Set the Delays, Gains and Phases: CACAL

Cacal will calculate the delay offsets for each baseline and then apply those corrections. To enable successful calculation of these (valid) corrections both the samplers and the delays have to be stabilised, ie. sampler should be to close to 17.3% and delays have to be `flat'. Note: It is necessary to run cacal at every observing frequency you use (even if it is only different by 1MHz). After you have run cacal you will need 3-5minutes of data on 1934$-$638 for calibration during your data processing.

For some narrow bandwidth correlator configurations (e.g. FULL_8_2048 and FULL_4_1024 you may find that the signal level is too low to run cacal on. To calibrate these correlator configurations use a correlator config with the same bandwidth, but fewer channels (e.g. FULL_8_32-aa or FULL_4_32-128). A list of available correlator configurations can be found in the top right corner of the CACOR gui.

Be Patient:^e wait a full minute from the last attenuator change after moving onto source. Do NOT run CACAL if the delays, amplitudes or phases are unstable. Likely causes of instability include:
$\bullet\,\,$antenna not yet on source,
$\bullet\,\,$samplers not yet stabilised,
$\bullet\,\,$RFI,
$\bullet\,\,$fundamental array problem. (If this seems to be the case, consult your Duty Astronomer.)

	CHECK delays and amplitudes are stable.
	VIS> tapd		To display amplitudes, phases and delays.
			Re-scale plots if needed.
			VIS> sca a [y1] [y2]
			VIS> sca d [y1] [y2]
			Where y1 and y2 bound the desired plot range.

	CHECK samplers have stabilised close to $17.3\%$.		See CAMON stats page.
			If after 1 minute on new source samplers haven't adjusted, see how to manually correct sampler attenuation and follow the directions therein.

	In an XBONES window:
	XBONES:$\sim$$ cacal		Select the automatic mode.

If the suggested corrections displayed on the XBONES screen have values significantly different than the others, you may consider re-doing the cacal.

After waiting for those corrections to take effect, the gain and phase offsets will be calculated and applied. A typical VIS display obtained while calibrating the array is shown, showing normal variation of the phases as CACAL searches for an optimum configuration.

Figure 5.4: A typical VIS screen display during setup with CACAL.

	CHECK:
	$\bullet\,\,$phases close to zero,
	$\bullet\,\,$for continuum observations,		ie. Bandpass $\delta\nu$ = 128MHz
	delays should be:
	- within $\pm$ 1ns,		When observing a strong calibrator.
	$\bullet\,\,$for spectral line observations		ie. Bandpass $\delta\nu <$ 16MHz
	delays should be:
	- less than $\pm$ 10ns,		When observing a strong calibrator.
	- as high as 100,000ns,		For a weak, extended source.
	$\bullet\,\,$amplitudes constant on all baselines.

NB: Occasionally, 3 millimetre observing modes require that the CAOBS $tsys$ calculations is disabled. In this situation, cm amplitudes will not be set correctly. If you have this problem, stop the scan type enable tsys, then re-do cacal. You may need to reset gains to 1 with CACAL in interactive mode before redoing a normal CACAL.

Note that calibration information is kept for the 16 most recent frequencies, which means that if you are observing in one of these frequencies, it may not be necessary to apply cacal.