VOEvent Update: JSON and data models

[Arnold Rots]

Roy,

Ha! You are back :-)

I equally respectfully submit that the choice of "HJD" and "UT" is unwise
(and actually foolish), for the following reasons:

- Heliocentric Julian Day is not an allowed representation of a time stamp,
either in FITS, or in STC
- HJD is a confusing term in that it entangles (presumably, but not for
certain) a time representation with a time reference position
- HJD is (probably) a misnomer (though, who knows?); I suspect it really
refers to the solar system barycenter, rather than the heliocenter
- UT probably (but who knows?) is intended to refer to UTC, rather than UT
- UT is a poor choice to combine with anything that has JD in it, since it
can become ambiguous
- If indeed the barycenter is intended to be the time reference position,
TDB is the obvious choice, since UTC is not  a linear timescale at the
barycenter

In short:
It's an ambiguous (and lazy) shorthand for metadata that should be
specified more precisely and explicitly.
Instead, use:
- Time scale = TDB
- Timestamp representation = JD (or MJD, or ISO 8601, if you like)
- Time reference position = BARYCENTER
It probably helps to also specify the solar system ephemeris used, like
DE405

I may refer to the Time Metadata 101 that I circulated in May.

Cheers,

  - Arnold

-------------------------------------------------------------------------------------------------------------
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 cfa.harvard.edu
USA
http://hea-www.harvard.edu/~arots/
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On Sun, Oct 15, 2017 at 11:02 AM, Roy Williams <roy at roe.ac.uk> wrote:

> Dear Colleagues
> We respectfully submit an IVOA Note about some proposed improvements to
> the VOEvent standard. We would appreciate some discussion on this email
> address and at the upcoming meeting in Chile.
> Thank you for your attention
> Roy Williams, Scott Barthelmy, Eric Bellm, Matthew Graham, Rob Seaman
>
> ==================
>
> VOEvent Update: JSON and data models
> Author(s):
> Roy Williams, Scott Barthelmy, Eric Bellm, Matthew Graham, Rob Seaman
>
> UTL:
> http://ivoa.net/documents/Notes/VOEventJSON/index.html
>
> Abstract
> We propose an extension of the VOEvent format, to translate the packet
> from XML to JSON – with no semantic change. We also propose to use the
> VOEvent data model system to define three data-model Groups: “Light Curve”,
> “Associated Sources”, and “Followup Imaging”. This straightforward update
> of VOEvent simplifies the syntax and provides simple, standard
> representation of common astronomical datasets.
>
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