Project Cyclops, A Design... - Department of Earth and Planetary ...

APPENDIXF
BASE
STRUCTURES
AZ-EL BASE STRUCTURES
The conventional az-el mount affords maximum sky
coverage, but this coverage is expensive due to increased
structural weight and expensive gearing and/or track and
wheel assemblies. Elevation rotation is generally provided
by gear or chain drives and generally requires
counterweighting of the dish to locate the dish center of
gravity on the elevation axis. The gearing on the
elevation drive must provide the necessary resistance to
wind torques. If the basic dish weight (including backup
structure) is W, then a weight penalty of roughly 0.3W
may be incurred in positioning of the center of gravity.
Dish attachments to the base structure are either near
the center (king-post design) or near the dish extremity
(Manchester mount). Azimuth positioning and dish
support are provided either by a hydrostatic bearing
coupled with a gear drive or wheel and track drive, or
solely by wheel and track support. Typical support
configurations are shown in Figure F-l.
MODIFIED AZ-EL BASE STRUCTURE
Available cost figures for contemporary designs indicate
that the base structure, which includes all drive
elements, is a significant fraction of the cost of the
telescope structure. Here we consider a support structure
that lends itself to mass production fabrication. Threepoint
support of the dish is provided, and the need for
counterweighting of the dish is eliminated. The assembly
uses a single piston for elevation control and a wheel
and track arrangement for azimuth positioning.
The use of the piston for elevation control eliminates the
need for rigid support on the backup structure to
support the bull gear segment required on conventional
designs. This rigid support structure and the bull gear
segment are comparatively expensive items. A novel
feature of the base structure is the use of a central
bearing to absorb all side loads; the wheel and track
arrangement need only provide vertical support and
therefore does not require close control of the track
radius.
BEARING FOR SUPPORT GEAR DRIVE
FOR AZIMUTH
WHEEL AND TRACK FOR
SUPPORT AND ROTATION
LARGE RADIUS WHEEL AND TRACK
ffOR SUPPORT AND ROTATION
CENTRAL BEARING POST OPTION
Figure F-I. Typical configurations.
Preliminary
Sizing
Figure F-2 shows two views (to scale) of the proposed
mount designed for a I00 foot dish with a maximum
zenith angle of 65 °. A preliminary calculation has been
made to obtain a weight estimate assuming the dimension
L_ '= 30 ft. With L_ and 0 given, the maximum
piston extension p is computed to be 42 ft, which
implies that a piston casing of about 45 ft is required.
Since the piston casing sits vertically when the piston is
fully collapsed (dish looking at zenith), the total height
of the base structure h must be at least 45 ft. The height
(and eventually the weight) depends strongly on the
desired angle 0. Once h is found, the lengths of the truss
members can be easily determined by simple geometry.
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