lower dispersion. Simkin (1975a) found zero-point shifts of up to 100 km/sec that varied with the orientation of the spectrograph during the observations. Such shifts are apparently not always present, however, since in other cases (e.g. Rubin & Ford 1970, 1971, Goad 1976, van der Kruit 1976b, van der Kruit & Bosma 1978b) checks of internal consistency and a comparison of the results of various optical and HI observations on the same object suggest zero-point errors of at most a few km/sec.

The application of Fabry-Perot interferometric methods to observations of the emission lines and the treatment of the resulting interferograms are described in detail by Courtes (1960, 1964) and more recently by Tully (1974a). Tully paid particular attention to distortions and to the the deconvolution of the instrumental and intrinsic spectral profiles; he concludes that measurements of radial velocities with these techniques can be made with an accuracy of 10 km/sec, which is comparable to that obtained from slit spectra.

The problem with radio observations of the distribution and motions of HI has mainly been the insufficient angular resolution of diffraction-limited radio telescopes. Useful information on several nearby galaxies of large angular size has been obtained from observations with the world's largest single-dish instruments: the 300-ft telescope at Green Bank, the 100-m telescope at Effelsberg, and most recently the Arecibo 1000-ft telescope. The methods of observation and some notable results have been described by Roberts (1975b), Roberts et al. (1978), and Huchtmeier (1975). For most galaxies, however, angular resolution even better than the 3'-10' provided by these instruments is needed, and the majority of the HI maps discussed here have been made with aperture-synthesis radio telescopes: the two-element interferometer at Owens Valley (e.g. Rogstad & Shostak 1971), the half-mile radio telescope at Cambridge (Baldwin et al. 1971), and the synthesis radio telescope at Westerbork (Allen et al. 1974). There are many instrumental problems peculiar to these data, and some of the difficulties associated with nonuniform coverage of the aperture plane, side-lobe effects, missing short-baseline information, subtraction of continuum emission, and analysis of the HI profiles are discussed by, for example, Rots (1974), Schwarz (1978), and van der Hulst (1977). The final accuracy with which the beam-smoothed radial velocity of the HI can be mapped is of the order of a few km/sec.