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The Telescope Optics and Pointing

The main reflector of the telescope is a , 305 m (1000 ft) in diameter and 51 m (167 ft) deep. Its surface consists of 38,778 perforated doubly-curved aluminum panels, each measuring about 3 ft 6 ft, supported by a network of steel cables strung across an underlying karst sinkhole.

Suspended 137 m (450 ft) above the reflector is a 900-ton triangular platform, supported by 18 cables strung from 3 reinforced concrete towers. Each tower is back-guyed to ground anchors via 7 3.25 inch diameter steel cables. Another system, consisting of 3 pairs of cables, runs from each corner of the platform to large concrete blocks under the reflector. These ``tie-downs'' are attached to giant jacks which allow adjustment of the height of each platform corner with millimeter precision, retaining the optimum focus and pointing despite changes in the ambient temperature.

Just below the triangular frame of the platform is a circular track on which the azimuth arm turns. The azimuth arm is a bow-shaped structure 100 m (328 ft) long, whose curved lower girders carry a second track, on opposite sides of which a carriage house and a geodetic (Gregorian) dome can be independently positioned anywhere up to from the vertical. Electric motors drive the azimuth arm, the Gregorian dome and the carriage house to point to and track any requested celestial position with millimeter precision (3 mm arcsec). Celestial tracking duration (rise to set) is given in Table 2. Tracking accuracy is about 5arcsec (rms) at night. The pointing corrections needed to achieve this include individual offsets for each feed, as well as analytic and tabular corrections for measured errors. Coordinate conversion from Galactic, B1950, J2000 and current coordinates to telescope azimuth and zenith angle is provided.

Table: Tracking Time

Two methods are used to correct the spherical aberration inherent in the use of a spherical primary reflector:

• A 96-ft long line feed tuned to 430 MHz, (and a 47-MHz point feed system), is located on the carriage house and used for ionospheric radar and some pulsar observations. This 430-MHz feed has the unique advantage of illuminating the entire reflector (at zenith), but has a bandwidth of only 10 MHz with system temperature and gain that degrade rather rapidly with increasing zenith angle.
• The system housed in the Gregorian dome provides continuous frequency coverage from 300 MHz to 10 GHz using a pair of shaped (secondary and tertiary) subreflectors, plus standard point feeds and receivers. The dome also provides weather protection. The feeds and receivers themselves are mounted on a rotating turret within the dome, allowing rapid changes (of order 30 sec) from one frequency band to another. Consecutive use of multiple receivers during an observing run is straightforward.

The available receiver systems mostly operate with considerable cryogenic cooling to minimize the noise level against which celestial sources are observed. For more details of these see Section 2.2. The 1-MW S-band planetary radar transmitter, located in a special room inside the dome, is used to transmit radio signals to objects within our solar system. Analyzing the echoes provides information about surface properties and object dynamics.

A powerful scripting language, Tcl, has been used to develop coordinated pointing and data acquisition procedures that are constantly evolving. Our experience has been that most observers desire some tailoring of the standard procedures to their specific needs, and a graphical user interface named CIMA is available for spectral-line, continuum and pulsar observing, and to perform calibration continuum cross-scans. CIMA considerably simplifies the observing process for a majority of astronomical observations. For full details on using CIMA, go to http://www.naic.edu/~cima.