IRAM Memo 2011-4
Description of the spectral axis handling in CLASS
J. Pety
& S. Bardeau
1. IRAM (Grenoble)
2. LERMA, Observatoire de Paris
July 6

, 2019
Version 1.2
(Version 1.1 released on October 10

, 2014)
(Version 1.0 released on October 20

, 2011)
An observer is interested by the physical properties (line frequencies,
velocity of the gas) of a source in its own reference frame. However, the
observations happen in a different frame, namely the topocentric frame
for ground-based observations. This implies a change of frame to infer
the physical properties in the source frame from the measured frequencies
in the measurement frame. This memo tries to explain the different
interpretation of the spectral axis, the additional subtleties related to
heterodyne receivers in radio-astronomy, the current description of the
spectral axis handling in CLASS, and the impact of various
short-comings that happen for historical reasons.