This eliminates vane diffraction and blockage, as well as secondary mirror scattering and absorption, thus improving image brightness and contrast.
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.
The hole in the front face of a Pfund tracking flat should only be large enough to pass the desired field of view with minimum vignetting (blocking of part of the light from the paraboloid) to minimize central obstruction.
[b] The front face should lie precisely in the plane of the elevation rotation axis to minimize the required flat mirror aperture.
This creates the need for counterweights extending forward from the mirror cell to balance the load on the elevation servo drive.
The aperture stop is the rim of the focusing paraboloid, thus the feed flat has to be slightly larger than the on-axis diameter required to maximize illumination over the desired field.
The McDonald Observatory Supernova Search Telescope used the Pfund configuration, and its feed flat diameter was 24″, while the focusing mirror was an 18″ f/4.5 paraboloid.
Examples of Pfund telescopes are the Infrared Spatial Interferometer Array at the University of California at Berkeley.
The Wheelchair Access Telescope is fully compliant with the Americans with Disabilities Act requirements.
[needs update] John O. Fundingsland was apparently unaware of Pfund's telescope design, and independently developed the same optical configuration.