VLA Pointing Page


VLA Test memo 233: Encoder upgrade at the VLA: mid-term status (Jan 2003).


Current status of pointing related hardware work at the VLA (8/2001).


Ken Sowinski's plot of pointing model history for all antennas.

Latest VLA Pointing results

Post-fit residuals

Measures the difference between antenna performance and the pointing model. These are long timescale errors which are usually removed with reference pointing on timescales of 30 to 45 min. The green curve shows elevation errors, the blue curve shows azimuth errors, and the magenta curve shows the rss.

PN3DB amplitudes

Measures short timescale pointing errors (10's of mins) by tracking a bright source at the 3db point of the primary beam in AZ and EL. These errors are not removed by reference pointing. Color coding is the same as above.

Reduced Chisquare

Measures the quality of the pointing model fit to the data. A high value (> 3) means a poor fit. A low value can either mean a good fit, or too few single-source scans were used in the fitting. The latter implies that many of the single-source measurements failed, and indicates poor pointing performance for the antenna. This can be checked by looking at the number of measurements plot below.

Number of Solutions

The number of single-source scans that have been used to determine the pointing model parameters. Each measurement corresponds to a 5-pointing raster centered on a calibrator. The pointing program then calculates the observed source position from the 5 point raster, and compares it to the nominal source position as commanded by the encoders. Sometimes the pointing performance is so poor that no positional fit is obtained from the 5-point raster, and the measurement is therefor rejected. Typically between 70 and 90 measurements are used in a given pointing run. If an antenna shows a number of measurements lower than the mean by 20 or so, then this is an indication of a major pointing problem for that antenna.

Maintained by C. Carilli (ccarilli@nrao.edu)