WCS question

[Arnold Rots]

Doug Mink wrote:
> Greg Hennessy wrote:
> > 
> > I am working on producing a set of images on CDROM that will have
> > astrometry according to the Greisen and Calabretta A&A 1996 paper, but
> > the four allowed values for RADECSYS are two for FK4, one FK5, and one
> > geocentric apparent place. The images I will be working on are
> > reduced against the ACT catalog. Are there any conventions to what
> > RADECSYS should be for this? I would assume 'HIP' but would like some
> > advice.
> 
> We talked about this at ADASS, and there was some agreement that the
> RADECSYS for Tycho- and ACT- (and Hipparcos-) based images could be
> HIPPARCOS (spelled out).  Pat Wallace has subroutines which convert
> between the Hipparcos system and J2000, and I will be adding them
> to my WCSTools package as soon as I get them from him.
> 
> -Doug Mink
>  WCSTools developer ( http://tdc-www.harvard.edu/software/wcstools/ )

Be careful!  There are three elements to the definition of a reference
frame: the epoch (2000), J or B, and the reference frame itself (FK5,
ICRS).  FK4 is the one commonly used with B1950.  Its J-equivalent FK5
has sofar been used with J2000.  However, "J2000" does not imply FK5
(as you imply above).  Instead, the IAU recommendation is to move from
J2000-FK5 to J2000-ICRS.  ICRS, based on Hipparcos and Tycho data, is
the official designation of the reference system and, hence, would seem
the logical choice for RADECSYS.

A similar issue comes up in the transformation from DE200 to DE405.
That transformation is close to FK5 -> ICRS.
Just for reference, this is the comment section for the function that
does that conversion:

 *  c200to405 converts a direction cosine vector from the DE200 frame
 *  (nominally FK5, but not quite) to the DE405 frame (ICRS).
 *  
 *  double dir[3]    direction cosine vector (in and out)
 *  
 *  The formula, according to Standish, is:
 *  
 *    dir(DE405) = dir(DE200) + eps x dir(DE200)
 *  
 *  where eps = (0.002, 0.012, 0.006) / 206265


  - Arnold Rots


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Arnold H. Rots                                         AXAF Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 81                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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