Polarimetric observation of 3C286 with Allen Telescope Array

Following my polarimetry experiments at Allen Telescope Array, on October 31 I did a polarimetric observation of the quasar 3C286 with two dishes from the array to use as a test-bed for polarimetric calibration. 3C286 is a bright, compact, polarized source, with a fractional polarization intensity of around 10% and a polarization angle of 33º over a wide range of frequencies, so it makes an ideal source for polarization calibration. It is the primary polarization calibrator for VLA. The observation duration was slightly more than 2 hours, and it was done around the transit of the source, so the parallactic angle coverage is large (around 90º).

My initial idea was to use this observation to perform a “single dish” polarization calibration of each of the dishes by separate (since the math is somewhat simpler) and then perform an interferometric polarization calibration. However, after initial examination of the data, the SNR doesn’t seem large enough to do a “single dish” calibration. The polarized signal from 3C286 is rather weak and is swamped by noise from other sources in the field and from the receiver, and also by gain variations in the receive chain.

On the contrary, the interferometric calibration has worked well, since correlating the signals from the antennas allows us to discard the uncorrelated receiver noise and to phase on the target and discard other signals from the field, by means of Earth rotation aperture synthesis.

In this post I give my analysis and results of the observation. I have done an ad hoc calibration in Python to determine the polarization leakage and measure the polarization degree and angle of the source, and also a full polarimetric calibration in CASA to compare my calibration with one obtained with professional software.

The data used in this post has been published in Zenodo as the dataset “Allen Telescope Array polarimetric observation of 3C286“.

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Tianwen-1 TCM3 final trajectory

A few days ago I posted about TCM3, the fourth trajectory correction made by Tianwen-1 so far. After some days, the Chinese DSN has performed precise orbit determination and updated the on-board ephemerides, so that we are now seeing the final trajectory in the telemetry state vectors.

The figure below shows how the state vectors have been updated a couple of times following the TCM, as the DSN computes and uploads an improved trajectory solution. I have plotted this graph in the following way: I have taken the first state vector received after TCM3, on the UTC afternoon of 2020-10-28, and used it to propagate a trajectory in GMAT. The plot shows the difference between the state vectors and the GMAT trajectory.

TCM3 happened on 2020-10-28 at 14:00 UTC, so the reference trajectory computed in GMAT corresponds to the trajectory of the state vectors immediately following the TCM. These are based on a prediction of the burn performance, rather than on the actual results. The graph above shows clearly two changes in the trajectory, one on 2020-10-29, and another one on 2020-11-01.

Since we have already seen the same trajectory for three days without updates, I am confident that this trajectory is now final. The latest state vector we have today is

[0163f5396aeb] 2020-11-04 03:25:57.092300 160383830.37880394 94290296.68475787 45404213.83552203 -9.17821106231485 22.96506090856966 10.332601544380973

As always, this gives the UTC timestamp and the ICRF heliocentric position and velocity coordinates in km and km/s respectively.

I have re-run the calculations in the previous post by back-propagating a state vector from the UTC evening of 2020-11-01, which already belongs to the final trajectory. The change in delta-V in comparison to what I should in the previous post is small. The new delta-V is 2.13 m/s rather than 2.09 m/s, and the components have changed around 5%. The detailed calculations and data can be found in the updated Jupyter notebook.