Poster Abstract

P.13 Sander ter Veen (ASTRON)

Calibration of a phased array using holography: the LOFAR case.

A well known technique to calibrate radio telescope dishes is Holography. This technique is used to compute and correct the irregularities in the surface of the dishes. A holography observation is performed using at least two telescopes: the target dish to be calibrated and a reference dish. The reference dish points to a calibrator source while the target antenna moves around to points to the calibrator source surroundings. By correlating the signals coming from the two telescopes it is possible to map the direction dependent response of the target antenna.

For phased arrays, composed of a collection of antennas on a flat surface grouped in stations, holography is a very powerful method for calibration also. Here, instead of mapping the shape of the main dish, we derive the antenna positions and the delays induced by the cables and electronics. Uncorrected delays deform the beam pointing in a similar way as a deformed surface area. Using holography, correction factors are derived for each antenna to calibrate the stations and obtain the optimum beam shape.

We are applying this technique to a phased array telescope, the Low Frequency Array (LOFAR). The holography method significantly improves our current calibration procedure as it is less sensitive to uncertainties due to ionospheric variation and local interference. Furthermore, it provides us with calibration factors to apply within a single station as well as for inter-station calibration in the dense core. Finally, it provides a direct diagnostic of system issues that affect the beam shape within minutes.