The MMA/LSA radiotelescope is currently planned to consist of a goal
of 64 antennas, each of 12 m diameter. The principal requirements
for the antennas are shown in Table 4.1.
|Configuration||Elevation-over-azimuth mount, Cassegrain focus|
|Frequency range||30 GHz to 900 GHz|
|Reflector surface accuracy||25 microns, rms|
|Pointing accuracy (9 m/s wind, 15 min between calibrations)||1/30 beamwidth at 300 GHz , rms (0.6 arcsec)|
|Fast switching (settle to 3 arcsec pointing)||Move 1.5 deg on sky in 1.5 seconds|
|Phase stability||15 microns rms|
|Close packing||1.25 dish diameters between azimuth axis|
|Transportability||Transportable on a rubber-tired vehicle|
|Vendors information meeting||September, 1998|
|Issue Request for Proposal (RFP)||March, 1999|
|Receive proposals||June, 1999|
|Evaluate proposals and place contract||Sept, 1999|
|Design completed by contractor||Sept, 2000|
|Deliver antenna to US test site||June, 2001|
The original concept for the MMA used 8 m diameter antennas. In 1997 the antenna diameter was increased to 10 m and in January 1999 it was further increased to 12 m, both of these increases being in response to the requirements of a larger project formed by merging the MMA and the European LSA projects (see MMA Newsletter, Jan 1999 for more information on the 12 m diameter decision).
This chapter presents a summary of the performance requirements detailed in the antenna Request For Proposal (NRAO, 1999) for the MMA antenna elements and describes the current strawman design concepts which will satisfy these requirements. Also described are the current plans for the procurement and testing of the antennas.
The "antenna" subsystem of the MMA is here defined to include the following equipment:
12 m diameter primary reflector including tripod or quadripod subreflector
Secondary reflector and its servo-controlled positioning platform, including nutation.
A receiver cabin and its HVAC system.
Alt/az mount, the drive systems on the mount and the servo-system controller for the drives.
Metrology instrumentation such as temperature probes, tiltmeters, laser metrology systems, etc.
Power distribution cabling on the antenna and the cable wraps for these cables.
Platforms for mounting auxiliary equipment such as cryogenic compressors.
Antenna foundation design but not fabrication.
Antenna transporter vehicle.
4.2. Specifications and Requirements
4.2.1 Operating Environment
The following operating environment defines the environment on the MMA site in Chile. The first few antennas will be tested at a location in the Continental US and it is possible that all antennas will be assembled and undergo preliminary testing at San Pedro de Atacama (altitude 2440 m). Before ordering the first antennas, these environmental specifications must be reviewed to ensure that they are adequate to ensure safety of the antenna at these alternate test location sites.
220.127.116.11 Location: Northern Chile, latitude -23d01m S, longitude 67d45m W.
18.104.22.168 Altitude: 5000 m (16400 ft) The barometric pressure at this altitude is 55% of its sea-level value.
22.214.171.124 Maximum Wind Velocity: The antenna must survive 65 m/sec (145 mph) without damage when positioned in its stow position.
126.96.36.199 Temperature: The antenna must operate correctly in the temperature range -20 C to 30 C. The annual average temperature on the site is -4 C.
188.8.131.52 Precipitation: Annual precipitation on the site is in the range 10 cm to 30 cm per year. Most of this falls as snow but thunderstorms do occur and so brief periods of heavy rain and hail are possible. The antenna must be designed to survive, without damage, the following conditions: maximum rate of rainfall 5 cm/hr, hailstones 2 cm diameter at 25 m/s, snow load 100 kg/sq.m on reflector surface, radial ice on all exposed surfaces 1 cm. Surface heating to prevent snow and ice buildup not required.
184.108.40.206 Humidity: The monthly average humidity in the summer (January) is 53% and in the winter (June) it is 31%. The annual average is 39%. The monthly average water vapor pressure in the summer (January) is 4.0 hPa ( 4 gm/sq.cm) and in the winter (July) it is 1.2 hPa. The annual average is 2.3 hPa.
220.127.116.11 Insolation: The site location on the southern tropic, the high altitude and low water vapor result in insolation rates amongst the highest in the world. The median midday solar flux in the wavelength range 0.3-60 micrometers for the months of December and June are 1290 w/sq.m and 840 w/sq.m respectively. Ultraviolet radiation will be approximately 70% higher than at sea-level.
18.104.22.168 Lightning. Thunderstorms occur on the site so the antenna must be equipped with a lightning protection system.
22.214.171.124 Dust and Grit. The site ground surface is volcanic soil and gravel with no vegetation of any kind to stabilize the surface. It is likely that wind-blown dust and grit will be a factor for machinery operating on the site but this problem has not yet been well quantified.
126.96.36.199 Earthquake. The MMA site is in a seismically active zone, but the source of the earthquakes, the tectonic plate interface, is more than 100 km below the surface so that the strength of the earthquakes is lower than the strength experienced closer to the Chilean coast. Design for 0.3G horizontal or 0.3G vertical acceleration.
4.2.2 General Configuration
The antenna will be a symmetric paraboloidal reflector, of diameter 12 m, mounted on an elevation over azimuth mount. Subreflector support legs will be either tripod or quadripod configuration. A reflector surface consisting of machined aluminum panels is preferred. The reflector surface will be mounted on a preferred carbon fiber reinforced plastic (CFRP) reflector backup structure (BUS). The BUS could be built completely of CFRP or could consist of CFRP struts connected by metal nodes. The tripod or quadripod will be made of CFRP. The reflector surfaces of the antenna will not be painted.
All drawings will have metric dimensions. All fasteners will be metric. The use of standard metric cross-sections for construction materials is preferred but will not be required if it results in a cost increase.
The antenna will be designed for a lifetime of 30 years. For design purposes it will be assumed that the antenna will execute not less than 270,000 complete cycles of elevation motion, where a complete cycle of elevation motion is defined to be movement of the reflector from its lower elevation limit up to its upper elevation limit and back down to its lower elevation limit. During its lifetime the antenna will execute not less than 200,000,000 degrees of total motion about each axis.
The supply voltage for the antenna will be the Chilean standards, 220 v (single phase), 380 v (3 phase). All electrical systems must operate correctly on both a 50 Hz or a 60 Hz supply.
The antenna will be designed so as to conform to all relevant Occupational Safety and Health Administration (OSHA) and Chilean safety codes.
4.2.3 Reflector Geometry
The receivers will be located at the secondary focus of a Cassegrain geometry. A strawman design for the Cassegrain geometry is shown in Figure 1 (taken from Lamb, 1999), but the final details will be negotiated with the antenna contractor so that the primary F/D can be optimized for close packing.
Figure 1. Strawman optics layout for MMA antenna.
4.2.4 Range of Motion
Antenna foundations will be constructed so that the azimuth axis of an antenna is parallel to local gravity at the pad. For observations close to the zenith this will result in a difference in parallactic angle between antennas.
Minimum elevation angle for observations: 2 deg
Maximum elevation angle for observations: 125 deg., but this will be limited to 90 deg if the cost of the "over-the-top" capability is significant in budget or performance. Cone of avoidance at the zenith: 0.2 deg in radius for normal sidereal tracking. Because of the high velocities and accelerations required for fast switching or on-the-fly mapping (see section 4.2.8 below) there will be a region around the zenith, probably about 30 deg in radius, where azimuth switching times are degraded .
Stow position for wind survival: elevation 15 deg (this position was chosen so that, during a winter storm, the reflector can be oriented with its back into the wind to prevent build up of snow and ice in the dish. Stow position for maintenance: zenith (this position was chosen to prevent an antenna undergoing maintenance from mechanically interfering with an adjacent antenna in the most compact array).
Range of azimuth motion: 270 degrees either side of due north.
4.2.5 Reflector Surface Accuracy
The surface accuracy goal is 20 microns and the hard specification is 25 micrometers rms, including the subreflector contributions. The specification will provide an antenna surface efficiency of 91% at 300 GHz and 41% at 900 Ghz. At night this accuracy is to be achieved in a wind of 9 m/s which is approximately the 90th percentile wind for nighttime (2000 hrs to 0800 hrs) observing. During the day this accuracy is to be achieved for any orientation of solar illumination in a wind of 6 m/s. During the day the focus can be calibrated astronomically every 30 min.
The final, precision measurement of the surface will be done by NRAO using holography. For the prototype antenna the contractor will provide a surface setting accuracy of 100 microns.The panel adjusters will be calibrated so that an adjustment point can be moved with a resolution of 5 micrometers. A full surface adjustment must require no more than 16 person-hours of work.
4.2.6 Pointing Accuracy
A pointing accuracy in "offset" pointing mode (calibrator 2 deg away every 15 minutes of time) of 1/30th primary beamwidth rms at 300 GHz is required. The antenna specification is 0.6 arcsec for offset pointing, 2.0 arcsec for absolute pointing. At night this accuracy is to be achieved in a wind of 9.0 m/s which is the 90th percentile wind for nighttime (2000 hrs to 0800 hrs) observing. During the day this accuracy is to be achieved for any orientation of solar illumination in a wind of 6 m/s.
Provision will be made in the antenna design for inclusion of metrology equipment which will allow antenna pointing to be corrected for structural deformation caused by wind or thermal loading. Metrology systems to be considered for including in the antenna include: a laser/quadrant-detector system to measure quadripod movement, tiltmeters, temperature probes, laser/retroreflector systems and an IR camera for offset-pointing on stars. The antenna contractor will provide any metrology that he considers essential for meeting performance specifications.
4.2.8 Fast Motion Capability
Three observing modes require the MMA antenna to have special fast motion capabilities: fast switching phase calibration, on-the-fly total power mapping and on-the-fly interferometric mosaicking.
Fast switching: The goal is to have the antenna move 1.5 degrees on the sky and settle to within 3 arcsec pointing error, all in 1.5 seconds of time. A possibly acceptable upper limit for this switching time is 2 seconds. It is expected that the switching acceleration profile will be carefully designed so as to avoid exciting the lowest structural resonant frequency of the antenna,, in which case the lowest resonant frequency should not be lower than approximately 8 Hz. The maximum velocity and acceleration required for fast switching are 3 deg/sec and 12 deg/sec/sec on the sky respectively, with both axes able to move at this rate simultaneously. It is expected that this velocity and acceleration will be achievable in azimuth only for zenith angles greater than 30 deg (this implies maximum azimuth velocity and acceleration of 6 deg/sec and 24 deg/sec/sec respectively).
Analysis of the expected use of this fast switching mode (Holdaway, 1997) indicates that the antenna should be designed to survive 30-50 million cycles of fast switching during an assumed 30 year life.
On-the-fly mapping: In this mode the antenna will scan at a rate of up to 0.5 deg/sec across a large object, several or many beamwidths in size, and then turn around as rapidly as possible and scan back across the source in the opposite direction. A maximum acceleration of 12 deg/sec/sec is required for the turn around. While the antenna is scanning across the source the antenna position must be recorded at a rate sufficient to provide an angular sampling interval on the sky of wavelength/(2D) radians. For 0.5 deg/sec motion and 900 GHz observations this requires antenna position readout every 2 msec. The antenna positions should be accurate to 1 arcsec. As the antenna tracks across the source it is not necessary for the position at any time to be precisely a precommanded position; it is sufficient to simply know where the antenna is actually pointing and all antennas need not point precisely at the same position.
On-the-fly interferometric mosaicking requires interferometry data to be taken while the antenna is continuously scanning across the source. It is expected that the antenna velocity will be only one-tenth of its mapping-on-the fly value (see previous paragraph), but in this case all antennas must point to the same position at the same time to within 1 arcsec rms.
4.2.9 Subreflector Position Control
The subreflector will be supported on a platform which allows movement in all 3 linear directions. The precision of the mechanism will be adequate to allow the subreflector to be positioned, under computer command, with sufficient accuracy to prevent gain loss of more than 1% at 900 Ghz due to focus, comatic or astigmatic aberration. Position will be correctable on timescales of tens of seconds. Total axial focus motion is 3.0 cm.
In addition to the above listed linear motions the first antenna will also be equipped with a subreflector nutator which will allow beam throws of three beamwidths at 86 GHz (4.3 arcmin) at rates up to 5 Hz in the cross-elevation direction only. The decision as to whether all antennas will be equipped with nutators will be made after testing the first antenna.
4.2.10 Phase Stability
Phase errors caused by variations in the propagation pathlength through the antenna can be rapidly or slowly varying. Fast phase changes are primarily caused by the wind and the peak pathlength variation in a 9.0 m/s wind must be no more than 15 microns. Slow phase changes are primarily due to variations in the temperature of the antenna and the goal is to keep these phase errors small enough so that the residual errors after an astronomical phase calibration every 3 min are small enough to allow observations at 900 Ghz.
4.2.11 Close Packing
In the smallest array the antennas must be placed close together. The goal is to be able to place the antennas within 15 m (1.25 D) of each other without any possibility of the antennas hitting each other, no matter what the relative orientation of the two antennas. An acceptable fallback on this requirement would be to have no possibility of interference for elevations above 20 deg. In the event that it proves necessary to have a higher elevation limit of this type when antennas are in the close packed configuration, an electronic interlock on the antenna pad will ensure that the higher limit is activated.
4.2.12 Solar Observing
Direct observations of the sun will be allowed. All surface accuracy and pointing requirements must be met while observing the sun and a suitable surface treatment of the primary reflector surface must be provided to prevent solar heating damage of the subreflector or its support legs. When observing the sun the solar heating of the secondary focal plane must be less than 3 kw/sq.m.
4.2.13 Low Antenna Noise
Contributions to system noise from the antenna, due to such mechanisms as scattering of ground noise into the feed and resistive loss of reflector surfaces, will be minimized as much as possible without compromising the surface accuracy and pointing requirements. Design features to be considered to achieve this goal include supporting the subreflector support legs close to the edge of the reflector, shaping the underside of the support legs to reduce ground pickup and locating the feeds at the Cassegrain focus to avoid the need for tertiary reflectors.
To move the antennas from one array configuration to another the antennas will be picked up and carried on a transporter vehicle which runs on a gravel road on rubber tires. The transporter with an antenna on board will be able to negotiate a 15 % grade, turn a corner with a minimum turning radius of 10 m and travel at 10 km/hr on the flat and 5 km/hr up a 10% grade. An unloaded transporter must be able to travel at 20 km/hr on the flat. The transporter must be able to safely move an antenna in winds up to 16 m/s (this is approximately the 95th percentile for the winds on the site at 1600 hrs local time, the time at which the winds are maximum each day). A stationary transporter with an antenna on board will survive winds up to 65 m/s; if necessary, structure carried on the transporter can be deployed to stabilize the transporter on the ground in this survival mode. To withstand the bumps and jolts of transportation and pickup/putdown the antenna will be designed to survive shock loads of 4 G vertical and 2 G horizontal acceleration.
The transporter will carry an auxiliary generator to keep all electrical systems on the antenna operational during a move. The transporter will pick up the antenna above its azimuth bearing so that the azimuth bearing and drive can be used to rotate the base of the antenna to simplify bolt hole alignment when an antenna is placed on a pad. It may be desirable to oxygenate the air in the transporter operator's cabin so the cabin must not have large air leaks.
When an antenna is picked up a time goal of 20 min is required from the time of arrival of the transporter to the time of departure with an antenna on board. When an antenna is placed down on a pad a time goal of 30 min is required from the time of arrival of the transporter until the transporter has departed and the antenna is ready to be pointed.
4.2.15 Receiver Cabin
A receiver cabin with dimensions approximately as shown in Figure 1 will be provided at the Cassegrain focus. Temperature in the cabin will be maintained by an antenna mounted HVAC system at 16 C to an accuracy of +- 1C. The electrical power consumption of equipment in the cabin will not be greater than 10 kw. The mass of equipment in the cabin will not be less than 1600 kg.
A built-in mechanism will be provided so that a receiver can be lifted from the ground, through the cabin door and into its observing location, all without significant man-handling of the receiver. Part of the installation of a receiver may involve the use of a separate special purpose vehicle, such as a high fork-lift, which lifts the receiver through the cabin door.
The cabin will be watertight and a thin RF-transparent membrane will cover the aperture through which the RF beam enters the cabin. A computer actuated shutter will be deployable to protect the membrane when necessary.
It may be desirable to oxygenate the cabin air when workers are inside so the cabin must not have large air leaks.
A typical layout for the receiver cabin is shown in Figure 2.
Figure 2. Typical layout for MMA receiver cabin.
4.2.16 Monitor and Control
The following functions on the antenna will be controllable under remote computer control:
Antenna position and scan rate
Subreflector position in x,y,z and nutation
Power distribution switching from normal to critical power and complete power down
Receiver cabin HVAC temperature set point
The following functions on the antenna will be monitored by a remote computer:
Antenna position and rate (velocity and direction)
Motor currents and all power supply voltages in the servo system
Readout from any metrology devices
Readout from any temperature probes
HVAC system performance
Limit switch status
The following fault conditions will be automatically sensed and acted on at the antenna:
Power down after smoke detection
Power down after loss of a phase of the power supply
Loss of a drive motor
Drive shutdown if antenna oscillation detected
Antenna stow if command link from control building lost
4.2.17 Interfaces to Other Subsystems
The following interfaces must be defined:
Monitor and control digital interface
Interface between antenna and transporter
Interface between subreflector support legs and the subreflector support mechanism
Interface between antenna and its concrete observing pad
Interface to the electrical power system
Interface to cable wraps
Interface to the receiver package in the receiver cabin
Interface to any equipment racks in the receiver cabin
Installation procedure for receiver package
Interface to cryogenic compressor
Interface to optical/IR pointing telescope
Interface to molecular sieve for cabin oxygenation
4.2.18 Maintenance and Reliability
Because of the remote site and large number of antennas the reliability and maintainability of the antennas are important. The antennas will be designed so that, with normal preventive maintenance, they should operate for 30 years without requiring elevation or azimuth bearing or reflector surface replacement. Although they should not be required, straightforward elevation and azimuth bearing replacement procedures must be included in the antenna design. All normal repair and maintenance actions should be able to be completed by a two- person crew in 4 hours. To the maximum extent possible all equipment on the antenna should be "modularized" so that a failure can be cured by simply swapping out the failed component without the need for any repair in place. Examples of equipment which should be designed for easy replacement includes gear boxes, drive motors, HVAC equipment, servo-system electronic components and the subreflector position control mechanism.
4.2.19 Manufacture and Assembly
The antenna will be designed for economic production costs.
It is expected that the first two antennas will be tested initially at a site in the US and later shipped to the MMA site so the ability to disassemble the antenna into pieces for overseas shipping is required.
The high altitude and remoteness of the MMA site make it desirable to minimize the amount of work required on the high site. It is expected that the antennas will be assembled, outfitted and tested at an Operations Support Facility 50 km from the MMA site at an altitude of 2400 m. They will be carried to the MMA site on the transporter vehicle or, in the event that this proves not to be feasible, they will be disassembled into just two pieces, the mount and the reflector, for transportation to the site on trucks. Thus the antenna should be designed for easy disassembly at the elevation axis and both the reflector and mount must have pickup points for handling as single units.
4.3. Design Concepts
Four antenna concepts that have the potential to meet the performance requirements have been developed:
A 10 m concept developed principally at NRAO and BIMA( Lugten
et.al., 1999 )
A 10 m concept developed principally at OVRO( Woody and Lamb, 1999 )
A 12 m concept developed principally at ESO ( Andersen, 1999 )
A 12 m concept developed principally at IRAM ( Plathner, 1999 )
4.4. Procurement and Construction Plans
(1) An RFP (Request for Proposal) for the design and fabrication of the first MMA antenna was issued on 30 March, 1999. The procurement will be a fixed-price contract to a performance specification. All information developed in the studies of the concepts described in Section 4.3 above is available to prospective bidders, and bidders can propose one of these concepts or an alternative design, but in either case it will be the responsibility of the contractor to meet the performance specifications. In the RFP response the bidders will be required to describe their proposed design in some detail, to provide the cost of the design and fabrication of the first antenna and an estimate for the cost of the antenna in production quantities.
(2) The European LSA project will issue an RFP for a separate prototype, designed for the same performance specification, about one month later than the MMA RFP.
(3) The two prototype antennas will be delivered in June, 2001, and then tested extensively for 1.5 years so that the design which is best on a cost/performance basis can be selected for quantity production.
(4) The contract for the production run will be issued in January 2003, with a required antenna delivery of 8 antennas per year. The antennas will be assembled and acceptance tested by the contractor at the Operations Support Facility (OSF) in San Pedro. NRAO will outfit and test the antennas at the OSF and then transport the antennas to the MMA site on the transporter.
4.5. Test Plans and Results
4.5.1 ON THE FIRST ANTENNA, BEFORE SECOND ANTENNA BECOMES AVAILABLE
NRAO Installation of:
This chapter represents the work of the MMA Antenna Working Group: J. Bieging, J. Cheng, D. Emerson, M. Fleming, M. Holdaway, J. Kingsley, J. Lamb, J. Lugten, J. Mangum, J. Payne, J. Welch, D. Woody
T. Andersen, "Feasibility Study for a 12 m Submillimeter Antenna", MMA Memo 253, Feb 1999.
M. Holdaway, "How many switching cycles will the MMA make in its lifetime", MMA Memo 174, 1997.
J. Lamb, 1999, Optimized Optical Layout for MMA 12-m Antennas, MMA Memo 246, Jan 1999.
J. Lutgen, J. Kingsley, J. Cheng, V. Gasho, M. Fleming, "A 10-m Antenna Design for the Millimeter Array", MMA Memo 240, Feb 1999.
NRAO, "Request For Proposal, A Prototype Antenna for the Millimeter Array/Large Southern Array Radio Telescope", http://www.cv.nrao.edu/~cwhite/mma/rfp/download.html, Mar 1999.
D. Plathner, "A 12m Telescope for the MMA-LSA Project", MMA Memo 259, April 1999.
D.P. Woody, J.W. Lamb, "A Design for a Precision 10-m Sub-Millimeter
Memo 241, Mar 1999.