Velocity-aligned spectra as a function of radius. This is a 2-D image
(equivalent to a Nrad column table) containing the spectra (averaged
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.
