Active Surface Compensation for Large Radio Telescope Antennas

Table 2

Modern antenna surface measurement methods.

Measurement methods	Characteristics	Remarks
Theodolite method	Less than 20 m under the accuracy 0.2 mm	The measurement process of large antennas is slow.
Total station method	Less than 120 m under the accuracy 0.2 mm	Compared with the former, it possesses lower cost, easier operation, faster construction of the coordinate system, and faster maintenance time.
Photogrammetric method	Less than 125 m under the accuracy 0.2 mm	It utilizes object images to reconstruct 3D shapes.
Laser tracking method	Less than 400 m under the accuracy 0.2 mm	It needs to be recalibrated when the instrument tilts, and dense sampling is slow.
Phase retrieval method	Tianma 65 m adopts this method to obtain the surface precision 510 μm.	It uses a single receiver and an astronomical source to measure the beam at a couple of different secondary focus positions.
Tower holography method	ALMA telescope, which is a reflector antenna array made up of antennas with the aperture of 12 meter and 7 meter, adopts this method to obtain the surface precision better than 20 μm.	It is a near-field measurement method using a tower transmitter.
Radio holography method	GBT adopted this method to obtain the surface precision 0.46 mm.	It requires a second reference telescope or receiver system, and both the telescope and receivers must have good phase stability.
Edge sensor method	Good stability and sensitivity on the order of 10 mm and CCAT use this method	It measures the real-time changes of panel-to-panel in the surface to implement real-time closed-loop control of the surface during astronomical observations.