Mary E. Putman , Brad K. Gibson, PASA, 16 (1), in press.
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Observations & Data Reduction
The HI Parkes All-Sky Survey (HIPASS) is a blind HI survey of the southern sky (
), over the velocity range -1,200 to 12,700 km s-1 (Staveley-Smith 1997). It therefore encompasses the Milky Way, the Magellanic Clouds, and the more distant Universe. The survey is conducted with the Parkes 64m radio telescope equipped with a multibeam receiver, a focal-plane array of 13 feeds set in an hexagonal grid. The spectrometer has 1024 channels for each polarisation and beam, with a channel spacing of 13.2 km s-1. The survey is conducted by actively scanning the telescope in 8
strips of declination, and displacing adjacent scans in RA such that sub-Nyquist sampling is achieved. The complete survey will scan the sky five times, with an effective integration time of 
minutes per sky point, corresponding to an rms brightness temperature sensitivity of 
mK. The data presented in this paper only incorporates the first set of scans (which still Nyquist sample the sky), corresponding to an rms sensitivity of 
mK.
The data were reduced using a modified version of the standard aips++ based HIPASS reduction software (which was designed for imaging discrete HI sources) (Barnes 1998). To recover the extended structure of HVCs, we used the entire 8 scan to calculate the bandpass correction for a particular channel and polarisation. Several statistical methods were tested, with the chosen method a compromise between recovering larger structures and the increasing variation in the correction for adjacent scans. The method chosen breaks each 8 scan into five sections, finds the median flux in each section and uses the minimum of the five medians for the bandpass correction. This greatly increases our ability to recover large scale structure, except in areas where the emission fills more than 90% of a given scan (e.g. near the Galactic Plane). To minimise the effect of the ripple from Galactic HI, the data is Hanning smoothed, resulting in a velocity resolution of 26.4 km s-1. Each individual scan was gridded using a combination of aips++, aips and miriad. The gridding process, and active scanning method used, introduce some striping into the final images. The gridding also increases the FWHP beamwidth from 
to an effective value of
.
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