Cookbook, Tricks, and Traps

It is difficult to produce a cookbook recipe for getting the pointing parameters. The following Traps should be avoided, however:

• Azimuth Elevation coverage
  It MUST be reasonable. Never believe the formal errors on MVE or MVN if the Azimuth coverage is less than 270 degrees. Never believe the formal error on COH if the elevation coverage is less than 60 degrees. From the pointing error equations, it can be seen that the best coverage consists in sources equally spaced in Azimuth, but clustered at high and low elevations.

• Radio Collimation COH
  This is essentially an antenna constant. It is better to fix COH to the previous value, rather than trying to fit it (see the INTER_BASE:GENERAL.ANi file to get this number). Accordingly, this is done by default. Do not believe COH has changed unless you have 1) many data points (>40) 2) a good sky coverage, and 3) a formal error on COH less than 0.5".
• Gauss fit formal error
  For radio pointing, if the formal error on the position from gauss fit is higher than 2.0", the data may be useless.
• Avoid long pointing sessions and systematic Az-El coverage
  At least in OPTICAL pointing. Time dependent effects, such as thermal behaviour of the optical telescope mount, may then become inextricably mixed with Az or El dependent terms.

Accordingly, the following Tricks should be used:

• Use SiO Masers in Pseudo Continuum
  They will help you get a good Az-El coverage. They are usually stronger than continuum sources.
• Use the beam switch
  Always for continuum sources, but also for SiO masers if the weather is poor.
• Fix the beam width for CLASS gauss fits
  This helps to reduce the formal error on the position, specially for weak sources. This can be done using the CLASS command ANALYZE $\backslash$METHOD CONTINUUM Width, where Width is in arc seconds. The beam width is 55" at 86 GHz. The PR:INIT_POINT.CLASS procedure executes such a command if the observing frequency is specified.

• Use the inclinometers
  At least just before and just after the pointing session, but also later from time to time to check any time dependence of MVE or MVN.

• Use the specially designed procedure POINT_INTER and POINT_MULTI
  These procedures have been written to produce a reasonable coverage, considering the functional form of the pointing model equations. Pointing sources have been selected to avoid excessive noise.