[daip] differential precession correction in AIPS

[Feng Gao]

Thank you Eric!

Best regards,

Feng

> On 30 Oct 2017, at 3:51 PM, Eric Greisen <egreisen at nrao.edu> wrote:
> 
> 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
>