a1 Kapteyn Astronomical Institute, Post bus 800, 9700 AV Groningen, The Netherlands; [email protected]
a2 University of Pittsburgh, Pittsburgh, PA 15260, USA
a3 DAEC, Observatoire de Paris-Meudon, 92195, Meudon Cedex, France
a4 USN, Observatoire de Paris-Meudon, 92195, Meudon Cedex, France
Driftscan methods are highly efficient, stable techniques for conducting extragalactic surveys in the 21 cm line of neutral hydrogen. Holding the telescope still while the beam scans the sky at the sidereal rate produces exceptionally stable spectral baselines, increased stability for RFI signals, and excellent diagnostic information about system performance. Data can be processed naturally and efficiently by grouping long sequences of spectra into an image format, thereby allowing thousands of individual spectra to be calibrated, inspected and manipulated as a single data structure with standard tools that already exist in astronomical software. The behaviour of spectral standing waves (multi-path effects) can be appraised and excised in this environment, making observations possible while the Sun is up. The method is illustrated with survey data from Arecibo and Nançay.
(Received September 04 1996)
(Accepted November 14 1996)