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Commissioning Progress of the FAST
Peng Jiang, Youling Yue, Hengqian Gan, Rui Yao, Hui Li, Gaofeng Pan, Jinghai Sun, Dongjun Yu, Hongfei Liu, Ningyu Tang, Lei Qian, Jiguang Lu, Jun Yan, Bo Peng, Shuxin Zhang, Qiming Wang, Qi Li, Di Li
TL;DR
FAST commissioning addresses the reliability and performance requirements of a huge, complex telescope. The paper presents system commissioning, measurement, and control approaches, reporting acceptance-index performance and initial pulsar-search activity while noting further calibration needs.
Problem
FAST commissioning must improve operational reliability and effective observation time while meeting acceptance indices for pointing accuracy, sensitivity, and system noise temperature.
Method
The paper reports commissioning of the active reflector and feed support systems, using real-time information, hierarchical safety procedures, and high-precision measurement and control.
Results
FAST reached 2,000 m2/K sensitivity, controlled system noise temperature below 20 K, achieved about 16″ pointing accuracy, and began batch pulsar searches.
Takeaways & Limitations
FAST achieved its reported telescope acceptance indices and completed functional commissioning for tracking, drift scanning, and basketweave scanning.
Takeaways & Limitations
Further testing, analysis, and considerable pointing calibration work remained necessary.
Abstract
from arXiv · showhide
The Five-hundred-meter Aperture Spherical radio Telescope (FAST) was completed with its main structure installed on September 25, 2016, after which it entered the commissioning phase. This paper aims to introduce the commissioning progress of the FAST over the past two years. To improve its operational reliability and ensure effective observation time, FAST has been equipped with a real-time information system for the active reflector system and hierarchical commissioning scheme for the feed support system, which ultimately achieves safe operation of the two systems. For meeting the high-performance indices, a high-precision measurement system was set up based on the effective control methods that were implemented for the active reflector system and feed support system. Since the commissioning of the FAST, a low-frequency ultra-wideband receiver and 19-beam 1.05-1.45 GHz receiver have been mainly used. Telescope efficiency, pointing accuracy, and system noise temperature were completely tested and ultimately achieved the acceptance indices of the telescope. The FAST has been in the process of national acceptance preparations and has begun to search for pulsars. In the future, it will still strive to improve its capabilities and expand its application prospects.
1. Introduction
The paper introduces FAST’s commissioning progress, addressing operational reliability, effective observation time, and high-performance acceptance indices through coordinated system control and measurement. Commissioning achieved key performance targets and enabled multiple observation modes and batch pulsar searches.
- 1. Introduction: FAST commissioning targets operational reliability, effective observation time, efficiency, pointing accuracy, and reduced system noise temperature.These goals address safety risks associated with failures in the active reflector and feed support systems.
- 1. Introduction: Tracking, drift scanning, and basketweave scanning were realized, completing the reported functional commissioning tasks.FAST had also begun batch pulsar searches during commissioning.
- 1. Introduction: 2,000 m2/K sensitivity, below 20 K system noise temperature, and about 16″ feed-receiver pointing accuracy were achieved.The system noise-temperature figure applies to the 19-beam 1.05-1.45 GHz receiver.
- 1. Introduction: The paper covers FAST’s implementation scheme, functional commissioning, technical indices, and performance commissioning progress.FAST’s main structure was installed on September 25, 2016, after which the telescope entered commissioning.
- 1. Introduction: Real-time information and hierarchical commissioning systems support safe operation of the active reflector and feed support systems.The FAST uses an innovative active reflector and rigid–flexible hybrid lightweight feed support system.
- 1. Introduction: The active reflector uses actuated cable-net cross nodes to form a 300-m illuminated aperture that moves with target zenith angle.The cable net comprises ~2,225 cross nodes and 6,670 steel cables, while actuators enable deformation from a spherical surface to a paraboloid.
2) Rigid–flexible hybrid lightweight feed support system
FAST combines a six-cable feed cabin with an A–B rotator and six-degree-of-freedom Stewart manipulator to control receiver position and orientation. Its real-time information system supports safety assessment during commissioning, while the feed-support design improves operational accuracy and telescope sensitivity prospects.
- Feed support architecture: Six cables about 600 m long and weighing 7 tons drive the feed cabin across 206 m at a height of 140 m above the reflector.
- Feed support architecture: The A–B rotator adjusts receiver orientation toward the celestial source because the cable-driven robot cannot simultaneously control position and orientation.
- Feed support architecture: A six-degree-of-freedom Stewart manipulator reduces wind-induced vibration and further adjusts receiver position and orientation for high-accuracy operation.
- Control and measurement: The feed-support system uses closed-loop control based on real-time measurements from four targets on both the feed cabin and Stewart manipulator.
- Real-time safety system: The real-time information system models key active-reflector structures and produces deformation results once every 10-16 s, fast enough for assessment.
- Real-time safety system: During commissioning, the system assessed actuator failures and large-ZA cable-force hazards, alerting operators to support safe telescope operation.
2) Functional commissioning of the feed support system
FAST functionally commissioned its feed support system through staged no-load, simulation, and combined testing, establishing a safe basis for telescope operation. By the end of 2017, it had realized multiple observation modes, completing functional commissioning.
- Functional commissioning methods: Staged functional commissioning tested the six-cable-driven robot under no-load, simulation, and combined feed-cabin conditions.The tests verified mechanism dynamics, control processes, wind-disturbance behavior, measurement simulation, and combined operation.
- Functional commissioning methods: The semi-physical simulation model verified combined operation of the feed support system and provided a safe, reliable basis for later performance commissioning.It combined the feed-cabin mechanism with the robot simulation and used terminal measurement equipment to emulate the FAST measurement system.
- Functional commissioning results: On August 27, 2017, FAST continuously tracked target source 3C286 twice, with each tracking run lasting about 10 min.This milestone indicated that the most serious safety risks had been solved.
- Functional commissioning results: By the end of 2017, tracking, drift scanning, and basketweave scanning had been realized, completing the telescope’s functional commissioning.Observation results from November 17, 2017, are shown in Fig. 2.8.
3. Performance Commissioning of the FAST
Performance commissioning decomposed FAST’s acceptance indices and established measurement and control requirements for efficiency, pointing accuracy, and system noise temperature. Under an assumed system noise temperature of 25 K, achieving 1600 m2/K sensitivity requires at least 57% telescope efficiency.
- Commissioning objectives: Performance commissioning tested and optimized telescope parameters against FAST’s acceptance indices.The work included technical-index decomposition and high-precision measurement and control commissioning for the active reflector and feed support systems.
- Technical-index decomposition: Telescope efficiency was decomposed into reflector, feed-cabin shielding, spillover, illumination, and miscellaneous efficiency factors.The decomposition relates efficiency to radio-telescope parameters, including reflector accuracy and observational wavelength.
- Technical-index decomposition: 1600 m2/K sensitivity requires at least 57% telescope efficiency when system noise temperature is assumed to be 25 K.Figure 3.1 relates reflector and feed-center accuracies to telescope efficiencies and identifies the accuracy requirements for 57% efficiency.
- Acceptance-index requirements: FAST’s telescope pointing-accuracy requirement was set to 16", corresponding to approximately RMS 12 mm feed-position measurement and control accuracy.The corresponding reflector surface precision was approximately RMS 8.2 mm according to Fig. 3.1.
1) Surface precision of the reflector system
FAST’s reflector-surface precision was analyzed through error decomposition and addressed with open-loop shape control plus high-precision measurement. The final reflector surface precision was expected to achieve RMS 6 mm.
- Error decomposition: RMS 3.75 mm was the estimated known panel-related error, which directly accumulates into reflector surface error.The estimate included design, manufacturing, temperature, wind-load, and installation contributions determined during construction design.
- Error decomposition: RMS 0.91–4.13 mm temperature-induced surface error was measured across ±20°C and zenith angles of 0°–26.4°; in most cases it was no more than RMS 2 mm.With a future database model, this error was expected to fall below RMS 1 mm, while RMS 2 mm was recorded for overall-system accumulation.
- Measurement accuracy: After calibration, shafting deviation was limited to 1", corresponding to RMS 0.8 mm over a reference-net side length of about 200 m.Atmospheric-refraction and measurement-reference-net errors were each expected to reduce to 1 mm; the active-reflector measurement accuracy index was set as RMS 2 mm.
- Control approach: A new open-loop reflector-shape control method was developed so real-time shape measurement was not necessary.The method was developed according to FAST’s operating conditions, with cable-net control precision assumed within RMS 3.5 mm.
- Surface-precision target: RMS 6 mm was the expected final surface precision of the reflector system after error decomposition.The target depended on high-precision open-loop control and measurement systems for the active reflector.
2) Accuracy of the feed support system
FAST established feed-support measurement and control requirements using total-station measurements and distance intersection. The resulting feed-support measurement error was expected to remain within RMS 4 mm.
- Measurement method: Distance intersection was adopted because substantially reducing total-station angle error was difficult over the feed’s large motion range and long measurement distance.This method uses distance information rather than relying on the difficult angle-accuracy reduction.
- Measurement method: The feed-cabin position and orientation, together with the Stewart manipulator’s configuration, were obtained by analyzing total-station distance information.This measurement approach supported feed-support-system control and accuracy assessment.
- Accuracy analysis: A 100 ms total-station delay was estimated to produce approximately RMS 1.2 mm error.The system’s measurement accuracy requirement was linked to the overall measurement and control accuracy through a one-third criterion, giving RMS 4 mm.
- Accuracy analysis: RMS 4 mm was the expected measurement-error limit for the feed support system after comprehensive error analysis.The analysis considered reference-net precision, total-station target and random errors, and system delay.
1) Measurement reference net
FAST established a high-precision measurement reference net to support reflector and feed-support operations, combining automated monitoring, mutual aiming, and network adjustment.
- The reference net’s core challenge is determining station positions with millimeter-level precision for telescope measurement.
- An auto-monitoring system was developed for high-precision calibration and periodic stability monitoring of the measurement reference net.
- Mutual aiming averages four distance-and-angle measurements between paired total stations, reducing atmospheric angle errors and offsetting coordinate and prism-installation errors.
- RMS 0.2 mm elevation residual error, 0.4" horizontal-angle residuals, and 0.3 mm horizontal-distance residuals were obtained after net adjustment.
- The 23-station reference net achieved 1 mm accuracy over a 500 m range, meeting the stated coordinate-accuracy and measurement-time requirements.
- Elevation-coordinate accuracy was better than RMS 0.1 mm, plane-coordinate accuracy better than RMS 0.2 mm, and measurement time under 30 minutes.
2) Measurement method of the active reflector system and feed support
FAST implemented measurement and control systems for its active reflector and feed support, using calibration databases, interpolation, and combined open- and closed-loop control.
- Active reflector system: The active-reflector measurement system uses ten total stations to measure 700 paraboloid targets and 2,225 cable-net targets for calibration and open-loop control.
- Active reflector system: The active-reflector open-loop controller uses a three-dimensional deformation database, retrieval, interpolation, and calibration modules to generate actuator commands.
- Active reflector system: The open-loop system avoids dependence on real-time measurement, supports all-weather operation, and does not produce radio-frequency interference.
- Active reflector system: RMS surface error was less than 1.7 mm for the basic spherical surface and less than 2.6 mm for the paraboloid, satisfying the RMS 3.5 mm requirement.
- Feed support system: The feed-support system combines open- and closed-loop control across a six-cable robot, A-B rotator, and Stewart manipulator.
- Feed support system: RMS tracking error of 5.4 mm and orientation error of 0.3° limited the overall feed-phase-center measurement and control error to RMS 7 mm.
4. Performance Testing of the FAST
FAST commissioning tested low-frequency ultra-wideband and 19-beam receivers, demonstrating cryogenic performance, broad frequency coverage, and confirmed pulsar detections.
- Receivers: The low-frequency ultra-wideband receiver combines a QRFH, cryogenic receiver, and room-temperature receiver, covering frequencies as low as 270 MHz.
- Receivers: Its cryogenic receiver reached 49 K and 9 K at the first and second cold heads, with equivalent noise temperature below 17 K.
- 19-beam receiver: The receiver’s channels support broadband 270–1,620 MHz and narrowband 1,300–1,620 MHz signals, with linear and circular polarization reception.
- Receivers: More than 60 high-quality pulsar candidates were observed with the ultra-wideband receiver, including 54 confirmed pulsars.
- 19-beam receiver: The 19-beam receiver covers 1.05–1.45 GHz, and its optical transmission system provides approximately 15 dB gain.
2) Digital backends
FAST developed digital backends for pulsar, spectral-line, VLBI, SETI, and related observations, enabling simultaneous multi-objective observing and tested sensitivity performance.
- Backend capabilities: FAST digital backends include pulsar de-dispersion, spectral-line, VLBI recording, SETI, and baseband-recording functions.
- Backend capabilities: The pulsar backend performs channelization and dispersion-delay compensation, while baseband processing supports coherent de-dispersion and high-time-resolution pulse recovery.
- Backend implementation: Backend processing uses ROACH2 hardware, FPGA polyphase filter banks, Ethernet transmission, and GPU clusters for further processing.
- Backend capabilities: The 19-beam backend supports simultaneous pulsar, narrowband spectral-line, wideband spectral-line, FRB, and SETI observations.
- Performance testing: Sensitivity ratio R remained about 2,400 m^2/K below ZA 20.4°, decreased to about 2,000 m^2/K at ZA 26.4°, and fell to about half that value by ZA 40°.
- Performance testing: Measured efficiency was 63% with gain 16.1 K/Jy, decreasing to gain 11.3 K/Jy at ZA approximately 40°.
- Performance testing: Pointing deviation was stable and less than 0.2ꞌ from the source in the reported scanning observations.
Drift
At large zenith angles, the receiver’s backward illumination mode leaves FAST’s beam shape and pointing performance requiring further testing and analysis.
- At large zenith angles, backward illumination requires further testing and analysis of FAST’s beam shape and pointing performance.
5. Future Prospects of the FAST Telescope
Future FAST development focuses on improving pointing, measurement precision, calibration, receiver sensitivity, and observational readiness. Planned upgrades address current measurement, calibration, and receiver-noise limitations while supporting broader applications.
- FAST is preparing for national acceptance while continuing efforts to improve capabilities and expand application prospects.
- Calibration priorities include pointing, baseline, beam shape, and timing systems for spectral-line, timing, and VLBI observations.
- A precise telescope pointing model is needed to further improve pointing accuracy.
- Total-station measurements offer high distance accuracy but are vulnerable to atmospheric effects, causing accuracy loss and low dynamic frequency.
- A fusion measurement system is necessary because single-measurement methods cannot meet feed-support measurement requirements.
- Total-station, inertial-navigation, and GNSS data fusion is planned to provide high-precision, high-dynamic, all-weather feed-support measurements.
- The ultra-wideband receiver’s low-frequency coverage makes its feed horn too large for Dewar cooling, adding more than 10 K of backend noise.
- Increasing the ultra-wideband receiver’s frequency coverage could enable Dewar cooling, reduce receiver noise temperature, and improve telescope sensitivity.