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... show ribbon 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 Camille
