Post-correlation ripple removal and RFI rejection for Parkes Telescope survey data
D.G. Barnes, The University of Melbourne
F.H. Briggs, Australian National University
M.R. Calabretta, Australia Telescope National Facility

The Multibeam receiver on the Parkes telescope has been used for many neutral hydrogen (HI) imaging projects, for example the HI Parkes All Sky Survey (HIPASS) and the Southern Galactic Plane Survey. In nearly all experiments to date, basic radio frequency inteference (RFI) rejection has been accomplished on a per-feed basis, using the median statistic throughout the processing, which eliminates outliers in a non-parametric fashion at the expense of increased statistical noise. The classic 21-cm baseline ripple problem, rife in Parkes 21-cm survey data, has been largely ignored, except for the subtraction of a coarse template which is independent of feed, time, telescope elevation, and receiver rotation. The ripple is especially strong during daytime, when the Sun acts as a broadband RFI noise source.

Here we report on new techniques that we are developing - for application to raw, post-correlation Multibeam data - which handle RFI rejection and ripple suppression in more sophisticated ways. RFI can be identified by the presence of coincident outliers on multiple feeds, by sharp increases or decreases in flux time series that are inconsistent with a beam-smeared sky, and by the presence of highly polarised flux. We present our progress with this approach and highlight some remaining difficulties. We also present a novel technique for modelling the variation during telescope scans of 21-cm baseline ripple caused by stray continuum radiation undergoing multiple reflections in the telescope cavity, and demonstrate its application to HIPASS data; this may increase the usefulness of daytime spectral line observing. Finally, we describe our on-going efforts to measure the response of the Parkes telescope and Multibeam system to on-axis continuum radiation sources.