East Asian VLBI Network (EAVN)

Tianma 65-m Telescope

The Tianma 65-m Telescope (hereafter Tianma65) has a shaped Cassegrain-type design with a 65-m diameter main reflector and a 6.5-m sub-reflector on Az-El mount. The main reflector consists of 1008 aluminum panels deploying an active surface control system with 1104 actuators. The prime mirror achieves a surface accuracy of about 0.3mm rms after compensating the gravitational deformation in real time by the active surface control system. The secondary mirror has a surface error of 0.1 mm rms. A rotatable receiver cabin with the feeds covering frequency range from S-band (2 GHz) to Q-band is mounted at the Cassegrain focus, while the L-band (1.6 GHz) feed is off focus mount separately. The slewing rates of the main reflector are 0.5◦/sec in azimuth and 0.3◦/sec in elevation, as shown in Table 2.

The aperture efficiency of Tianma65 is above 50% at both K- and Q-bands with the active surface control system (see Table 3). The main reflector panels were assembled to give the maximum surface accuracy at the elevation angle of 52◦. The antenna efficiency goes down to less than 10% at low (< 10◦) and high (> 80◦) elevation angles, mainly due to the gravitational deformation. The active surface control system is used for compensating the gravitational effect at different elevation angles, making the gain curves as a constant over the elevation. Figure 3 shows the elevation dependence of the aperture efficiency at Q-band with or without the active surface control. The active surface control system is set 'ON' by default at K- and Q-band observations.

Dual-beam receivers are installed in Tianma65 at both K- and Q-bands. These two beams have a flxed separation angle of 140 arcsec at K-band and 100 arcsec at Q-band. One of the beams is placed at the antenna focus for VLBI observations. Typical sidelobe levels are 13-15dB at both K- and Q-bands. The measured beam sizes (HPBW) are listed in Table 3.

Figure 3: Elevation dependence of the aperture efficiency (ηeff) for Tianma telescope at Q-band. The red and blue colors represent ηeff with or without the active surface control, respectively.
Table 2: Driving performance of EAVN antennas.
Driving axis Driving range Max. driving speed Max. driving acceleration
AZa -60◦ ~ 510◦ 0.3◦/sec 0.3◦/sec2
EL 12◦ ~ 80◦ 0.3◦/sec 0.3◦/sec2
AZa -60◦ ~ 425◦ 0.5◦/sec 0.27◦/sec2
EL 8◦ ~ 88◦ 0.3◦/sec 0.16◦/sec2
AZa -90◦ ~ 450◦ 2.1◦/sec 2.1◦/sec2
EL 5◦ ~ 85◦ 2.1◦/sec 2.1◦/sec2
FRa -270◦ ~ 270◦ 3.1◦/sec 3.1◦/sec2
AZa -90◦ ~ 450◦ 3.0◦/sec 3.0◦/sec2
EL 5◦ ~ 85◦ 3.0◦/sec 3.0◦/sec2
aThe north is 0◦ and the east is 90◦.
bFR is 0◦ when Beam-1 is at the sky side and Beam-2 is at the ground side, and CW is positive when an antenna is seen from a target source.

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EAVN ( East Asia VLBI Network )