Overheads and Data Collection

For scans, the main constraint on the telescope scanning rate is the maximum rate at which the correlator can take data. For the HIPASS surveys, it is expected that the smallest integration time of 5 s will be used. In order not to smear the 14.4' beam by more than 35%, a scan rate is required. For all scans, apart from drift scans near the South Pole, many passes are required to build up the effective integration times listed in Table 1.

On the other hand, the point-and-shoot mode results in no beam-smearing but, as already mentioned, may involve significant telescope overheads. The existing Parkes control system has a delay of around 20 s even for short drives. Although this may soon change, such an overhead implies an unacceptable loss of observing time for integrations of around 2 min, or less. Very long observations in point-and-shoot mode require `parallactification' because of the alt-az nature of the Parkes telescope. However, it is important to note that changing parallactic angles during an observation, or between a `source' and a `reference' position, may adversely affect baseline stability because of subtle changes in the standing wave characteristics of the telescope.

In both cases (scanning and point-and-shoot), collecting data every 5 s is advantageous for the suppression of time-variable interference (HIPASS's greatest enemy). However, this creates a large volume of data ( TB for the whole survey). It is therefore important to have an automated data pipeline with as much processing performed online as is practical. Such a pipeline is being written using aips++.

© Copyright Astronomical Society of Australia 1997