Subject: WCS question

From: gsh at tantalus.clark.net (Greg Hennessy)
Date: Mon, 2 Nov 1998 17:57:59 GMT

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.

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From: Arnold Rots <arots at head-cfa.harvard.edu>
Date: Thu, 5 Nov 1998 10:04:27 -0500 (EST)

The logical next value for RADECSYS is "ICRS".  That is what we will
be using for AXAF.

  - Arnold Rots

> RADECSYS should be for this? I would assume 'HIP' but would like some

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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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From: Doug Mink <dmink at cfa.harvard.edu>
Date: Thu, 05 Nov 1998 14:51:11 -0500

> RADECSYS should be for this? I would assume 'HIP' but would like some

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/ )

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From: Arnold Rots <arots at head-cfa.harvard.edu>
Date: Mon, 9 Nov 1998 09:42:03 -0500 (EST)

>  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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From: Patrick Wallace <ptw at star.rl.ac.uk>
Date: Mon, 9 Nov 1998 20:31:54 +0000 (GMT)

On Mon, 9 Nov 1998, Arnold Rots wrote:

> 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.

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
____________________________________________________________________________

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From: Arnold Rots <arots at head-cfa.harvard.edu>
Date: Mon, 9 Nov 1998 16:07:01 -0500 (EST)

> 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

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From: Patrick Wallace <ptw at star.rl.ac.uk>
Date: Mon, 9 Nov 1998 23:48:52 +0000 (GMT)

On Mon, 9 Nov 1998, Arnold Rots wrote:

> ...However, in practice (for observing), one

Proper motion apart, an "ICRS position at epoch 1998.85" is precisely the
same as an ICRS position at any other epoch, because the ICRS-defined
axes don't move.  There is no such thing as mean place any more.

Putting it another way, precession isn't involved until you want to
point a telescope.  Even then, in the new deal there is never any reason
to apply precession alone;  only the orientation of the Earth matters, and
the artificial separation into secular/long-period terms (precession) and
short-period terms (nutation) is avoided except perhaps as a computing
convenience.

Patrick Wallace

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From: Doug Mink <dmink at cfa.harvard.edu>
Date: Mon, 09 Nov 1998 19:06:12 -0500

> short-period terms (nutation) is avoided except perhaps as a computing

So if all we want to know is the position of an object in the image in
the
coordinate system of the reference stars, the only reasons to worry
about
the date of the observation are proper motion and conversion to another,
time-dependent coordinate system, such as FK4 or FK5?

-Doug

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From: Patrick Wallace <ptw at star.rl.ac.uk>
Date: Tue, 10 Nov 1998 00:19:12 +0000 (GMT)

On Mon, 9 Nov 1998, Doug Mink wrote:

> the coordinate system of the reference stars, the only reasons to worry
> about the date of the observation are proper motion and conversion to
> another, time-dependent coordinate system, such as FK4 or FK5?

Affirmative.

Patrick Wallace