2 Need for the scheme

It is well known that the phase and amplitude errors inferred from observations of a calibrator source may not apply to a source observed at a different time and in a different part of the sky because of the phase errors due to propagation effects. Also in the case of a dipole antenna array like the GRH [10, (Ramesh et al. 1998)] where one has to introduce phase/delay gradients across the array to tilt the antenna beam, the instrumental phase errors do not remain constant. Apart from this external calibration scheme, the other method by which one can obtain the correction terms independant of knowledge about the sky brightness distribution is by the use of a calibration scheme based purely on redundancy in the length and orientation of the baseline vectors. But there are some practical difficulties in using this scheme for a dipole array.

In an array like the GRH, the phases of the different antenna groups along either arm of the "T'' can be found by using the redundant baselines along the respective arms. These phases can then be used to correct the observed visibilities on the cross baselines (E-W $\times$ S or E-W $\times$ N) since in a T-shaped interferometer array only the visibilities corresponding to the multiplication between the antennas in either arm are necessary for making a 2D map. But the phase/delay shifter settings for tilting the antenna beam will be different for the two arms of a "T'' array and in general it will not be possible to compensate exactly the path length differences between the individual elements in a group for different source locations. Due to these, the visibilities on the cross baselines will have residual phase errors. In view of the above, we present a hybrid imaging scheme based on closure and redundancy techniques to get the position of the structures in the source and then use the self-calibration for a proper treatment of the noise [1, (Cornwell & Fomalont 1989).] According to [8, Perley (1989),] self-calibration reduces the noise in regions of no known structure and increases the source brightness. It is well known that self-calibration converges quickly particularly for complex fields if the initial model closely approximates the actual source brightness distribution [2, (Cotton 1979;] [14, Wieringa 1992)] and also it fits the closure phases well [7, (Pearson & Readhead 1984).]

  

Figure 1: Radio map of the Sun at 75 MHz made by elimination of phase errors using redundancy and closure techniques. The open circle at the centre is the optical Sun and the filled circle at the bottom right is the beam of the instrument. The peak brightness in the map is $\sim 1.5 \ 10^{6}$ K and the rms noise is $\sim 2.5 \ 10^{4}$ K