[daip] IMAGR position shifting

[Eric Greisen]

Mark Reid wrote:
> 
> Eric,
> 
>     I'm still struggling to figure out if I need to make some
> corrections to proper motion data and have a question
> about IMAGR position shifting.  The data I'm concerned with
> are VLA A-configuration images of the Galactic Center, containing
> SgrA* (pointing and phase-reference source) and some red giant stars 
> with SiO masers that are between 5 and 40 arcseconds from SgrA*.
> 
> I've been calibrating on SgrA* and then mapping with IMAGR with
> multiple small images centered on SgrA* and several maser stars
> using (RASHIFT,DECSHIFT)s.
> 
> When I specify (RASHIFT,DECSHIFT) how does IMAGR interpret these
> values?  I can think of two possibilities:
> 
>   1) treat them as apparent offsets; correct them for
>      precession, nutation, and aberration; apply them in
>      equinox J2000 coordinates.
> 
>   2) treat them as J2000 offsets directly (ie, making no
>      corrections for precession, nutation, aberration).
> 
> Ultimately, I'm trying to figure out if positions measured
> in the maps, should be corrected for the differential effects
> of nutation and aberration (between SgrA* and a maser star
> position) before calculating proper motions.  (I presume that
> no correction for differential precession is needed as the maps
> are in mean (catalog) coordinates.)
> 
>
I am amazed at how competent you think AIPS is.  Let me tell you, we are 
just lucky that programs end more or less naturally.

The case 2 above applies.  RASH and DECSH are simple shifts and you get 
the image centers quoted in J2000 - no precession (How would that apply 
anyway?), nutation, aberration etc etc are applied.

Eric Greisen