[daip] some aips issues

[Eric Greisen]

Cao Hongmin wrote:
> hi Eric,
> 
> i used TASK "clcor" (opcode "ANTC") to correct the source position. The 
> phase, delay and rate were also corrected. but i am not sure which 
> antenna was chosen as reference antenna during the process (?) I checked 
> the "cl" table after running the task. it seems that antenna 1 was the 
> reference antenna ( its phase, delay, and rate are all zero) automatically.
> 
> when removing the phase of the parallactic angle using TASK "clcor" 
> (opcode pang),  it seems there is no reference antenna chosen in the 
> process while the expected results, that the phases of alt-az mounted 
> antennas were corrected with no changing done for the equatorial mounted 
> antennas, were got.
> 
> whether it is proper to choose another antenna ( not antennas 1) as 
> reference antenna in the following steps, such as Fringe-fitting? (it 
> seems that the phases (delay and rate) corrected before fringe-fitting 
> are just added to the phase (delay and rate) got by task "fring" after 
> referencing to a common antenna).
> 
> whether the source position correction using task "clcor" with opcode 
> "antc" will introduce big source postion error if the old position 
> (uncorrected one or the prior phase centre) is too far from the new one 
> such as large than 1 arcsec?

The use of a reference antenna in ANTC is not actually required when all 
the antennas are widely spaced (as in VLBI).  We found for the VLA that 
the very long absolute baselines meant that answers were not adequately 
accurate so we introduced a reference so that the relative baselines 
would be short.  It does not matter what antenna is chosen.  I suppose 
if a VLBI array had several close antennas and one very distant it would 
be better not to use the distant one.

It sounds as if all the tasks have done what you need.  ANTC is very 
accurate, but stores its answers in single precision.  If the total 
phase change is very large in an absolute sense, then it is possible 
that precision needed for phase differences may be lost (hence the 
reference antenna use for VLA).  It should be good up to 1 asec but not 
much more.

Eric Greisen