Bandwidth and Correlator Configuration

As described in the Introduction, the ATCA receiver chains and correlator offer a large number of observing modes. There are two frequency chains (Frequency 1 and Frequency 2). The bandwidths available on Frequency 1 are: 4, 8, 16, 32, 64 and 128MHz. Frequency 2 offers 16MHz, 64MHz and 128MHz only. Usually 128MHz would be used for continuum and the narrower bandwidths used for spectral line observations. The table below lists ATCA correlator details. All configuration options currently supported are listed.

Some standard examples of correlator options using 2-bit sampling are described below:

FULL_128_2 Two frequencies, four polarisation products (XX, YY, XY, YX) per antenna pair, with each product providing 32 channels across a bandwidth of 128MHz. This option is appropriate for continuum observations.

FULL_8_512 For one frequency there are 512 channels across a bandwidth of 8MHz on both orthogonal polarisations. This option is suitable for spectral line observations of external galaxies.

FULL_8_512-128 The same output as FULL_8_512 for frequency 1 and a similar output from FULL_128_2 but for only frequency 2. This option is `the best of both worlds' in that you can have spectral line and continuum observations at the same time.

With each configuration it is possible to use all correlator channels but record only half of them. This allows you, for example, to save diskspace when using 2048 channels with the 8MHz bandwidth.

Chromatic aberration (bandwidth smearing) is not usually a consideration with the ATCA, except perhaps for 21cm, wide field observations. For more information about chromatic aberration, refer to the next section or Killeen 1993.

Table 3.4: AT Compact Array Correlator Specifications.

BANDWIDTH (MHz)	128	64	32	16	8	4
Max. Channels/baseline	128[a]	256	512	1024	2048	4096
Resolution (kHz)	1000	250	62.5	15.6	3.9	1.0
BAND	VELOCITY RANGE (KM/S)
20 cm	25600	12800	6400	3200	1600	800
13 cm	16600	8300	4150	2075	1037	519
6 cm	7680	3840	1920	960	480	240
3 cm	3840	1920	960	480	240	120
BAND	MINIMUM VELOCITY INCREMENT (KM/S)
20 cm	200	50	12.5	3.1	0.78	0.20
13 cm	130	32	8.1	2.0	0.51	0.13
6 cm	60	15	3.8	0.94	0.23	0.059
3 cm	30	7.5	1.9	0.47	0.12	0.029

¹ Actually 256, but only 128 independent channels because of windowing.

Notes:
(a) Actually 256, but only 128 independent channels because of windowing.