[daip] differential precession correction in AIPS

[Eric Greisen]

On 10/30/2017 05:25 AM, Feng Gao wrote:
> Dear Eric,
> 
> I have a question related to differential precession correction in AIPS.
> 
> For a VLBA dataset, when I saw "Coordinate equinox 2000.00" in the 
> header, I would assume all the RA, DEC positions I see in different 
> tasks (e.g.  in imstat or jmfit) are defined reference to the J2000.0 
> epoch. Then when I make an image away from the phase center, does AIPS 
> also corrects the precession from the actual observed date back to J2000 
> (so that the coordinates in that image are also defined within epoch 
> J2000, not the epoch of the observed date) ?

No additional precession is done.  None is needed since the uvw 
coordinates are computed at J2000 rather than at observation date.
The coordinates in an off-center field should be correct.

Eric Greisen