GASS: The Parkes Galactic All-Sky Survey. Update: improved correction
  for instrumental effects and new data release
  The Galactic All-Sky Survey is a survey of Galactic atomic hydrogen emission
in the southern sky observed with the Parkes 64-m Radio Telescope. The first
data release (GASS I) concerned survey goals and observing techniques, the
second release (GASS II) focused on stray radiation and instrumental
corrections. We seek to remove the remaining instrumental effects and present a
third data release. We use the HEALPix tessellation concept to grid the data on
the sphere. Individual telescope records are compared with averages on the
nearest grid position for significant deviations. All averages are also
decomposed into Gaussian components with the aim of segregating unacceptable
solutions. Improved priors are used for an iterative baseline fitting and
cleaning. In the last step we generate 3-D FITS data cubes and examine them for
remaining problems. We have removed weak, but systematic baseline offsets with
an improved baseline fitting algorithm. We have unraveled correlator failures
that cause time dependent oscillations; errors cause stripes in the scanning
direction. The remaining problems from radio frequency interference (RFI) are
spotted. Classifying the severeness of instrumental errors for each individual
telescope record (dump) allows us to exclude bad data from averages. We derive
parameters that allow us to discard dumps without compromising the noise of the
resulting data products too much. All steps are reiterated several times: in
each case, we check the Gaussian parameters for remaining problems and inspect
3-D FITS data cubes visually. We find that in total ~1.5% of the telescope
dumps need to be discarded in addition to ~0.5% of the spectral channels that
were excluded in GASS II.The new data release facilitates data products with
improved quality. A new web interface, compatible with the previous version, is
available for download of GASS III FITS cubes and spectra.
