Pfund telescope

The Pfund telescope, originated by A. H. Pfund, provides another method for achieving a fixed telescope focal point in space regardless of where the telescope line of sight is pointed. This configuration utilizes a two-axis feed flat mirror to reflect starlight into a fixed paraboloid of revolution (paraboloidal) mirror, usually with a horizontal optical axis. The paraboloid focuses through a central hole in the feed flat to a convenient distance behind the flat. No spider vanes or Newtonian secondary fold mirrors are required in this configuration. This eliminates vane diffraction and blockage, as well as secondary mirror scattering and absorption, thus improving image brightness and contrast.

The feed flat is mounted on a two-axis azimuth/elevation mount. The azimuth and elevation drive servos must be continuously controlled as objects move across the sky, using vector addition to calculate the mirror motion in real time. One vector (V₁) is stationary and points from the center of the feed flat to the center of the fixed paraboloid mirror. The other vector (V₂) points from the center of the feed flat to the object to be tracked, which of course moves across the sky in time. The surface normal of the feed flat mirror is the 3D bisector of vectors V₁ and V₂, normalized to unity length. If (N_k,N_l,N_m) are the instantaneous unit vector components of the mirror surface normal, then the mirror elevation angle is arcsin(N_l), and the mirror azimuth angle is arcsin[N_k/cos(Elevation)]. The field of a Pfund telescope rotates at a nonuniform rate during tracking, precluding it from long-exposure astrophotography unless a derotation control matrix and optics are used to compensate field rotation.

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