Illustration of the reference frame carried by a physical observer in PyGRO. Angles of observations in the observer’s reference frame are defined as standard longitude, ϕ_obs, and latitude, θ_obs, with the origin on the f₁ direction and reference plane identified by f₁ and f₂. Helper functions built into the Observer class allow one to fire geodesics with any given pair of angles, defined in this way, from any of the frame vectors.