Moderate winds have a very strong effect on both pointing and antenna figure. The advertised maximum wind speed for precision operations is 15 mph (7 m/s), and careful observations have demonstrated this to be the practical maximum wind speed for useful observing at K and Q bands. Observations at these bands in winds significantly in excess of this limit are not advisable, and users should consider moving to a lower frequency. Wind speeds near the stow limit (45 mph) will have a similar negative effect at X and U bands.
To achieve better pointing, we have added a capability for repeated calibration and correction of the local pointing error during astronomical observations. In this observing mode, known as `referenced pointing', a nearby calibrator is observed in interferometer pointing mode every hour or so. The local pointing corrections thus measured can then be applied to subsequent target observations. Tests show that this mode reduces rms pointing errors to typically 2 - 3 arcseconds if the reference source is within about 10 degrees (in azimuth and elevation) of the target source, and the source elevation is less than 70 degrees. The JObserve program is aware of this observing mode.
Use of referenced pointing is highly recommended for all K- and Q-band observations, and for X- and U-band observations of objects whose total extent is a significant fraction of the antenna primary beam. It is usually recommended that the referenced pointing measurement be made at X-band, regardless of what band your target observing is at, since X-band is the most sensitive, and the closest calibrator is likely to be rather weak. Proximity of the reference calibrator to the source position is of paramount importance. The calibrator should have at least 300 mJy flux density and be unresolved on all baselines to ensure an accurate solution.
Measuring the pointing offsets at K-band for subsequent K-band observing is usually successful, but should not be attempted on objects of less than 300 mJy flux density. However, attempting to measure these offsets at Q-band directly generally is not recommended, since the blind pointing error is often larger than the Q-band half-power half-width - which will cause the referenced pointing to fail.
Secondary referenced pointing (i.e., using X-band referenced pointing to permit subsequent Q-band or K-band determination of the residual pointing errors), is recommended only for high precision observations at Q and K bands when observing extended objects which fill the antenna primary beam. For general observing of small objects, simple referenced pointing using the offsets determined at X-band is nearly always sufficient to keep all antennas pointing to within 5 arcseconds.
Consult with Rick Perley, Ken Sowinski, or Claire Chandler for more information about referenced pointing. The information above is largely taken from the guide to high-frequency observing on the VLA, found at http://www.vla.nrao.edu/astro/guides/highfreq/.