[daip] CLCOR

[Eric Greisen]

Mark Reid wrote:
> Dear Eric,
> 
>    We've been thinking about position shifting with CLCOR, since we've 
> (successfully) analyzed a lot of data this way.   Here is what I think 
> is correct.  Please let me know if I'm wrong.
> 
> CLCOR with "ANTC" takes CLCORPRM(5),(6) and calculates a new apparent 
> position and puts values in the CL table to shift interferometer data to 
> the new position.  Since it uses apparent positions, it does make 
> corrections for precession, nutation, aberration, G.R. bending, etc (by 
> calling subroutine JPRECS).   We've had recently been worrying that it 
> might not have done corrections for aberration, which could introduce 
> bad (yearly signature) effects in parallax measurements.
> 
> If we then map the source with IMAGR, there is potential for problems 
> with uncorrected u,v,w values as you have pointed out.  However, since 
> we have shifted our (nearly point) sources to the center of the field, 
> the slightly incorrect u,v,w values should have little to no effect.
> 
> Do I understand things correctly?
> 
> Mark
> 
> -------------------------------------------------------------
> Mark J. Reid                 Phone: 617-495-7470
> Harvard-Smithsonian CfA      Fax  : 617-495-7345
> 60 Garden Street             Email: reid at cfa.harvard.edu
> Cambridge, MA 02138, USA     Web  : www.cfa.harvard.edu/~reid
> -------------------------------------------------------------

CLCOR uses JPRECS to compute a new coordinate at EQUINOX due to the 
shift of the apparent position.  In that sense, the difficult things 
come out in the new RA, Dec J2000 values.  Differential aberration 
appears only in UVFIX as a scaling of U and V.  The phase corrections 
are made simply by computing the delay with the old apparent coordinates 
and the new apparent coordinates and differencing.  Nothing fancy there.
Note that if you shift Equinox coordinates JPRECS is used to find the 
shift in apparent which then enters the phase shifting part.

Cheers,

Eric Greisen