On-the-Fly Calibration Correction

Many tasks will apply calibration corrections `on-the-fly'. This means that often there is no need to form a calibrated dataset. Normally this calibration correction step is performed by default. However, the correction stage can be disabled using some fairly standard switches in the options parameter. These switches are:

nocal:

Do not apply antenna gain and delay corrections.

nopol:

Do not apply polarisation leakage corrections.

nopass:

Do not apply bandpass function corrections.

For example, to disable gain and bandpass correction in uvspec, use:

     % uvspec options=nopass,nocal

It is always possible to form a calibrated dataset by using either of the visibility copying tasks (uvcat or uvaver). Whether or not you choose to do this may be often a matter of personal taste. However there are three VERY important situations where you should form a calibrated copy of your dataset (i.e. where `on-the-fly' calibration is inadequate).

• A still significant, but decreasing, number of tasks do not perform `on-the-fly' calibration. Tasks which do apply calibration corrections will invariably have nocal, nopol and nopass options. Additionally all tasks which are performing `on-the-fly' calibration will issue messages when they are performing these steps.

• Many Miriad tasks allow `on-the-fly' averaging of spectral channels (using the line parameter with a width greater than 1 - see section 5.4) in addition to bandpass correction. However the technique used when simultaneously bandpass correcting and averaging is only approximately correct. For arcane reasons, rather than applying the bandpass and then averaging the channels, the tasks correct the average of the channels with the average of the bandpass. This can be significantly in error if the bandpass functions vary significantly over the range of channels being averaged. Tasks performing this dubious operation issue warning messages to alert you of your possible folly. If this is a real problem, then it is best to form a copy of the bandpass-corrected data, and then use these corrected data for channel averaging.

• The self-calibration tasks produce gain tables in the same format as the calibration tables derived from observations of calibrators. Thus if you self-calibrate a dataset containing you initial calibration, you will overwrite your initial calibration - a generally undesirable step. Additionally the self-calibration tasks do not apply some calibration corrections (e.g. bandpass and sometimes leakage corrections). Generally it is best to self-calibrate a dataset which has had the initial calibration applied.