The Tianma 65-m Telescope (hereafter TMRT65) 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 0.3 mm 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 TMRT65 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 aperture 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 TMRT65 at both K- and Q-bands. These two beams have a ﬁxed separation angle of 140 arcsec at K-band and 100 arcsec at Qband. 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 TMRT65 at Qband. The red and blue colors represent ηeﬀ with or without the active surface control, respectively.|