Thanks Eric, I got it to work now.  To summarize the code to get the axis information from an atom spec string  looks like this. Please let me know if there is a better or simpler way to do it.

#-----------------------------------

atomspec = '#0:14-37.A@CA'

atoms    = chimera.selection.OSLSelection(atomspec).vertices()

xyzs     = chimera.numpyArrayFromAtoms(atoms)

myaxis   = StructMeasure.axis(xyzs)

#-------------------------------------

Note that I had to import StructMeasure.

Best

H.