Recent Galactic plane observations at 843 MHz, using the Molonglo Synthesis Telescope (MOST) (Gray1994a) and at 2.4 GHz, using the Parkes 64-m single dish (Duncan et al.1997b), have revealed a number of new candidate SNRs. However many of these objects were in complicated regions of the Galaxy and these observations also suffered from contamination due to thermal emission at these high frequencies. Although the MOST observations were at a relatively low frequency, the side-lobes due to nearby strong sources severely limited the fidelity of images. With only single frequency data available for most of these fields (these observations did not have any overlapping fields), the identification of the candidate SNRs in these fields remained inconclusive.
A sample of candidate SNRs, accessible to the GMRT, were selected from these observations. Continuum observations of this sample were done with the GMRT at 325 MHz during the period of 1998 to 2000. The telescope hardware, on-line and off-line software were all in a state of being debugged during this period and observations were invariably intermingled with debugging activity and related observations. Consequently, the data acquired was often unusable, sometimes requiring several observing sessions involving the same field to get good data. This chapter describes the procedure used for on-line data monitoring and identification of a potential source of data corruption, off-line data flagging and finally imaging using the AIPS package.
Section 4.1 gives the general description of the observations done at 325 MHz. Imaging at these low frequencies requires the use of algorithms and techniques not usually used for mapping at higher frequencies. These low frequency specific problems for inversion of visibilities and image deconvolution, and the techniques used to overcome these problems are discussed in Section 4.2. Section 4.2.2 discusses the problem of bandwidth smearing of sources away from the phase centre, which necessitates the multi-channel mode of observations. The procedures used for on-line system and data monitoring are described in Section 4.3. The detailed procedure used for off-line data editing and calibration is described in Sections 4.4. Section 4.5 describes the procedure used to finally map the data and while Section 4.6 presents the full primary beam images after correcting for the primary beam attenuation.