Time series data in VOEvent

[Arnold Rots]

Anita,

There is nothing wrong with using "HELIOCENTRIC" if that is what's
being used (although I agree with Steve that its use should not be
encouraged). The real problem occurs when people say "HELIOCENTRIC"
when they really use "BARYCENTRIC" - the difference is significant for
some applications.

Btw, when you say "J2000", do you mean "FK5/J2000" or "ICRS"? ;-)

  - Arnold

Anita M. S. Richards wrote:
> On Tue, 12 Jun 2007, Frederic V. Hessman wrote:
> > On 12 Jun 2007, at 3:49 pm, Steve Allen wrote:
> >> Please don't use adjectives "heliocentric" or HJD anymore.
> >> Everything IAU-approved is currently "barycentric".
> >
> > It may be "IAU-approved", but "barycentric" is not unambiguous: a 
> > "barycenter" is simply w.r.t. SOME (but not any particular) center-of-mass, 
> > so if you want the Solar System we'll have to come up with something like
> >
> > 	ucd="time.julian;time.barycentric;SolarSystem"
> >
> > (note that NONE of these are UCD1+).   The IAU may not used "HJD" anymore, 
> > but I know zillions of astronomers who do ;-)
> >
> > Rick
> 
> Yes, we have to be careful not to get confused between what we might be 
> trying to enforce as journal editors, supervisors/students, etc. etc., and 
> what the VO should do.  The VO should describe data as accurately as 
> necessary - just about everything possible is in STC, so it isn't hard to 
> figure out what to use.  If significant data use a heliocentric time frame 
> then we need to know what it is.  I don't know what is worse, to 
> mistakenly thnk that there are not data for something because the VO would 
> not publish data in the 'wrong' coordinate frame, or if the VO makes an 
> assumption that everything is in our favourite frame when in fact it's in 
> something different.  I am a scrupulous J2000, SI user as approved by the 
> IAU, but I guarantee that if I talk about a magnetic field in Tesla or a 
> number density in m-3, but did not specify the units, most astronomers 
> over 30 think I mean Gauss and cm-3.
> 
> We also run into possible conversion problems - I don't know if this 
> applies to time, so the VOEvent people can probably ignor the rest of this 
> para, but I am often faced with converting Heliocentric velocities into 
> VLSR, or B1950 positions into J2000.  But in both cases you need to know 
> the epoch of observation which is rarely available for old data, if you 
> want to be fully accurate.  A rule of thumb conversion is good enough for 
> registry searches but the actual data returned should be in the original 
> units unless the user specifies otherwise  - and only converted if the VO 
> can do it accurately to within the precisioin of the data (e.g. for IRAS, 
> the epoch is less important than for VLBI...)
> 
> cheers
> a
> 
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Dr. Anita M. S. Richards, AstroGrid Astronomer
> MERLIN/VLBI National Facility, University of Manchester, 
> Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K. 
> tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).
> 
> 
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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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