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D. G. Barnes
School of Physics, The University of Melbourne,
Parkville, VIC 3052, Australia; Email: dbarnes@physics.unimelb.edu.au
L. Staveley-Smith, T. Ye and T. Oosterloo
Australia Telescope National Facility,
Marsfield, NSW 2122, Australia
In Barnes (1998) we give an overview of the Parkes Multibeam Software, and the motivation for its development, that is, the desire to have robust, realtime processing software for the neutral hydrogen HI Parkes All Sky Survey (HIPASS). In this paper, we provide more details on the bandpass correction algorithm, and its implementation, based on the AIPS++ toolkit.
Traditionally, the bandpass that is present in single-dish 21 cm spectra is removed by observing in a ``signal-reference'' mode. In this mode, an extended integration (typically 3 min) is acquired while the telescope is pointed at the target position, or signal. A second extended integration is then acquired while the telescope is pointed towards a nearby position free of line or continuum sources--the reference. The bandpass is removed by dividing the signal spectrum by the reference spectrum. If longer on-source integration times are required, this process can be repeated many times, and the quotient spectra averaged.
For the HIPASS Project, where the telescope is actively driven across
the sky at a rate of , the traditional
bandpass removal technique is not suitable. Instead, a statistical
estimate of the bandpass at the position and time that a particular
spectrum--the target spectrum--was acquired is made from a set
of earlier and later spectra observed by the same beam--these are the
reference spectra. The bandpass is then removed from the target
spectrum by dividing it by the statistical bandpass estimate.
This process can be formalised as follows: a particular integration
will be denoted as Ib,p,c, where the subscripts b and p
represent a beam-polarisation combination,
and c is the cycle number of the integration. Ib, p, c
is a 1024-channel flux density spectrum.
For a given beam and polarization pair, the target spectrum is , and the reference spectra,
, are a set of
spectra which can be used to estimate the bandpass for the target
spectrum. The suitability of a particular spectrum depends on a
number of factors. Most importantly, a reference spectrum must be
measured on a part of the sky which is independent (with respect to
the telescope beam) of the part of the sky measured by the target
spectrum. However, the bandpass can be expected to vary with time, so
spectra taken nearby in time to the target spectrum are more useful
reference spectra than those taken at greater time separations.
Furthermore, any movements of the receiver with respect to the
telescope dish, including rotations and axial movements, were found to
introduce large phase changes in the bandpass. This led to extreme
ripple in the baseline of spectra corrected with reference spectra
having varying parallactic angles or axial offsets. Thus, valid
reference spectra must be from the same right ascension (or Galactic
latitude) scan as the target spectrum.
Having selected the reference spectra , the estimate of the
bandpass (
) is given by
![]() |
(1) |
With determined, calibration of the
spectral data can be done concurrently with bandpass removal. The
system temperature for the target spectrum,
, is
provided in the correlator file, whilst that for the bandpass
estimate,
, is obtained in a fashion analogous to
the calculation of a single channel in the bandpass estimate. Thus
the bandpass-corrected, calibrated target spectrum is
![]() |
(2) |
The BANDPASS CALCULATOR, which is our AIPS++ implementation of
Equation 2, stores incoming spectra in a four
dimensional buffer, of the form
Matrix<Matrix<Float>>.
The row and column in the parent matrix select a beam-polarization pair, and
the row and column in the sub-matrices select cycle and channel
numbers. The spectra are arranged this way to optimise access speeds,
yet still provide row() and column() methods to
efficiently extract spectra or time series of individual channels.
Both these operations are required for bandpass correction. Since
shuffling the data through the matrices would be inefficient, an
indexing system is utilised to keep track of the newest spectrum added
to the buffer. Spectra are discarded (overwritten) when they are no
longer required.
Figure 1 shows the uncalibrated and
calibrated flux densities for a selected channel of a HIPASS scan
through a galaxy with the central beam of the Multibeam.
![]() |
![]() |
The feature observed near cycle 69 in the raw bandpass (Figure 1) is the continuum source PKS 1307-403 which generates an increased flux density in all line channels. To first order, continuum sources are removed by the bandpass removal algorithm, since the entire spectrum is rescaled according to Equation 2, and then the median value of all channels is subtracted from each channel as a simple means of baseline removal. For strong continuum sources though, there are still serious baseline ripple and curvature effects which remain in the calibrated spectra.
The robust bandpass removal algorithm implemented for the HIPASS
Project successfully produces calibrated HI emission line
spectra for most of the survey data. The technique is robust to the
presence of contamination by radio frequency interference and strong
on-axis and off-axis continuum sources in the reference spectra.
Unfortunately though, sources which emit in a particular channel over
more than of declination, such as the Galaxy or
associated high-velocity clouds, are not well calibrated. This is
because a trade-off between a larger set of reference spectra
(yielding a better estimate of the emission-free bandpass in such
cases) and a smaller set of reference spectra (yielding robustness to
time-variability of the bandpass) was necessary. Conversely, the
calibrated spectra of unresolved sources, which will form the bulk of
the sources detected by the HIPASS, are invariably flat and of
excellent quality.
We were delighted with the frequent and high-quality help provided by the many AIPS++ programmers during the planning and programming of the Parkes Multibeam Software. We are grateful to the ATNF and collaborating institutions for supporting the development of the Parkes Multibeam Receiver. We acknowledge the large investment of time and personnel towards the design and construction of the receiver by the CSIRO Division of Telecommunications and Industrial Physics. DGB acknowledges the support of an APA, and is grateful for the financial assistance to attend ADASS '97 provided by the ADASS POC and a Melbourne Abroad Scholarship.
Barnes, D. G. 1998, this volume
Next: RVSAO 2.0 - A Radial Velocity Package for IRAF
Up: Applications
Previous: The Co-Addition Technique Applied to Images of Galaxy Cores
Table of Contents -- Index -- PS reprint -- PDF reprint