A Study of the Fitting Accuracy of the Active Reflector for a Large Spherical Radio Telescope

We propose a spatial three-degree-of-freedom (DOF) parallel mechanism combining two degrees of rotations and one degree of translation to support the active reflector units of a large spherical radio telescope. The kinematics, workspace and accuracy of the mechanism are analyzed. One-dimensional and two-dimensional fitting errors to the working region of active reflector are investigated. Dimensional parameters of the mechanism and active reflector unit are examined with respect to the requirement of fitting accuracy. The result of accuracy analysis shows the effectiveness and feasibility of the proposed mechanism, and gives a design rule to guarantee the highest working frequency required by large radio telescope.

EMC design for actuators in the FAST reflector

An active reflector is one of the three main innovations incorporated in the Five-hundredmeter Aperture Spherical radio Telescope（FAST）.The deformation of such a huge spherically shaped reflector into different transient parabolic shapes is achieved by using 2225 hydraulic actuators which change the position of the 2225 nodes through the connected down tied cables.For each different tracking process of the telescope,more than 1/3 of these 2225 actuators must be in operation to tune the parabolic aperture accurately and meet the surface error restriction.This means that some of these actuators are inevitably located within the main beam of the receiver,and Electromagnetic Interference（EMI）from the actuators must be mitigated to ensure the scientific output of the telescope.Based on the threshold level of interference detrimental to radio astronomy described in ITU-R Recommendation RA.769 and EMI measurements,the shielding efficiency（SE）requirement for each actuator is set to be 80 d B in the frequency range from 70 MHz to 3 GHz.Therefore,Electromagnetic Compatibility（EMC）was taken into account in the actuator design by measures such as power line filters,optical fibers,shielding enclosures and other structural measures.In 2015,all the actuators had been installed at the FAST site.Till now,no apparent EMI from the actuators has been detected by the receiver,which demonstrates the effectiveness of these EMC measures.

Directly modulated active distributed reflector distributed feedback lasers over wide temperature range operation(-40 to 85℃)

We experimentally demonstrated that the distributed feedback(DFB) lasers with the active distributed reflector achieved a 25.8 Gb/s operation over a wide temperature range of -40 to 85℃. The DFB lasers can achieve additional feedback from an active distributed reflector with accurately controlled phase, and single-mode yields are not related to the position of cleave. The threshold currents of the fabricated laser are 6 mA and 20 mA at -40℃ and 85℃, respectively. The side mode suppression ratio of the fabricated laser is above 50 dB at all temperatures. Transmissions of 25.8 Gb/s after 10 km single-mode fibers with clear eye openings and less than 0.8 dB power penalty over a wide temperature range have been demonstrated as well.