Calibration

For 12-mm data, calibration is essentially identical to the centimetre bands. Primary flux calibration can be done using gpboot and observations of 1934-638, in much the same fashion as at the lower frequencies. 1934-638 is about 1 Jy at 12-mm wavelengths: the Miriad calibration software contains a built-in model of 1934-638's change of flux density with frequency. For information about this flux scale, see the memo by Bob Sault ``ATCA flux density scale at 12mm'' (2003).

As 1934-638 is comparatively week at 12-mm wavelength, it is not an good bandpass calibrator. If you require a bandpass calibration, you should take observations of a strong source such as 1921-293 or 1253-055.

Because atmospheric opacity affects the amplitude gain calibration step, it is important to consider it when high precision flux calibration is required. In the flux bootstrapping step, gpboot determines the boot factor to be applied to the gains of the secondary calibrator to make them the same as the primary. As the opacity correction done by atlod is only correct to first order, ideally the comparison between primary and secondary gains would be made at the same elevation and under the same weather conditions. Selecting the range of times that the gains should be compared is a more important issue at 12mm than at lower frequencies. To select the time range of the secondary to compare, use time selection in the select keyword.

Some judgement will need to be made about the set of amplitude gains of the secondary to use in the comparison. In stable weather conditions, it is probably best to select the time range of the secondary where its elevation corresponds most closely to the elevation during the primary's observation. When the weather changes significantly during the observation, it is probably best to use a time range of the secondary which is close to the primary's observation. Tasks uvplt and varplt can plot source elevation as a function of time.

Typical inputs to gpboot are:

GPBOOT
vis=vela.uv	Input visibility dataset.
select=time(1:00,2:00)	Select a time range in the ``vela.uv''
	dataset where the amplitude gains should be
	comparable to those in the primary calibrator.
cal=1934-638.18001	Primary flux calibrator.