11.4.1. Observing Procedure
The radiometer system commonly used consists of a multi-channel (or swept-frequency) receiver, whose overall bandwidth covers the range of velocities to be found in the observed galaxy and is centered on or near the systemic velocity of the galaxy. The receiver is frequency-switched in order to establish a reference outside the hydrogen line, or observations are made on and off the source to find a frequency baseline for the spectrum. Scans are made across the galaxy, and the intensity in each frequency channel is recorded as a function of RA and declination. The sensitivity of the frequency channels may be calibrated by observing continuum sources or by injecting broad-band noise into the system. After calibrating the data, intensity-versus-frequency (or velocity) spectra may be drawn on a grid of points on the sky. These are the basic data. Integration under all the profiles yields a map of the hydrogen emission brightness temperature. Radio continuum emission from the galaxy may be subtracted, on the assumption of little interaction with the line radiation, by using channels outside the hydrogen-line emission or by using an off-frequency observation.
Because the resolution is limited, the frequency profiles are usually single-peaked, and it is an easy process to establish a velocity at the peak, median value, or leading edge of the profile. Thus a single velocity at each grid point can be derived and a map of iso-velocity contours of the line-of-sight velocity can be plotted.
Most galaxies subtend only a few beam areas, but for the nearest galaxies there are many beam areas and the maps derived give beam-smoothed estimates of the overall hydrogen density and velocity distribution.