The technique

The designs of the Arecibo and Nançay telescopes make them particularly well suited to taking data in a driftscan mode. Both have large collecting areas and are therefore sensitive survey instruments. They obtain the large collecting area by having much of their structure fixed to the ground. When they are used to track specific celestial coordinates, the on-axis gain changes and the far out sidelobes move in unpredictable ways, causing spill-over on the ground and RFI to be time variable. These instabilities increase the level of systematic uncertainties, thus increasing the difficulty of detecting weak signals. Driftscan observations avoid these problems since all components of the structures are fixed relative to the ground, thus achieving the full sensitivity of the large reflecting areas. Similar arguments apply to more conventional radio telescopes, when variation in the spillover causes fluctuations in the spectral baselines and when RFI entering the receiving system is modulated by gain variations in the far sidelobes as the antenna tracks celestial sources. This report is based on experience obtained in extragalactic HI surveys using the Arecibo and Nançay Telescopes.

Sorar (1994) and Briggs conducted the Arecibo HI Strip Survey in the driftscan mode in order to determine the HI-mass function for nearby extragalactic objects by surveying long strips at constant declination. The observations covered approximately 6000 independent sightlines to a depth of 7500 km s, but since the strips were retraced on many days in order to increase the integration time on each sightline, nearly a million individual spectra had to be calibrated, regrouped and averaged. The details of the observational technique were developed by Sorar (1994), and the results and followup to the survey are summarized by Zwaan et al (1996, these proceedings).

 
Figure 1: Nançay Raw Data Image. Passband calibrated spectra have been loaded into the image in time sequence increasing from right to left. There are two slightly overlapping, 6.4 MHz wide spectral bands, as marked at the right border. A trace of the continuum as a function of right ascension is drawn under the image.

The method is also in use for surveys at Nançay. One project, being conducted by Kraan-Korteweg, van Driel, Binggeli, and Briggs, will test the completeness to a depth of 2300 km s of deep optical catalogs of dwarfs and low surface brightness galaxies in the CnV I cloud (Binggeli et al 1990). A sample of the raw data from the HI survey are shown in Figure 1. The spectral passband calibration for this data has been performed by computing the average spectrum of the entire 3.5 hour dataset, and each spectrum was divided by the average spectrum as it was loaded in time sequence into the columns of the image. Passage of a telescope beam over a background continuum source is registered in the image as a dark band. The residual Galactic HI emission causes the splotchy horizontal band across the image around the HI rest frequency.

Once the data is loaded in image format, images become the units in which the data is stored, manipulated and displayed. For example, continuum subtraction, averaging of data from different days and smoothing can be accomplished using procedures in familiar astronomical image processing packages. Figure 2 shows the processed spectra for a total of 2500 sightlines in three adjacent declination strips. The figure results from approximately 82,000 individual spectra. There are 11 detected extragalactic signals resulting from 9 separate galaxies, plus a number of interesting features associated with Galactic HI.

A second project now in progress at Nançay will observe % of the sky to a depth of 4500 km s with noise level mJy (5) for velocity resolution 20 km s. The survey is well matched to detecting nearby examples of gas-rich systems such as HI 1225+01 (Giovanelli & Haynes 1989, Chengalur, Giovanelli & Haynes 1995) and the circum-galactic ring in Leo (Schneider 1989), as well as detecting a sample of several hundred normal galaxies.

 
Figure: Processed Nançay images for three declinations. Spectra are loaded in horizontal lines in this image, with right ascension increasing upwards. Overlap of the spectral sub-bands has been removed, and labels on the horizontal axis indicate velocity in km s. Eleven detected signals are marked.

© Copyright Astronomical Society of Australia 1997