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

feeds must correct for spherical aberration and as
a result tend to be narrow band. The Arecibo
telescope is an example of this arrangement.
Bandwidths can be increased by the use of triple
mirror
configurations.
2. Fixed parabolic troughs with north-south axes and
line feeds along the line focus. Again the feeds are
difficult to broadband, and without east-west
tilting of the trough the instrument can only aim
at the meridian.
3. Fixed paraboloidal sectors pointed at tiltable flat
mirrors. Again, unless the flat mirror can rotate in
azimuth, only near meridian observation is possible,
and the sky coverage in declination is limited.
In addition to Earth-based elements, large single
antennas have been proposed for space use (ref. 1). None
of these has had the accuracy required for operation in
the microwave region. Although not subject to gravity
and wind stresses, space antennas are subject to thermal
gradients from the sunlight. In addition, a large single
dish 3 km or more in diameter would have to be rather
rigid to avoid large amplitude, very low frequency modes
of vibration, which would be excited under repointing
maneuvers. Even the flimsiest of structures-a balloon of
1 mil mylar 10 km in diameter (which would permit a
3-km diameter spherical reflector as part of the surface)-weighs
about 8000 metric tons. At a cost of $100
per pound to put it in synchronous orbit, the cost would
be $1.8 billion. This does not include cost of assembly in
space, nor the weight of receivers, transm!tters, pointing
rockets, servos, and all the other needed equipment. We
do not mean to exclude space antennas from consideration,
but we cannot consider a microwave antenna of
3-km diameter or more in space to be within the present
or near-future state-of-the-art.
For these reasons we largely confined our thinking to
more or less conventional steerable dishes to be used as
elements of an array. If further study should reveal a less
costly approach, the effect will be to reduce the cost of
the Cyclops
TYPES
system.
OF MOUNTS
Steerable dishes, particularly in the smaller sizes, are
often mounted equatorially. The equatorial mount has
the advantages that (1) tracking of sidereal motion
requires a constant rate of rotation of the polar axis
only, and (2) there is no singularity in the sky through
which tracking must be interrupted. For reasons of
economy, large fully steerable dishes are almost always
mounted in ait-azimuth mounts. In this form of mounting,
rotation about both the azimuth (vertical) axis and
the elevation (horizontal) axis occurs at nonuniform
rates while a star is being tracked. In addition, a star
passing directly overhead requires an abrupt rotation of
180 ° in azimuth to maintain tracking. These, however,
are minor disadvantages. The axis motion is readily
programmed into a computer and the zenith singularity
can almost always be avoided. Because the az-el mount
requires less counterweighting and permits a lighter, less
complicated base structure and because more experience
has been gained with large az-el mounts than with large
equatorial mounts, the az-el mount has been selected for
Cyclops.
Az-el mounts fall into two rough categories: the
king-post design and the wheel and track design. In the
former, a very massive rigid column, or king post, carries
the azimuth bearings and supports a yoke or crosspiece,
which serves as the elevation axis bearing support. In the
latter, the elevation bearings are widely separated and
carried by a truss structure, which in its entirety rotates
on trucks carried on a circular track. Lateral constraint
can be supplied by a simple central bearing at ground
level.
The king-post design is suitable for small dishes or for
dishes enclosed in radomes. For large dishes exposed to
the wind, the wheel and track design appears to offer the
required stiffness and strength against overturning wind
moments with much lighter members and smaller bearings.
In addition, the large-radius track permits simple
angle encoders in azimuth while the open structure
allows a large radius elevation drive and simple encoders
on this axis as well. Sandstorms and ice pose problems
for the wheel and track design but these appear
controllable with appropriate seals or pressurization or
both. Thus, if large dishes are used for Cyclops, wheel
and track air-azimuth mounts are probably indicated.
The element design group gave some thought to some
novel base structure (see Appendix F). Because of the
time limitations of the study, careful costing and, in
certain cases, careful evaluation of the structural stability
problems involved could not be carried out. Hence,
the economies of these designs could not be ascertained,
particularly if sky coverage is sacrificed.
SURFACE
TOLERANCE
The function of the backup structure of the reflecting
surface is to hold the shape of the surface within
prescribed tolerances under all operating conditions of
wind, gravitational, and thermal stress. In addition,
buckling or misalignment of the surface panels can take
their toll from the error budget. The allowable surface
errors are proportional to the minimum wavelength and
hence inversely proportional to the highest operating
frequency at which a given efficiency is desired.
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