Pfund telescope
Fixed focal point telescope using a two-axis feed flat.
The Pfund telescope, invented by A.H. Pfund, offers a way to keep a telescope's focal point fixed in space no matter where it is aimed. It uses a flat feed mirror mounted on two axes to direct starlight into a stationary paraboloidal mirror, which typically has a horizontal optical axis. The paraboloid then focuses the light through a central hole in the feed flat to a convenient spot behind it. This setup avoids the need for spider vanes or a Newtonian secondary fold mirror, eliminating vane diffraction and blockage, as well as secondary mirror scattering and absorption, which boosts image brightness and contrast.
The feed flat sits on an azimuth/elevation mount. Its drive servos must be continuously adjusted as objects move across the sky, using vector addition to compute the mirror's motion in real time. One vector is fixed, pointing from the feed flat's center to the center of the fixed paraboloid. The other vector points from the feed flat's center to the tracked object, which moves over time. The mirror's surface normal is the 3D bisector of these two vectors, normalized to unit length. From the normal's components, the mirror's elevation angle is the arcsine of the vertical component, and its azimuth angle is the arcsine of the horizontal component divided by the cosine of the elevation.
The field of a Pfund telescope rotates at a nonuniform rate during tracking, making it unsuitable for long-exposure astrophotography without a derotation control matrix and optics to compensate. The hole in the feed flat's front face should be just large enough to pass the desired field of view with minimal vignetting, reducing central obstruction. This hole must be conical, opening outward toward the back with at least a 45° cone, to prevent vignetting from the flat's back at high tilt angles. The front reflective face must be extremely flat, smooth, and zone-free, ideally within about 25 nanometers peak-to-valley error. It should lie precisely in the plane of the elevation rotation axis to minimize the flat's required aperture, which then requires counterweights extending forward from the mirror cell to balance the load on the elevation drive.
The Pfund flat's diameter is generally larger than the focusing paraboloid's, balancing fully illuminated field coverage against cost and weight.
- Originator
- A.H. Pfund
- Feed flat diameter example
- 24″
- Focusing paraboloid diameter example
- 18″
- Focusing paraboloid focal ratio example
- f/4.5
- Flatness tolerance
- 25 nanometers peak-to-valley error
- Minimum flat diameter ratio
- √2 = 1.4142 times paraboloid diameter
Lore & Background
The Pfund telescope was originated by A.H. Pfund. Its design employs a two-axis flat feed mirror mounted on an azimuth/elevation mount, with drive servos continuously controlled as objects move across the sky using vector addition to calculate mirror motion in real time. The surface normal of the feed flat mirror is the 3D bisector of two vectors: one stationary from the feed flat center to the fixed paraboloid center, and one from the feed flat center to the tracked object. The mirror elevation angle is arcsin(N_l), and the azimuth angle is arcsin[N_k / cos(θ_el)].
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. The hole in the feed flat must be conically shaped, opening outward with at least a 45° cone to prevent vignetting at high tilt angles. The front reflective face must be polished extremely flat, smooth, and zone-free, ideally to within about 25 nanometers peak-to-valley error. The front face should lie precisely in the plane of the elevation rotation axis to minimize required flat aperture, necessitating counterweights. The feed flat diameter is generally larger than the focusing paraboloid; for fully illuminated field coverage at a 90° flat angle, the minimum flat diameter must be at least √2 times the paraboloid diameter.
Reader's Guide
The Pfund telescope is notable for achieving a fixed focal point in space regardless of where the telescope is pointed, which simplifies instrument placement. By eliminating spider vanes and Newtonian secondary mirrors, it reduces diffraction, blockage, scattering, and absorption, improving image brightness and contrast. However, its nonuniform field rotation during tracking limits its use for long-exposure astrophotography unless compensated. The design requires precise real-time vector calculations for the feed flat's azimuth and elevation servos. The aperture stop is the rim of the focusing paraboloid, so the feed flat must be slightly larger than the on-axis diameter to maximize illumination. Examples include the Infrared Spatial Interferometer Array at the University of California at Berkeley, the George B. Wren Supernova Search Telescope at McDonald Observatory, and the Wren-Marcario Wheelchair Access Telescope at the McDonald Observatory Visitor Center. John O. Fundingsland independently developed the same configuration, unaware of Pfund's design, and published a description of his 4″ aperture prototype in 1999.
Did You Know?
- The Pfund telescope uses a two-axis flat feed mirror to reflect starlight into a fixed paraboloidal mirror, usually with a horizontal optical axis.
- The feed flat's surface normal is the 3D bisector of a stationary vector and a moving vector to the tracked object.
- John O. Fundingsland independently developed the same optical configuration, unaware of Pfund's design, and published a 4″ aperture prototype in 1999.
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