WCS question

[Arnold Rots]

Patrick Wallace wrote:
> On Mon, 9 Nov 1998, Arnold Rots wrote:
> 
> > 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).
> 
> No longer true for ICRF.
> 
> > However, "J2000" does not imply FK5 (as you imply above).
> 
> Maybe not, strictly speaking, but in practice "J2000" is always taken
> to mean "in accordance with the IAU 1976 recommendations", which, as
> well as introducing J2000 as the fundamental epoch, brought in a
> specific precession model (Lieske's) and frame (FK5).
> 
> > Instead, the IAU recommendation is to move from J2000-FK5 to J2000-ICRS.
> 
> The J2000 is superfluous in the case of ICRS.  You only need epochs
> when the system depends on a model of precession.  ICRS has no ecliptic
> or equinox.  It is fixed for all time in the extragalactic background,
> liberated from considerations of the Earth's orientation or orbit.  If
> you wrote J2050-ICRS it wouldn't mean anything.

Yep, I spoke too hastily.  However, in practice (for observing), one
may want to precess the ICRS position to a different epoch.  Doesn't
it then become an ICRS position at epoch 1998.85?

  - Arnold

> 
> 
> Patrick Wallace
> ____________________________________________________________________________
> Starlink Project Manager                        Internet:  ptw at star.rl.ac.uk
> Rutherford Appleton Laboratory                       Tel:    +44-1235-445372
> Chilton, Didcot, Oxon OX11 0QX, UK                   Fax:    +44-1235-446667
> ____________________________________________________________________________
> 
> 

--------------------------------------------------------------------------
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/
--------------------------------------------------------------------------