Overview and terminology

The DAS is highly versatile; the electronics can be re-configured using software (ICL) commands to produce spectra of different resolutions/bandwidths. In addition, the system can be split up to sample different parts of the IF passband(s) at various resolutions. However, there are rules limiting the allowed configurations. The simplest configurations are listed in section 6. These are the standard options selected by chosing a bandwidth when the DAS is started up. The more unusual configurations are given in section 7; these are set up in advance in a text file, which includes the configuration number as well as other setup parameters. Additional configurations can be added to the library on request. Contact a support scientist in advance if you want to use a operating mode not listed in section 6 or section 7.

Depending on the configuration loaded the DAS produces one or more separate subsystems, each of which has continuous frequency coverage. A normal dual-polarisation observation has two subsystems. However, it is equally possible to configure for two subsystems from a single- polarisation receiver, by sampling two sections of the passband.

Each subsystem is composed of between 1 and 8 subbands. A subband is one section of the correlator, and is 160 MHz wide. However, because of the necessary overlap between subbands to make up a subsystem, typically only 125 MHz of this is used. Thus while the subbands are contiguous in channel space, they overlap in frequency (or velocity) space. This situation is illustrated in Figures 1 and 2. To construct a single contiguous spectrum from a subsystem, the edges of the subbands must be knitted together to form a single spectral array. See section 10 for more details on this.