The pass equations are
{(x=rcostheta),(y=rsintheta):}
then
(rcostheta-h)^2+(rsintheta-k)^2 =a^2 or
r^2-2rhcostheta-2rksintheta+h^2+k^2=a^2 or
r^2-2r(hcostheta+ksintheta)+h^2+k^2=a^2
Making now
h/k=tanphi_0 we have
r^2-2rk(tanphi_0costheta+sintheta)+h^2+k^2=a^2 or
r^2-2r(k/cosphi_0) (sinphi_0costheta+cosphi_0sintheta)+h^2+k^2= a^2
but cosphi_0=k/sqrt(h^2+k^2) so
r^2-2rsqrt(h^2+k^2)(sinphi_0costheta+cosphi_0sintheta)+h^2+k^2=a^2
Finally
r^2-2rsqrt(h^2+k^2)sin(theta+phi_0)+h^2+k^2=a^2 or also
r = sqrt[h^2 + k^2] Sin( theta+phi_0) pm sqrt[
a^2 - h^2 - k^2 + (h^2 + k^2) sin(theta+phi_0)^2]