Band Specific

Continuum | Spectral Line | Band-Specific

Overview

The general approach for EVLA data processing will be similar regardless of band, and will consist of (for continuum observations):

  • Flagging of any obvious bad data (flagdata, plotms, viewer)
  • Fixing the flux and spatial distribution of the flux density calibrator (setjy)
  • Performing a bandpass calibration (gaincal for initial phase, bandpass)
  • Performing gain calibration on calibration sources (gaincal)
  • Setting the flux density scale (fluxscale)
  • Transferring the gain calibration to the science target (applycal)
  • Imaging (clean)

A good example of this approach can be found in the EVLA Continuum Tutorial 3C391. Of course, there are many variations on this theme: at some point, the data could be averaged (in frequency and/or time) to reduce its size; this, of course, would depend on the tolerance for bandpass smearing, and ... Furthermore, one may wish to perform Self Calibration if the science target is sufficiently strong.

Low-frequency bands (L, S)

The greatest concern at low frequencies is radio frequency interference (RFI). A detailed description of this phenomenon, as well as guidelines for how to avoid it (prior to obtaining and observation) and mitigate its effects (if it is present in an observation) can be found on the Radio-Frequency Interference page.

 

Mid-frequency bands (C, X, Ku)

High-frequency bands (K, Ka, Q)

At high frequencies, there is generally little radio-frequency interference, but the atmosphere can be of particular concern. This will be dependent on the weather conditions during the observation (check the weather table), but generally, the shorter radio wavelengths will be substantially more prone to atmospheric effects.

One important consideration is atmospheric absorption. Since this is highly dependent on the elevation of the source, the greatest errors will be introduced when the calibration sources (especially the flux calibrator) are at a substantially different elevation from the science target. In generally, an opacity correction should be applied.

Note that this much occur during every step of calibration. In order to do this, the opacity parameter must be set in bandpass, gaincal and applycal.

Connect with NRAO

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