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