The 5.5-meter telescope was constructed in 1968. It served for many years as an interferometer component at JPL's Table Mountain Observatory. In collaboration with JPL, the telescope was moved to OVRO for microwave background observations in the late 1980s.
The dish is a commercial type built by the Whittaker Corporation with a diameter of 18 feet. The surface consists of 16 radial segments, leaving a central hole of 2 feet diameter. The front of these panels is covered by a solid aluminum skin that is attached to the panels' internal honeycomb structure with an adhesive. No rivet or screw heads disturb this surface. The panels abut each other tightly, leaving a gap of typically less than 1 mm. The backup structure of the dish is very rigid. As a result, the antenna surface does not deform measurably as the telescope tilts, so that the antenna gain remains constant at all zenith angles.
A hole with a diameter of 2 feet cuts through one of the top panels at a radius of about 2 meters from the apex. It is used by a television camera whose optical axis is aligned approximately with the telescope's radio axis, and which serves to find the initial radio pointing offsets. During normal observing, this hole is covered by an aluminum plate to prevent ground spillover.
The telescope is mounted in a standard altitude-azimuth (altaz) configuration. Since it was originally mounted equatorially, the geometry of the counterweight arms is peculiar and makes balancing the telescope difficult at some zenith angles.
The telescope pedestal is a commercial type as well, built by Scientific Atlanta in 1968, and uses two variable-speed DC motors for each axis. In slew mode, both motors have the same direction, while in track mode the directions are opposite to eliminate backlash and to allow slow and accurate motion of the dish. When moving to a position, the telescope first moves there at high speed in slew mode. Once the telescope has come within 2° of the target, the drive system switches to track mode and pulls slowly toward the requested position, reducing the track speed as it closes in. In this way, the telescope overshoots only minimally during acquisition, saving valuable observing time. It is also helpful that the telescope slews very fast; it can complete a 360° turn in less than two minutes.
The angular encoders are thermally controlled Baldwin encoders of somewhat advanced age that use a germanium photodiode array illuminated by a flash bulb twice a second, the frequency of the telescope drive servo cycle. Because of the encoders' age and their germanium electronics, encoder problems have been encountered frequently. The encoder resolution is 0.005° (18 arc seconds).
The guiding design consideration for the telescope's optical system was the minimization of ground pickup by the antenna, within the constraints of an obstructed aperture. We use the dish in a Cassegrain configuration to accommodate the large and heavy receiver and to reduce direct ground spillover. The hyperbolic secondary has a diameter of 2 feet and is constructed of cast aluminum. During the replacement of the feed legs, this mirror was retrofitted to contain a vertex plate whcih dramatically reduces the illumination of the center of the dish, where the receiver front and its mounting brackets can scatter radiation in unwanted directions.
The secondary support leg structure is a tripod whose legs are led over the edge of the dish. The configuration is an inverted Y. The angle separating the bottom legs is 90°, giving an angle of 135° to the top leg. The width of the legs in the radiation direction is 1.5 in, giving very low geometrical obstruction. The dish edge of the legs is modified by a baffle arrangememnt to direct any secondary reflections away from the ground. (See the paper Reduction of ground spillover in the Owens Valley 5.5m telescope by C. R. Lawrence, T. Herbig, & A. C. S. Readhead 1994, Proc. IEEE, 82, 763-767.)