ASSISTANCE

This task provides online diagnosis of the system, checking the most vital functions required for obtaining useful data. Besides checking the working of the array and correlator it also performs a number of checks on the visibilities obtained. Checks are performed every integration cycle ($\approx 10$ seconds) and whenever you press the Return key.

ASSISTANCE can display a help screen with a more detailed explanation of an error found and suggest a solution. Disregard messages which do not persist, as incorrect values are received from the monitor points occasionally.

Note that ASSISTANCE can be told to ignore some messages. However, all messages will stop being ignored whenever there is a program change.

The following things are checked by default at all times:

• the status of the Antenna Control Computers (ACCs);
• the cycling of the integration clocks for each antenna;
• the correlator blocks and delay unit;
• the temperatures of various modules in the antennas;
• the pedestal and vertex rooms' ambient temperature.

The following points are checked when on source:

*

tracking (and cable wrap limits);

*

sampler levels and statistics;

*

IF power levels: total power and synchronous demodulator output (cross-polarisation power due to noise source);

*

noise source amplitude: signal level in the AB channel of each baseline;

*

noise source stability: signal variation in the AB channel of each baseline;

*

the cross-polarization delays;

*

presence of Early-Normal errors (8ns jumps in cross-polarisation delays).

The following points are checked for strong ($\approx > 1$ Jy) sources:

*

relative difference between polarization channels,

*

baseline delays (from slope of spectrum).

The following points are checked for calibrators: (Label calibrators as calcode=c in your schedule files.)

*

equality of amplitudes on all baselines;

*

baseline delays;

*

presence of closure errors (both amplitude and phase);

*

for 1934-638: the measured average amplitude compared with the interpolated flux density.

An ASSISTANCE display with no errors is shown below. A command summary is displayed on the bottom line, together with the help, bell and log status. The screen is divided in 3 regions:

• the top region gives the source name and position, time and observing frequency(ies);
• the middle region gives a summary of errors encountered or the average amplitude if all is ok;
• the bottom region shows error messages with `+' indicating a good state, `-' an error state and `*' a disabled state for a particular antenna or interferometer. For some checks the actual monitor values are displayed if outside limits, with good values indicated by `.' only.

Figure 8.8: Example of an ASSISTANCE display showing the ATCA working normally.