Reference Pointing

``Reference pointing,'' effectively pointing calibration, involves using a nearby calibrator to make adjustments to the global pointing model to increase the pointing accuracy of the target observations. Reference pointing is necessary for 3mm observations and, if pointing accuracy is important, can be required at 12mm and even 3cm (8GHz).

In good weather, a bright calibrator 5-10° from the target source will reduce pointing errors to about 2-5" rms. This is a substantial improvement on the global pointing solution which is often in error by 20", however the achieved accuracy depends on many parameters - source strength, atmosphere, wind, frequency, azimuth/elevation, distance to target, global pointing solution accuracy, and antennas used; to name the well-known variables.

Generally, significant (at higher frequencies) pointing improvement can be had by pointing on a quasar with a flux greater than 2 Jy within 20° of the target field. To maintain accurate pointing during the observation include a pointing scan every hour throughout the observing program.

It is common to use your phase calibrator as your reference pointing source (assuming that it is strong enough) however, the phase calibration scan should be separate from the pointing scan.