VLBA 7mm performance impacted
On July 14, 2021, a long theoretically understood method for improving telescope performance at 2cm, 7mm and 3mm was turned on. This was made possible by an upgrade to the motors controlling the rotation of the Focus/Rotation Mount (FRM).
This method attempts to correct for the elevation dependent deformation in the dish structure by rotating the subreflector and correcting azimuth and elevation offsets induced by this rotation in real time.
Unfortunately, an error in the amount of correction applied and an offset in the 'zero point' introduced has affected all 7mm observations since that date until the error was detected and 7mm observations were paused on Nov 20. 2cm and 3mm observations do not appear to have been affected.
The issue was initially undetected as the telescope performance did not appear to degrade in SEFD monitoring. This was due to said monitoring using assumed gains as no accepted flux density calibrators were in the test observations.
Telescope performance as a fraction of nominal performance during this time period (July 15 2021 to November 20 2021) is given in the table below:
| Station | Fraction of nominal performance |
|---|---|
| SC | 0.30 |
| HN | 0.52 |
| NL | 0.66 |
| FD | 0.60 |
| LA | 0.49 |
| PT | 0.70 |
| KP | 0.62 |
| OV | 0.64 |
| BR | 0.60 |
| MK | 0.46 |
Expected effects on data are a ~40% sensitivity loss and flux density scale change for 7mm observation between July 15 and November 20. The flux density scale can be corrected by using a newly calculated set of gains that can be provided on request, however the sensitivity loss is unrecoverable.
Please reach out to the helpdesk (VLBA Observing department) if your data have been affected and you wish to apply these new gains.
The errors were corrected on December 9, 2021, and telescope performance has returned to nominal values except at:
- SC (long standing 7mm sensitivity issues);
- PT, fixed on December 17.
- MK (recent rotation encoder slip), fixed January 6.
A further regression in performance occurred between 20 Dec 2021 and 04 Jan 2022 affecting BR, KP and LA and rendering data from these telescopes useless. Only three projects appear to have been affected. This was due to a loss of the interconnect between rotation and azimuth/elevation corrections.
