Velocity-aligned spectra as a function of radius. This is a 2-D
image (equivalent to a Nrad column table) containing the spectra (aver-
aged brightness temperature) for each of the Nrad radii defined by the
sampling KEPLER_RINT, KEPLER_RMAX and KEPLER_STEP.
The velocities are defined in KEPLER_SPECTRUM[1], but also derived
from the axis description of this 2-D image.