Tied-Array observing

This note describes the procedures required to operate the Compact Array in tied array mode in its provisional configuration. In this configuration the tied array adder is capable of operation at only one frequency and at bandwidths of 16, 8 and 4 MHz only.

The adder is controlled by the process CATIE which is available only on ATRIA. To activate it, start a DECTERM on MANUEL ( i.e. login to ATRIA, user OBSERVER ) and type CATIE.

On startup, CATIE obtains the default setup from the file COR$MENU:TDA.FILE.

An example of a TDA.FILE is:

; Dataset address
  31 
; Active antennas - frequency 1
  1 2 3 4 5 6
; Active antennas - frequency 2
  1
; Functions - for the four tied array ouputs
;   Freq. 1               Freq. 2
;    1           2          1          2
   XX-YY       XX+YY       XX         YY       
; Reference levels
    24         24          8          8
; Old correlator delay setting
  1

Phasing up the tied array.

CACAL has the option of continuously calibrating the phase to keep the tied array phased up. This mode can only be used if you are observing a calibrator: mark the source with CALCODE=C in the schedule. You need to run CACAL in interactive mode to be presented with this option.

Producing circular polarizations.

Another feature of CACAL is the option to put in a 90 degree phase shift between the X and Y channel, effectively converting the linears to circulars. Run CACAL in interactive mode to activate this mode.

Pulsar binning mode.

Pulsar binning mode is enabled if:

The mode variable is set to PSR in the schedule file.
The correlator is configured with a suitable configuration file.
A special file, containing a timing prediction for the pulsar for the epoch of observations, is present in the area COR$PULSAR on ATRIA.

Setting the mode variable to PSR causes the software to look for the ephemeris file for that source - thus, for calibration sources, set the mode to be STANDARD. A default period of 0.2 seconds will be used for the calibration sources

Currently the following configurations are available.

PSR8_128_2
Similar to FULL_128_2 but with 8 bins in each frequency.
- 2 Frequencies, both 128MHz bandwidth,
- 4 polarisations per frequency,
- 32 channels and 8 bins.
PSR8_4_1024-128
Similar to FULL_4_1024-128 but with 8 bins in the second frequency.
2 Frequencies.
- 1st frequency has:
  - 4MHz bandwidth
  - 2 polarisations
  - 1024 channels for each polarisation and no binning.
- The 2nd frequency has:
  - 128MHz bandwidth
  - 4 polarisations
  - 32 channels and 8 bins.
PSR16_128_1
Similar to FULL_128_1 but with 16 bins.
- 1 Frequency, 128MHz bandwidth,
- 4 polarisation,
- 32 channels and 16 bins.

The COR$PULSAR area on ATRIA must contain a file SOURCE.POLYCO, where SOURCE is the source name given in the schedule file. If this file is not present or is invalid an error message will appear on the CACOR screen.

Using VIS in pulsar binning mode

Currently, the CA visibility viewing program, VIS, can display only one bin at a time. This is because the data for only one bin is sent from CACOR to VIS. The choice of which bin is transferred is made using the COR_DISP process. In COR_DISP, return to the top level of the menu and choose:

2 - Time Variables
1 - Setup Variables
4 - Default setup
then, in the ensuing question and answer session, accept the defaults and the bin number of choice.

Note that it takes a few cycles for the new bin to appear on VIS (but the time of the change is correctly represented).

Notes about the pulsar binning mode

With our standard pulsar binning mode, there is a lower limit of 2048 us on the bin time. There is another non-standard binning mode which allows bin times down to around 250 us, but then with a maximum of 8 bins. In an example case, with a period of, say, 33.4 ms, it is limited to 16 bins. This requires a new configuration. If you believe you will need new configurations, you should get in touch with Warwick Wilson ( warwick.wilson@csiro.au )

Original: Warwick Wilson (5-MAY-1995)
Modified: Mark Wieringa (2-AUG-1996)