The Nançay Decameter Inteferometer

The first decametric instrument installed at the radio observatory of Nançay, which stays in the Sologne forest (France) under the responsability of the Observatory of Paris, was a 2-antenna radio interferometer built in 1969 to observe solar decametric bursts at high spectral and temporal resolution (Boischot, 1974). The first observations were acquired in January 1970 and the telescope remained in operation until 1977.

It quasi-daily observed the Sun but also Jupiter, so that we named it Nançay Decameter Inteferometer (NDI) in the frame of this project. Solar radio emissions decompose in a variety of component which probe the solar corona which expands in the solar system and forms the solar wind. Jovian radio emissions are produced in the auroral regions, above the planetary magnetic poles, and diagnose the auroral activity of the large magnetosphere of Jupiter. Those two radiosources are the most intense ones observed at decametric wavelength and can in turn be easily and routinely detected by radiotelescopes of modest sensitivity.

The interferometer was made of two Yagi antenna mounted on 10-m high steerable booms. One antenna was fixed and the other one could move along an East-West railtrack to change the baseline. Several radio receivers (wave analysers) have been used. The characteristics of the instrument are reminded below.

(Boischot, 1974) :
– Each antenna consists of an array of 4 log-periodic dipoles, vertically polarized [...] The antennes can be used in the range 20 to 80 MHz with a nearly constant gain of 10 to 12 dB. One antenna is movable on an East-West 1500 m-long railtrack, while the other is in a fixed position. In standard configuration, the antennes are placed 1300 m apart, on an EW basedline, to perform simple interferometry.
– Two swept-frequency receivers cover the ranges 20-40 and 40-80 MHz, with bandwidths of 20 kHz and 100 kHz, respectively. The sweeping rate can be chosen between 1Hz and 500 Hz. The two receivers have two channels. One corresponds to the total power, the other one to correlation measurement used to perform interferometery.
– A multichannel spectrograph is alternately used to increase the temporal resolution. It consists of 50 adjacent channels of 20 kHz bandwidth covering a total range of 1 MHz, whose central frequency can be chosen anywhere in the meter and decameter ranges.
– Four fixed frequency receivers, at 30, 36, 60 and 72 MHz with a 250 kHz bandwidth, can be used for intensity calibration purposes or to track the temporal profile of radio bursts.

Reference :

Radioastronomy on Decameter Wavelengths at Meudon and Nançay Observatories, A. Boischot, Solar Physics, 36, 2, 517-522, 1974.