Thanks for the helpful emails everyone.
What I did to get the figure I wanted (CA trace with every 10th ca
highlighted) was as follows...
use your normal PDB file and do...
set ribbon_sampling, 0
..as suggested by Robert L. Campbell
...and then I just showed the sphere for specific CA atoms and reduced
the radius with..
alter n, vdw=1
! where n is the name of the CA