Low (P/4) Band Antenna Collimation
From May 22, 2019, pointing collimation offsets for all VLA antennas for P-band (220 - 500 MHz) are being applied by the online observing system. This pointing term has not been included since June 27, 2007; which likely means that the P-band pointing was offset in the period from June 27, 2007 to May 22, 2019. The table below shows the corrections that were put into the system in arcminutes for each antenna. Most of these corrections are quite small relative to the FWHM of the P-band primary beam (~135 arcmin at band center). However, a few of them are large enough to have a noticeable effect on some observations; e.g., antennas 02 and 21 have corrections > 10 arcmin.
Moreover, the overall pointing at 4/P band was modified by shifting the pointing center an additional +6.5 arcmin in elevation in observations after May 22, 2019. All these changes now align the center of the primary beam with the boresight of the antenna.
Currently the only mitigation to address the impacted observations is to flag antennas with the largest offsets during post processing. We encourage those with P-band data in the noted 12-year period to compare images made with and without such flagging to determine whether the flagging has helped.
The effect of the pointing offset would be most visible in the primary beam corrected flux densities, in in-band spectral indices, and it particularly affects combination of multiple configurations and mosaicking. We are working on being able to correct these errors for CASA tclean imaging but that solution is not yet available. In the meantime, the only solution to minimize errors is to flag grossly mis-pointed antennas or accept larger error bars on flux densities.
If you have any further questions regarding this issue, please contact the helpdesk.
| Antenna (ea#) | Azimuth collimation offset (arcmin) | Elevation collimation offset (arcmin) |
|---|---|---|
| 1 | -6.0 | -2.0 |
| 2 | -1.5 | -15.0 |
| 3 | -1.5 | 5.0 |
| 4 | 1.5 | 3.0 |
| 5 | 2.0 | 1.0 |
| 6 | 0.5 | 1.0 |
| 7 | 0.0 | -7.0 |
| 8 | 1.5 | 3.5 |
| 9 | 1.0 | -2.0 |
| 10 | -1.5 | 0.0 |
| 11 | 0.0 | 3.0 |
| 12 | 1.0 | 0.0 |
| 13 | 1.0 | 0.0 |
| 14 | 0.0 | -2.5 |
| 15 | 0.0 | 0.0 |
| 16 | 1.5 | 2.5 |
| 17 | 0.0 | 0.0 |
| 18 | 0.0 | -1.5 |
| 19 | 0.0 | -2.0 |
| 20 | 0.0 | -4.0 |
| 21 | 2.0 | -26.0 |
| 22 | -1.0 | -4.0 |
| 23 | 0.5 | 7.0 |
| 24 | 0.0 | 0.0 |
| 25 | 0.0 | 4.0 |
| 26 | -1.5 | 0.0 |
| 27 | -0.5 | -7.0 |
| 28 | -0.5 | 4.0 |
