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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 () 2) a good sky coverage, and 3) a formal error
on COH less than .
- Gauss fit formal error
For radio pointing, if the formal error on the position from gauss fit is
higher than , 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 ANALYZEMETHOD 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.
Next: POINT Language Internal Help
Up: Determination of Pointing Parameters
Previous: Inclinometers
Contents
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Gildas manager
2014-07-01