For the first time, a radio telescope has been constructed with sufficient directivity for the production of detailed ‘radio pictures’ of the brightness distribution on the solar disc at decimetre wavelengths.The aerial system, which operates at 21 cm wavelength, consists of two long mutually-perpendicular arrays, each made up of 32 paraboloid antennae, 19 ft in diameter, spaced uniformly along a 1 200 ft base-line. The polar diagram of each array is a set of fan-shaped beams, with high resolution in one direction only. A pencil-beam response (with multiple beams) is produced by combining the signals from the two arrays by a phase-switching method. With the dimensions used, these pencil beams are 3' of arc in diameter and about 1° apart. West-east scanning is provided by the earth's rotation, and the north-south position of the beams can be adjusted by a phase-shifting mechanism. A series of parallel profiles across the sun is obtained; these profiles may be combined to give a ‘picture’ of the solar disc.For satisfactory performance, side lobes in the arrays must be kept at a very low level. To achieve this, the current distribution is tapered from the centre to the ends of each array, and very close tolerances are maintained on the phase relationships between the elements. Phase errors due to thermal expansion of the long twin-wire feeder system are avoided by arranging the lines so that all the elements are connected to the receiver through equal lengths of feeder.The design and construction of the instrument are described in detail. The paper concludes with an account of the techniques used in testing and adjusting the arrays.