Owens Valley Radio Observatory

OVRO 40-meter telescope

40-Meter Telescope

The 40-meter (130-foot) telescope was designed in 1963-64 and constructed in 1965-1966 by Westinghouse Electric Corporation, with funding from the National Science Foundation.

The telescope is an alt-azimuth instrument, consisting of an f0.4 40-m paraboloidal dish reflector, feed and supports, mounted on an alidade and base pedestal. The reflecting surface consists of a mosaic of 852 precision-contoured, lightweight aluminum, individually-positionable panels attached to the steel dish structure by means of leaf spring mounting brackets designed to accommodate differential thermal expansions without affecting the reflector performance.

The dish support structure consists of a welded steel tubing trusswork and includes counterweights and supports, the main 180°, 5.5-m pitch diameter elevation gear sectors, and catwalks and ladders for access to the dish and feed platform. The entire counterweighted dish structure is rotated in elevation on the alidade by means of two 0.74-m diameter roller bearings at the ends of the alidade cross tube.

The alidade consists of a horizontal steel tube about 3 m in diameter and 9 m long and a conical base section. The tube forms the elevation axis and mounts the elevation bearings and drive gear boxes at each end. It also houses the secondary gear boxes, the elevation drive system, electrical and cryogenic equipment. The alidade rotates on the azimuth bearing at the top of the upper base pedestal. Electrical and instrumentation cables are routed through the west elevation bearing to the dish structure by means of a floating-spider torsional cable wrap-up system.

The pedestal is a steel tower 12 m high, 5.6 m in diameter at the top, and 12 m square at the base. The upper pedestal is surmounted by the 4.3-m diameter azimuth bearing of the X roller type, and contains the azimuth gear boxes and drive system. In the bottom of the pedestal is a control room, electrical controls, etc. Electrical power and electronic cables are routed through the azimuth bearing to the alidade by means of a semi-spiral cable wrap-up drum which permits 410° of azimuth rotation without damage to the cables.

The pedestal rests on a movable base carriage: the telescope was originally intended to be one element of an eight-element interferometer array, and was therefore designed to be movable on a rail-track, but construction of the remainder of the array was not funded.

The telescope is steerable in elevation from a low point of 5° above the south horizon through the zenith to a point 55° past the zenith (but at present the computer control program does not permit observations beyond the zenith). In azimuth the telescope is steerable through an angle of 410° with the 50° overlap in the north-west quadrant. This permits uninterrupted tracking of any source for a period of 12 hours or more, except that sources cannot be followed through a zone of about 1° radius around the zenith. Maximum slew rate is 15°/minute in both coordinates. Each axis is powered by a 10 h.p. DC servo motor which is used for both tracking and slewing; the servo motors are under computer control.

The pointing angles of the telescope are indicated by Inductosyn digital encoders mounted directly on the azimuth and elevation axes. These are 21-bit electromechanical encoders with nominal resolution about 0.7 arcsec, and they were installed in 1983 to replace the older photoelectric encoders. The signals generated by the encoders are converted to digital readings by an NPL antenna data converter system mounted in the computer rack; the digital encoder readings are displayed on the front panel of this box. For local manual control, two three-speed synchro systems indicate both azimuth and elevation at the control panel in the base of the telescope. Sense switches are provided so that the telescope may be driven automatically to the stow position at the zenith or to the service position, pointing at the southern horizon.

The angles read from the encoders are fed to the computer, in which the conversion from equatorial to alt-azimuth coordinates is performed. The computer closes the servo loop by supplying the drive signal to servo amplifiers which drive the DC motors. The original digital control system utilized an SDS 920 computer, which was replaced in 1980 by an LSI-11 computer, which was in turn replaced by the present MicroVAX computer in 1991.

Surface Accuracy and Efficiency

The original specification on the reflector panels was for a 1.6 mm rms deviation from the required paraboloid on individual panels. Sample panels showed an rms deviation of 0.36 mm. Initially, the telescope was adjusted at zenith distance 0° and the panels set to an accuracy of ±0.8 mm in that position. For an alt-azimuth mount, a much better procedure is to adjust the telescope for best fit parabola at a zenith distance of approximately 40°, thus allowing a smaller gravitational deformation over the useful range of zenith distances and hence a smaller change in the effective area of the telescope. The panels are currently set within ±0.25 mm at zenith distance 40°. To a great extent the gravitational deformation of the telescope as the antenna is moved in zenith distance is homologous, that is the paraboloid deforms into a new paraboloid. The focal point of the paraboloid of best fit moves, however, in both the axial and lateral directions as the zenith distance is changed, and for optimum performance the feed may be moved in both these directions under manual or computer control.

The overall surface accuracy is estimated to be about 1.1 mm rms, giving efficiencies (at 40° zenith distance) ranging from 42% at 10 GHz to 20% at 24 GHz.

Drive Performance

The specified pointing accuracy for the telescope in 20 mph wind was 20 arcsec. The rms tracking error (i.e., the difference between the shaft encoder reading and the demanded encoder position) actually achieved is considerably better than this except in high winds, but there are non-repeatable pointing errors of up to one arcmin which are not fully understood at present. The drive systems provide sufficient torque and have adequate stiffness to maintain precision tracking (< 20 arcsec) in winds up to 20 mph, usable tracking to 30 mph, and capacity to slew to “stow” position in winds above 40 mph.

Moonlighting

The 40-meter telescope played a major role in the 1996 film The Arrival, written and directed by David D. Twohy and starring Charlie Sheen as “noted radio astronomer” Zane Ziminski. “Twohy's film is just smart enough to qualify as more than a guilty pleasure.” — Jeff Shannon (amazon.com).


Tim Pearson (tjp·astro.caltech.edu)
2000 July 22