Coordinates on a sphere

[sphere]

Hello,

It's probably a simple question and yet, I can't get to solve it.

Given the observer location (x,y,z) and a sphere (with radius 0) location and orientation (x,y,z,heading(angle in relation to z axis),pitch(angle in relation to y axis),roll(angle in relation to x axis)).
How do I determine the exact coordinates on the sphere, that the observer see. i.e. if the sphere is build of 1 degree on 1 degree tiles. should the observer shoots toward the center of the sphere, which tile will he hit?

Thanks and have a greet weekend.