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
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.
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.
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.
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)