Atmospheric Optical Depth Correction

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4.3.3 Atmospheric Optical Depth Correction

The troposphere is not completely transparent. At high radio frequencies (>15 GHz), water vapor and molecular oxygen begin to have a substantial effect on radio observations. According to the physics of radiative transmission, the effect is threefold. First, radio waves from astronomical sources are absorbed (and therefore attenuated) before reaching the antenna. Second, since a good absorber is also a good emitter, significant noise-like power will be added to the overall system noise. Finally, the optical path length through the troposphere introduces a time-dependent phase error. In all cases, the effects become worse at lower elevations due to the increased air mass through which the antenna is looking. In CASA, the opacity correction described here compensates only for the first of these effects, tropospheric attenuation, using a plane-parallel approximation for the troposphere to estimate the elevation dependence.

Opacity corrections are a component of calibration type ’T’. To make opacity corrections in CASA, an estimate of the zenith opacity is required (see observatory-specific chapters for how to measure zenith opacity). This is then supplied to the opacity parameter in the calibration tasks, which can be a single value or a list of opacities with entries for the different spectral windows.

ALERT: The opacity parameter must be supplied to any calibration task that allows pre-application of the prior calibration (e.g. bandpass, gaincal, applycal). This should be done consistently through the calibration process. In future updates we will add the capability to gencal (§ 4.3.5) to create a calibration table for this. Furthermore, you currently can only supply a single value of opacity, which will then be pre-applied to whatever calibration task that you set it in. Generalizations to antenna- and time-dependent opacities, including derivation (from weather information) and solving (directly from the visibility data) capabilities, will be made available in the future.

For example, if the zenith optical depth is τ = 0.1 nepers, then use the following parameters:

gaincal(’data.ms’, ’cal.G0’, solint=’inf’, combine=’’,refant=11, opacity=0.1)

The calibration task in this example will apply an elevation-dependent opacity correction (scaled to 0.1 nepers at the zenith for all antennas for this example) calculated at each data sample before solving for gains on an “infinite” (up to scan boundaries) timescale.

If you do not have an externally supplied value for opacity, for example from a VLA tip procedure, then you should either use an average value for the telescope, or leave it at zero and let your gain calibration compensate as best it can (e.g. that your calibrator is at the same elevation as your target at approximately the same time. As noted above, there are no facilities yet to estimate this from the data (e.g. by plotting Tsys vs. elevation).

Below, we give instructions for determining opacity for JVLA data from weather statistics and VLA observations where tip-curve data is available. It is beyond the scope of this cookbook to provide information for other telescopes.

4.3.3.1 Determining opacity corrections for JVLA data
4.3.3.2 Determining opacity corrections for VLA data

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