[fitswcs] Draft WCS Paper V: Time

[William Thompson]

The time paper includes the keyword OBSORBIT to point to an orbit ephemeris 
file.  I'd like to draw your attention to the paper "Coordinate systems for 
solar image data"

http://adsabs.harvard.edu/abs/2006A%26A...449..791T

which includes keywords for specifying ephemeris information.  The basic method 
is to specify a set of keywords

abcX_OBS
abcY_OBS
abcZ_OBS

where "abc" is a 3-letter code for the coordinate system in use, as given in 
Table  3, e.g. GEI for Geocentric Equatorial Inertial.

To make this system truly complete, one would have to specify how the coordinate 
system is calculated.  To some extent, this is handled by the proposed keyword 
PLEPHEM (e.g. DE405), but this still leaves a question as to how the ecliptic 
coordinates are defined.  There are three possible meanings:

Ecliptic of epoch:  An inertial coordinate system tied to a specific epoch, 
typically J2000.

Mean ecliptic of date:  Ecliptic coordinates corrected for precession.

True ecliptic of date:  Ecliptic coordinates corrected for both precession and 
nutation.

At first blush, this appears to be similar to the EQUINOXa keyword of Calabretta 
and Greisen, except that we're talking about the position of the spacecraft, and 
not the coordinates along any of the data axes.  However, I'm not entirely 
certain how one distinguishes between the mean ecliptic of date and the true 
ecliptic of date in Calabretta and Greisen.

Bill Thompson


Arnold Rots wrote:
> FITS WCS Paper V: Time
> ======================
> 
> Draft is available for comment
> ------------------------------
> 
> 
> I have the pleasure, on behalf of the team of authors, of (finally)
> announcing the draft of WCS Paper V on Time coordinates.
> 
> The group of authors has worked quite hard over the winter to get to
> this point where we feel it is ready for public discussion.
> During this time our dear and respected friend and co-author Pete
> Bunclark passed away. He put a lot of effort into this paper
> during the last months of his life and without him it would not have
> been where it is today.
> 
> You may find the draft (Version 0.71) and some useful links at:
> 
>     http://hea-www.harvard.edu/~arots/TimeWCS/
> 
> This version still needs some polishing and there are a few issues
> that are yet to be resolved. You will notiice editorial comments
> printed in red.
> 
> The intent is that this discussion be carried on through
> fitswcs at nrao.edu even though this announcement also goes out on
> fitsbits.
> 
> 
> Enjoy!
> 
>   - Arnold Rots
> 
> 
> --------------------------------------------------------------------------
> Arnold H. Rots                                Chandra X-ray Science Center
> Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
> 60 Garden Street, MS 67                              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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-- 
William Thompson
NASA Goddard Space Flight Center
Code 671
Greenbelt, MD  20771
USA

301-286-2040
William.T.Thompson at nasa.gov

(Note changed email address)