Available Graphs

The signal display provides a number of key visualisations of the data. The plots are separated between two tabs; “Visibilty Receive” housing the real time visualisation of incoming data and “Calibration Data” holding the outputs of the calibration pipelines.

Visibilty Receive

Descriptions of each plot in the Visibilty Receive tab and how they respond follows.

Spectrum

The incident flux on the telescope is distributed over a finite receiving band, and is a function of frequency. The spectrum is the flux per unit bandwidth. The broad continuum spectrum of a radio source may contain a number of spectral lines, whose profile are the subject of detailed study. However, a receiver bandpass is usually wide enough to contain one or more spectral lines, and so we sub-divide the band into a number of filter channels.

A digital autocorrelation spectrometer samples the input signal at the Nyquist frequency, producing a series of binary packets representing the signal in time. The signal is then delayed by a series of identical values, or lags, such that the inputs to the multipliers are the signal itself and a series of delayed signals.

Sampling of the output provides an estimate of the discrete autocorrelation function. Once noise has been accounted for, the autocorrelation function is related to the power spectrum by the Discrete Fourier Transform.

Here, we present a plot of the mean autospectrum averaged over all channels and baselines, per polarisation.

Power vs Phase

For each baseline and polarisation a plot of the amplitude of the visibility spectrum is displayed, along with a plot of the phase of the visibility spectrum. From the plot of the amplitude it is possible to discern whether a source is resolved, along with information about its shape. The phase information allows us to determine the source’s offset from the phase center.

Spectrum Waterfall

A visibility is the correlation between two antennas over a time and frequency interval. A lag or XF correlator multiplies (X) the signals from each antenna together as a function of lag.

This can be integrated for multiple time steps and is what an XF correlator outputs. However, the contributions from all the channels are mixed together, and so to extract the information about the power in each channel, we Fourier transform (F) this signal (and this is where the F in XF comes from).

This is the Cross-Correlation power as a function of frequency and it is what we get from our correlator.

For each baseline and polarisation, we present a waterfall plot of the phases of the visibilities as a function of frequency. A flat spectrum of phases is synonomous with zero residual delay. This is due to the ‘Shift Theorem’ which states that a delay in the time domain corresponds to a linear phase term in the frequency domain.

Cross-Correlation Power vs Time Lag

The output of our Correlator is the Cross-Correlation power as a function of frequency (see above), and furthermore it is an FX correlator, performing the Fourier transform before the multiplication. To change this back to “Cross-Correlation power as a function lag” we need to calculate the inverse Fourier transform (iFFT) of the visibilities for each baseline.

We present this calculation in the form of a Waterfall plot. For each baseline and timestep, the iFFT of the complex visibility spectrum is calculated.

Any residual delay will manifest itself as a shift of the peak of the lag plot away from zero. I.e., if the signals have been correctly delayed before their Cross-Correlation the peak power in Cross-Correlation will be at zero lag.

Calibration Data

Descriptions of each plot in the Calibration Data tab and how they respond follows.

The pointing offset calibration pipeline fits 2D Gaussian primary beams to the visibibility or gain amplitudes. Each scan is split into a number of frequency chunks, and the primary beam is fitted for each frequency chunk and dish. The weighted average of the fitted parameters for each frequnecy chunk is provided for each antenna.

Elevation and Cross-elevation offset

The fitted parameter representing the centre of the primary beam provides the elevation and Cross-elevation offsets, along with their uncertainties. If a calculated pointing offset exceeds a threshold percentage of the expected value, then it is discounted. These discounted pointing offsets are indicated by the red shaded regions in the graphs.

Beam width

The expected and fitted widths of the 2D gaussian primary beam are displayed, along with their uncertainties.

Beam height

The expected and fitted heights of the 2D gaussian primary beam are displayed, along with their uncertainties.

Gain Calibration

In radio telescopes, the complex receiver gains are initially unknown and need to be calibrated. Measured intrerferometer data is generally corrupted by instrumental and atmospheric effects, which can be corrected for through a process known as gain calibration. Gain calibration enhances the quality of astronomical images and improves the effectiveness of signal processing techniques.

Since the antenna gains are unknown prior to observing the field of interest, science scans are typically interspersed with calibrator scans of high SNR, well-modelled objects. By determining the major factors influencing the antenna gains, and applying the inverse to the target field, we can produce corrected data that can act as the starting poing for self calibration.

In order to assess the stability of the gain calibration solution with time, we present a time-series plot of the amplitude and phase of the complex gains, for each antenna. Currently, only the first frequency channel is displayed.