The locus of the point (rsecαcosβ,rsecαsinβ,rtanα) is
x2+y2−z2=r2
x2+y2+z2=r2
x2+y2−z2=2r2
x2−y2+z2=r2
We have x2+y2-z2=r2sec2αcos2β+r2sec2αsin2β-r2tan2α =r2