Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~phil/mbeam/hipass.pdf Дата изменения: Fri Mar 14 17:44:01 2003 Дата индексирования: Tue Oct 2 01:06:14 2012 Кодировка: Поисковые слова: п п п п п п п п п п п п п

Mon. Not. R. Astron. Soc. 322, 486 ± 498 (2001)

The H i Parkes All Sky Survey: southern observations, calibration and robust imaging
D. A. M. R. M. D. M. R.
1 2

Australia Telescope National Facility, CSIRO, PO Box 76, Epping, NSW 1710, Australia Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia 3 Netherlands Foundation for Research in Astronomy, PO Box 2, 7990 AA Dwingeloo, the Netherlands 4 Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH 5 Department of Physics and Astronomy, University of Wales, Cardiff CF24 3YB 6 Department of Physics, University of Western Sydney Macarthur, Campbelltown, NSW 2560, Australia 7 Department of Physics, University of Bristol, Tyndall Avenue, Bristol BS8 1TL 8 School of Physics, University of Melbourne, VIC 3010, Australia 9 Research School of Astronomy and Astrophysics, Australian National University, Weston Creek, ACT 2611, Australia 10 School of Physics, University of Sydney, NSW 2006, Australia 11 Institute for Astrophysics, University of New Mexico, Albuquerque, NM 87131, USA 12 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 13 Departamento de Astronomia, Universidad de Guanajuato, Guanajuato, GTO 36000, Mexico 14 Anglo Australian Observatory, PO Box 296, Epping, NSW 1710, Australia 15 ц Observatoire de la Cote d'Azur, Nice, 06304 Nice Cedex 4, France

G. Barnes,1,2wІ L. Staveley-Smith,1 W. J. G. de Blok,1 T. Oosterloo,3, I. M. Stewart,4 E. Wright,1 G. D. Banks,5 R. Bhathal,6 P. J. Boyce,7 M. R. Calabretta,1 M. J. Disney,5 J. Drinkwater,8 R. D. Ekers,1 K. C. Freeman,9 B. K. Gibson,2 A. J. Green,10 F. Haynes,1 P. te Lintel Hekkert,1 P. A. Henning,11 H. Jerjen,9 S. Juraszek,10 J. Kesteven,1 V. A. Kilborn,8 P. M. Knezek,12 B. Koribalski,1 R. C. Kraan-Korteweg,13 F. Malin,14 M. Marquarding,1 R. F. Minchin,5 J. R. Mould,9 R. M. Price,11 х E. Putman,9 S. D. Ryder,14 E. M. Sadler,10 A. Schroder,15 F. Stootman,6 8 1 1 L. Webster, W. E. Wilson and T. Ye

Accepted 2000 October 2. Received 2000 September 28; in original form 2000 June 1

The acquisition of H i Parkes All Sky Survey (HIPASS) southern sky data commenced at the Australia Telescope National Facility's Parkes 64-m telescope in 1997 February, and was completed in 2000 March. HIPASS is the deepest H i survey yet of the sky south of declination 128, and is sensitive to emission out to 170 h21 MpcX The characteristic root 75 mean square noise in the survey images is 13.3 mJy. This paper describes the survey observations, which comprise 23 020 eight-degree scans of 9-min duration, and details the techniques used to calibrate and image the data. The processing algorithms are successfully designed to be statistically robust to the presence of interference signals, and are particular to imaging point (or nearly point) sources. Specifically, a major improvement in image quality is obtained by designing a median-gridding algorithm which uses the median estimator in place of the mean estimator. Key words: instrumentation: detectors ± methods: observational ± methods: statistical ± techniques: image processing ± surveys ± radio lines: galaxies.
1
E-mail: dbarnes@swin.edu.au І Present address: Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, VIC 3122, Australia.
w

A B STRA CT

I N T R O DU CTI O N All Sky Survey (HIPASS) is a survey for neutral emission from extragalactic objects in the radial 21280 , cz , 12 700 km s21 Y over the entire south of declination 128, with an effective
q 2001 RAS

The H i Parkes hydrogen (H i) velocity range southern sky


HIPASS: southern observations, calibration and robust imaging
Table 1. Parameters for the H i Parkes All Sky Survey. The H i mass limit is calculated for a galaxy profile width of 200 km s21. Sky coverage Integration time per beam Average system temperature Central beam efficiency Central beam FWHM Average beam FWHM Velocity range Channel separation Velocity resolution 3s positional accuracy 3s detection limit 3s H i mass limit

487

d , 28 450 s 19 K (35 Jy) 63 per cent 14.0 arcmin 14.3 arcmin 21280 , cz , 12 700 km s2 13.2 km s21 at z 0 18.0 km s21 3 arcmin 40 mJy beam21 6 10 б d2 M( Mpc

1

integration time of 450 s per beam. The survey parameters are shown in Table 1. HIPASS observations utilize the new 21-cm Multibeam receiver installed at the prime focus of the Parkes 64-m radio telescope,1 began in 1997 February, and were completed in 2000 March. The scientific potential of the survey is tremendous: the H i mass function for the nearby Universe will be determined better than ever before. HIPASS will provide completely new information on the distribution of galaxies, the cosmological density parameter, the space density of rare and optically invisible galaxies, and on group and supercluster dynamics. New results from the all-sky survey have included the discovery of a leading arm to the Magellanic system (Putman et al. 1998) and the identification of 10 new members of the Centaurus A group of galaxies, adding to the 28 already known within the area surveyed (Banks et al. 1999). Present work is directed towards the generation of a complete catalogue of southern sky H i sources. This paper describes the observing, calibration and imaging techniques used for the survey. In Section 2 the Multibeam hardware and the survey observing technique are described. In Section 3 the algorithms designed for the on-line processing of the data are described; these algorithms are directed at robust bandpass removal and flux calibration, residual baseline subtraction, spectral smoothing and Doppler tracking. Technical details on the implementation of some of the algorithms are given in Barnes (1998). Images of the H i sky are generated using the median-gridding technique introduced in Section 4, and are postprocessed into final survey images using the techniques given in Section 5. The primary survey product is a set of data cubes, measuring 88 б 88 on the sky, and having 1024 individual maps of H i emission at mean increments of 13.4 km s21 covering the velocity range given in Table 1.

Figure 1. Multibeam receiver configuration on the sky. The radii of the inner and outer rings of beams are 29.1 and 50.8 arcmin respectively.

Fig. 1; see also Staveley-Smith et al. 1996, Wilson et al. 1997 and Sinclair et al. 1997). Physically, the 13 feed horns are identical, having diameters of 240 mm at the focal plane, narrowing in steps towards the receiver end. The 26 receivers (two per feed horn) are sensitive to orthogonal linear polarizations of radiation in the frequency range 1.27 ± 1.47 GHz. The dewar is cooled to ,80 K surrounding the feed horns, and to less than 20 K at the low-noise amplifiers. At 1.42 GHz, the mean beamwidth (full width at half power) is 14.3 arcmin, and the peak responses of the 13 beams are projected on to the sky with typical separations of 30 arcmin, or just over two beamwidths. The Multibeam correlator has an instantaneous bandwidth of 64 MHz divided into 1024 channels for all 26 receivers. This wide bandwidth offers a velocity range of 21280 to 12 700 km s21 (when the receivers are tuned to 1394.5 MHz), and a mean channel spacing of 13.4 km s21. The correlator chip is identical to that used in the new Arecibo system (Canaris 1993). 2.2 Scanning the sky Observations for HIPASS commenced on 1997 February 27, and are taken by scanning the telescope in Dec. strips of length 88. The Parkes telescope has an alt-azimuth mount, and while in principle the receiver could be rotated during each scan to compensate for the apparent rotation of the beam pattern on the sky, this is not done, as it has a detrimental effect on spectral baseline stability. Therefore, prior to each scan, the receiver assembly is rotated by the parallactic angle at the scan mid-point, plus another 158, to obtain approximately uniform coverage of the sky (see Fig. 1 and also Staveley-Smith 1997). The footprint of the receiver on the sky is ,18 7, so that each scan maps out an 88 б 1X78 area of sky with X reasonably uniform coverage. The exact coverage depends on the rate of change of parallactic angle (h ) during the scan. This is highest near the zenith dhadd sin h cot zY where d is Dec. and z is zenith angle). Generally, observations within 208 of the zenith have been avoided, as significant changes in parallactic angle jDhj . 108 become possible. To obtain full coverage of the sky at full sensitivity, subsequent scans are displaced by 7 arcmin in RA, as shown in Fig. 2. Since the Nyquist rate is la2D 5X7 arcmin (where l is the observing wavelength and D is the telescope diameter, in the same units), this scan displacement ensures that the sky is mapped at close to the Nyquist rate by each of the 13 beams, and gives sufficient redundancy that data can be edited by purely automated, statistically robust procedures. The scan rate is 18 min21, and the correlator signal is recorded every 5 s. Measurements of the widths

2 2.1

O B S ER VING The Multibeam system

The Parkes 21-cm Multibeam system comprises a cooled, 13 beam receiver and a digital correlator. The receiver was installed at the prime focus of the Parkes 64-m radio telescope on 1997 January 21. The 13 circular feed horns of the Multibeam are positioned in a hexagonal arrangement on the focal plane, with a single central feed, and inner and outer rings of six horns each (see
1

The Parkes telescope is part of the Australia Telescope which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO.

q 2001 RAS, MNRAS 322, 486 ± 498


488

D. G. Barnes et al.
Coordinate information from the telescope is generally not synchronized with data from the correlator. As a result, coordinates in the data files refer to the position of the telescope a few seconds before the mean time-stamps applied to the data. A linear interpolation of telescope positions has been applied throughout the survey, typically resulting in corrections of a few arcmin in the Dec. direction. A comparison of the positions of more than 200 sources selected from the PKSCAT90 2700-MHz continuum source catalogue (Wright & Otrupcek 1990) with positions measured from the final survey images exhibits a standard deviation of 0.8 arcmin in RA, and slightly less than this in Dec. There appear to be no significant systematic position offsets. 3 3.1 S PECT RAL PR OC ESSING Requirements

Figure 2. HIPASS scan pattern on the sky for an 88 б 88 field. Five sets of 15 scans each are acquired, with adjacent scans within each set separated by 35 arcmin, and each set displaced from the previous set by 7 arcmin in RA. The solid lines with arrows depict one set of 15 scans; the arrows indicate the direction of each scan. The final scan density, for a single beam of the Multibeam receiver, is shown by the dot-dashed lines near the left-hand edge of the field.

of the beams give full widths at half power of 14.0 arcmin for the central beam, 14.1 arcmin for beams in the inner ring, and 14.5 arcmin for beams in the outer ring. The mean observing beamwidth is therefore 14.3 arcmin, but is extended to 14.7 arcmin in the Dec. direction because data are acquired while the telescope is scanning. The total integration time of the survey is 7 б 103 s deg22 Y or 450 s beam21. Although the Dec. scans are separated by 7 arcmin, the observations are made in five separate sets, with the scans in each set spaced by 35 arcmin. Care was taken to make the observations of a given part of the sky at well-separated times so that interference, if present, did not corrupt all data for that direction. This seems to have worked well, in general, with little sign of any solar or narrowband interference in the final images, except in a few unfortunate cases where more than ,60 per cent of the data have been acquired during the daytime, over a period of just a few days. In terms of narrowband interference, only the 11th harmonic of the 128-MHz sampler clock at 1408 MHz cz . 2640 km s21 consistently appears at moderate levels in the final cubes. This harmonic was removed in early 1999 by enclosing the correlator in a Faraday cage. 2.3 Data acquisition

Spectra generated by the Multibeam correlator contain the usual structure seen in spectra from other H i instruments: a bandpass spectrum which is the sum of the sky, ground and receiver temperatures multiplied by the product of the filters in the receiver chain; superimposed on this is a noise whose amplitude depends inversely on the square root of integration time and channel bandwidth; occasional baseline ripple whose amplitude is a complicated function of the telescope geometry and the location and strength of 20-cm continuum sources in the sky; and other internal effects such as ringing due to strong, sharply peaked sources. Any H i line emission ± whose amplitude is typically a few thousandths of that of the system bandpass spectrum ± lies on top of the structure described above. For HIPASS, algorithms were designed: to remove the system bandpass spectrum, to calibrate the residual spectrum, to remove any remaining baseline (DC) offset, to suppress strong ringing effects, and to shift the resultant spectrum from the topocentric observing frame to a fixed frame of reference. Since radio-frequency interference (RFI) had the potential to seriously contaminate a substantial fraction of the survey data, the algorithms were developed to be statistically robust to a moderate fraction of bad data. Furthermore, as the Parkes beam is significantly larger on the sky than the typical extragalactic H i source, the algorithms were designed specifically to identify compact emission regions. 3.2 Robust bandpass estimation

The Multibeam correlator is programmed to write data directly into a disk file. The correlator cycle time is 5 s, and spectra are written for each beam and polarization at the end of each cycle. There are two polarizations per beam, so a total of 26 spectra are written each cycle, each with 1024 channels. Each channel value is stored as a single-precision floating point number, occupying four bytes of storage. Thus the HIPASS raw data rate is 104 kb cycle21, or 1.2 Mb min21. The correlator file is closed and reopened each cycle to ensure that the very latest data can be read for near real-time processing. All HIPASS data are archived in their unprocessed state.

The dominant component in a raw correlator spectrum is the system bandpass spectrum, which varies with time because of, e.g., slow fluctuations in the physical temperature of the receivers, and external influences, such as atmospheric conditions which of course change with pointing direction. Traditionally, bandpass removal is accomplished by observing in a signal/reference mode. In this mode, an extended integration (e.g., 200 s) is acquired while the telescope tracks the target position, yielding a signal spectrum. A second integration, usually of the same duration, is then acquired while the telescope is pointed toward a nearby position (hopefully) free of line or continuum emission, yielding a reference spectrum. The reference spectrum is assumed to be a good estimate of the bandpass spectrum, and is subsequently removed from the signal spectrum by division. For longer onsource integrations times, this process is repeated many times, and the quotient spectra averaged. It is normal to spend as much time off-source as on-source for this observing mode, since division by the (noisy) reference spectrum increases the noise in the resultant
q 2001 RAS, MNRAS 322, 486 ± 498


HIPASS: southern observations, calibration and robust imaging
quotient spectrum accordingly. It is also normal to select a reference position such that the sidereal telescope track with respect to the ground is similar to the track for the signal observation, in order to re-create the ground spill-over contribution to the bandpass spectrum as well as possible. For HIPASS, where the telescope is actively scanned across the sky at 18 min21, the signal/reference method of bandpass removal needs to be modified. The basic method for removing the bandpass from a selected (target) HIPASS spectrum is first to estimate the shape of the bandpass spectrum at the time the spectrum was acquired, and then to divide the target spectrum by the bandpass estimate. The bandpass estimate is determined from a set of earlier and/or later spectra observed by the same feed of the Multibeam receiver. These reference spectra are selected individually for the target spectrum, and must satisfy a number of criteria to be suitable. Suitable reference spectra should be independent measures of the H i sky to that of the target spectrum, but should be acquired nearby in time so that temporal variations of the bandpass are kept to a minimum. Since the bandpass estimate is made from a number of spectra, the increase in spectral noise caused by bandpass removal can be made negligible. Actual estimation of the bandpass is done by taking a channelby-channel median of the reference spectra, i.e., the bandpass estimate is the median reference spectrum, not the mean reference spectrum. Despite the standard error on the median statistic being 25.3 per cent greater than that on the mean statistic (for normal distributions; Kendall & Stuart 1963; Freund 1971), the median statistic is robust to a high fraction of outlying data points, and is independent of the magnitude of deviation of the outlying points. The mean statistic, on the other hand, has the characteristic that no data point can be arbitrarily distant from the general trend without affecting the statistic. Formally, the HIPASS bandpass estimate is determined independently for all 26 feeds (13 beams, two polarizations). Since the 13 beams track out nearly parallel paths on the sky, it is sufficient to generate a single list of cycle numbers, or equivalently integration time-stamps, which refer to integrations which contain spectra that are valid for estimating the bandpass spectrum at the instant of the target spectrum. This set of cycle numbers, wT, is calculated once for the central beam, and assumed to be good for all other beams. Only in the case of unusual observing circumstances would this assumption be invalid, e.g., spiral scanning, or rapid rotation of the receiver while acquiring data. The suitability of a particular cycle for inclusion in the set wT is determined as follows. (i) Only spectra from the same Dec. scan as the target spectrum are used as reference spectra. Between scans, the receiver is rotated slightly, thus changing the observing geometry, and hence the sidelobe radiation pattern. (ii) The displacement on the sky from the target integration position must be at least 15 arcmin, in order to measure an independent part of the sky to that viewed by the target integration. Even though the beam half-width is only 7 ± 8 arcmin, depending on the position of the beam on the focal plane, this minimum distance is set conservatively to prevent a particularly strong point source, or instead a nearby and extended source, from being partially removed during bandpass removal. (iii) The displacement in time from the integration time-stamp of the target cycle must be less than 120 s. This is principally a restriction to enable near real-time processing of the data, and provide immediate feedback to the user on the data quality.
q 2001 RAS, MNRAS 322, 486 ± 498

489

However, this criterion also ensures that the spectra used to generate the bandpass estimate are not drawn from a long period of time over which the bandpass may have varied significantly. Once the set of cycle numbers (wT) is determined, the bandpass estimate is generated channel by channel, for each polarization of each beam of the receiver. For beam b 1ј13 and polarization p (1 or 2) of the target cycle cT, the bandpass estimate as a function of channel number n 1ј1024 is: BcT
YbYp

n median{ScYbYp n : c [ w T }Y

1

where Sc, b, p is the raw correlator spectrum for cycle c, beam b and polarization p. There are two points worth special mention regarding equation (1): first, in this formalism the bandpass estimate for the nth channel is independent of the bandpass estimate for any other channel and, secondly, an immediate consequence of the choice of the median statistic is that spatially unresolved spectral line sources will be absent from all bandpass estimates, provided that the set wT is sufficiently large (compared to the source density on the sky), independent of (ii) above. In particular cases, though, the median estimate may be slightly biased by the presence of a strong source in a subset of wT ; see Fig. 3. 3.3 Bandpass removal and calibration Calibration of the spectra is done concurrently with bandpass removal, since system temperatures are recorded for every

Figure 3. An example of negative bandpass sidelobes generated by bandpass correction north and south of a strong H i line source: the source at the centre is ESO 214 2 G017Y and the fainter source to the north east is ESO 264 2 G035X The contours are at 250 and 220 mJy (dashed), and 20, 50 and 200 mJy (solid).


490

D. G. Barnes et al.
3.4.1 Spectral smoothing A characteristic of digital correlators is that their spectra often suffer from what is commonly known as the Gibbs ripple. The spectral response function of a digital correlator is the Fourier transform of the time-lag weighting function used in the correlator. In the Multibeam correlator, the lag weighting is nearly constant as a function of lag so that the spectral response function is, to first order, a sinc function. When narrow lines are present in the data, this spectral response causes strong ringing in the spectra that decays roughly as n21, where n is the number of channels away from the narrow line. Given that in many positions on the sky the Galactic H i emission is strong and contains (in terms of the spectral resolution of the Multibeam receiver) narrow emission lines, this ringing can seriously affect the data over a large velocity range (see Fig. 4 for an example). A standard procedure to suppress this ringing is to Hanning-smooth the data. Such a smoothing very effectively suppresses the sidelobe level (the first spectral sidelobe decreases from 26.7 to 215.9 dB) and more importantly, the sidelobe level decays as n23. Because of these effects, the region of the spectrum affected by the ringing is much smaller, and a larger fraction of the spectrum can be used.
Unfiltered

spectrum written to the correlator files. The system temperature, T cT YbY p , of the raw target spectrum, in Jy, indicates the total power measured by the correlator, calibrated against a calibration diode which is constantly switched in and out of the signal path. The diode itself is occasionally calibrated against an extragalactic radio source of known amplitude, e.g., 19342638 (14.9 Jy at 1420 MHz) or Hydra A (40.6 Jy at 1395 MHz). T cT YbYp is read directly from the correlator file. The system temperature of the bandpass estimate EcT YbYp must be estimated. This is accomplished by treating the system temperature in exactly the same way as a single channel, and taking the median of the system temperatures for the cycles in wT, i.e. E
cT YbYp

median{T

cYbYp

: c [ w T }X

2

Then the bandpass-removed, calibrated target spectrum ScT YbYp is obtained by comparing the raw correlator spectrum to the bandpass estimate, scaled by the ratio of the system temperatures: ScT YbYp EcT YbYp ScT YbYp T cT YbYp б б 21 BcT YbYp T cT YbYp ScT YbYp 3 2 T cT YbYp X EcT YbYp б BcT YbYp In practice, some H i sources are sufficiently bright that bandpass estimates for target cycles just prior to and just after the cycle when the source makes its closest approach to a beam axis are elevated by the source itself. This results in bandpass sidelobes which are depressions in the spectra north and south of strong H i sources. The depressions are generated during the division of the target cycle by a bandpass estimate containing positive H i flux. Note, though, that depressions do not occur in the sources themselves, unless they are spatially extended (compared to the Parkes beam and the restrictions on wT) or brighter than ,1 Jy: this is prevented for all but the strongest sources by the second criterion in generating wT. In Fig. 3 an example of the sidelobes associated with a strong source, as they appear in a gridded sky image (see Section 4), is given. HIPASS bandpass sidelobes are generated during the bandpass removal stage of the HIPASS spectral processing. Thus the H i emission from extended H i sources, including the Galaxy, the high-velocity clouds (HVCs), the Magellanic Clouds and Stream, and other nearby galaxies such as Circinus, is corrupted in the standard HIPASS calibrated data, and therefore also in the HIPASS sky images (see Section 4). However, since all unprocessed HIPASS spectra are archived, the spectra can be reprocessed using different techniques, e.g., algorithms which preserve spatially extended emission. Such an approach has already been applied to the data in order to successfully image HVCs and the Magellanic Stream (Putman et al. 1998). 3.4 Residual baseline removal

30

S (arbitrary units)

20

10

0

500

0

500 Vhel (km s )
1

1000

1500

Tukey 25%

30

S (arbitrary units)

20

10

Whilst bandpass removal and calibration ordinarily yields spectra that are flat, excluding receiver noise and spectral line sources, there are cases where the baseline is not flat. The two predominant causes of non-flat baselines for the HIPASS project are ringing associated with strong Galactic H i emission, and continuum emission extending over the wavelength range 20 ± 22 cm which can produce standing wave patterns in the telescope structure, thereby causing residual ripple after bandpass removal. The first of these ± spectral ringing ± is relatively straightforward to correct, by smoothing every spectrum with a well-chosen kernel.

0

500

0 V

500
hel

1000
1

1500

(km s )

Figure 4. An unfiltered (top) and Tukey 25 per cent smoothed (bottom) HIPASS spectrum containing strong Galactic emission near 0 km s21 and extragalactic emission at 1000 km s21. The ringing in the unfiltered spectrum is essentially removed in the smoothed spectrum. q 2001 RAS, MNRAS 322, 486 ± 498


HIPASS: southern observations, calibration and robust imaging
However, the spectral resolution after Hanning smoothing is degraded by 67 per cent, and some detail in the signal is lost. In order to suppress the Gibbs ringing in Multibeam spectra, while retaining most of the information in the signal, a Tukey filter was selected for smoothing purposes. This filter is applied in the lag domain, and can be written as V for jxj , f б xmax b1 ` T f x 1 1 b 1 cos p jxj 2 fxmax for fxmax # jxj # xmax Y X xmax 2 fxmax 22 4 where xmax is the maximum lag, and 1 2 f is the fraction of the lag spectrum that is tapered. The Tukey filter is very similar to a Hanning filter (in fact, it is identical for f 0Y and the sidelobe level of a filtered spectrum also decays as n23. Hence the region of the spectrum affected by the ringing is limited. The main difference with a Hanning filter is that the tapering is performed only on part of the lag spectrum as controlled by the parameter f. This means that the sidelobe level is higher, but also that the spectral resolution is degraded much less. For HIPASS spectra, 1 2 f is set to 0.25, and the resolution is degraded by only 15 per cent compared to the full resolution (and the first sidelobe is at 26.9 dB). Fig. 4 also shows an example smoothed spectrum, illustrating that the ringing is much reduced and only a very small part of the spectral is not usable because of the ringing. Fig. 5 shows that the spectral resolution is much less affected by this Tukey filter than by the Hanning filter, and that more detail in the signal is retained. 3.5 Doppler tracking

491

yields a spectrum in the selected frame. The velocity frame conversion is applied after bandpass removal, since the bulk of the bandpass spectrum is internally generated in the observing frame. 4 I M AG I N G

4.1 Requirements The result of the spectral processing of a single HIPASS scan is a set of calibrated H i spectra for 1300 unique positions on the sky. The scanning approach delivers typically 1500 spectra per square degree of sky, ,55 of which are acquired by each Multibeam feed. For the most part, these spectra are free of continuum ripple, of baseline curvature, and residual bandpass effects. Indeed, they are mostly free of H i line emission too, given the sparsity of galaxies on the sky. Furthermore, since the integration time is only 5 s, the root mean square (rms) noise level in the individual spectra is typically 72 mJy (see Section 4.3.2), and so only the very brightest H i sources are ever seen in single spectra. Some operation is therefore required to compile the individual spectra into high signal-to-noise images of the H i sky. As well as dramatically improving the visibility of low-flux sources, such an operation produces a more compact and natural representation of the H i sky. This operation is the primary task of the Multibeam imaging software. Projection centres have been defined for 388 images covering the southern sky. The projection used is the orthographic projection (see Kellaway 1946) with reference (tangent) point at the centre of each image. Spatially the images measure 88 б 88Y where the width in RA is measured at the centre of the image. For the HIPASS pixel size of 4 H б 4 H (see Section 4.2.2), the images have dimensions of 170 б 160X Spectrally, each image comprises 1024 planes (channels), extending in cz from 21280 to 12 700 km s21. Each image is generated from 75 HIPASS scans, and has typically a 10 per cent sky overlap (by area) with adjacent images. 4.2 Robust gridding Conversion of the individual spectra into position-positionvelocity cubes requires placing the spectra on a regular grid, i.e., gridding. For each pixel in the map, the gridding process must: (1) determine which spectra will contribute to the pixel; (2) reject those spectra which appear to contain corrupt data; (3) ascribe a weight to each remaining spectrum, and (4) calculate the value of the pixel based on the input data and weights. The gridding algorithm used for the survey is optimized for point sources: more than ,90 per cent of HIPASS sources are expected to be small in angular size compared to the Multibeam resolution. 4.2.1 Algorithm The objective of gridding is to reconstruct the flux at a certain position (pixel) on the sky, given an arbitrary number of single beam spectra measured at irregularly distributed positions near the target pixel. Fig. 6 sh