Spectral 1.1 to 2.0 "cheat-sheet"

[Markus Demleitner]

Dear DM Group,

On Fri, Aug 08, 2014 at 02:22:06PM -0400, Laurino, Omar wrote:
> It was at some point requested that a "cheat-sheet" on how to upgrade
> implementations from Spectrum 1.1 to Spectral 2.0 be created to help the
> transition from one version to the other.
> 
> Jamie Budynkiewicz and Mark Cresitello-Dittmar produced such document, and
> here is a pointer to it:
> http://wiki.ivoa.net/internal/IVOA/Spectral2RFC/map-document-new.pdf

Thanks for that -- most useful, in particular as  I've not really
followed the development of the standard.

However, I'm a bit confused on one point (though I suspect that's
mainly due to utypes): The changes document says the
Char.TimeAxis.Coverage.Bounds.* fields are mandatory.  However, the
standard, in 7.1.2, says that both TimeAxis and SpatialAxis are
optional ("None may be defined) -- as they must be, given that a large
part of the spectra currently available in Spectral 1.1 are
theoretical spectra that did not result from an observation and hence
are not characterised in either time or space.

So, I guess the answer to that little riddle is that the Bounds
fields are "conditionally mandatory" ("*if* you have a TimeAxis,
then..."), right?

Slightly related: If we make Target.Name mandatory (and I'm always
for doing away with optional things), I guess some (non-normative)
advice on what to do in case of generated spectra (in 2.11.1) would
be useful.  As I'm not quite certain what the intended use of
Target.Name is, I don't feel confident as to what a good solution
would be, but possibilities that come to my mind include:

* Assign different names to the outcomes of different computations OR
* Assign identical names to the outcomes of computations with the
  same physics OR
* Have a constant target identifying the code ("Spectrum computed
  with Magic 3.23pl1") OR
* Just have "Artificial" in there always if there's no physical
  object

Each has advantages and disadvantages for certain scenarios.  Without a
strong advice here I expect the providers of theoretical spectra will
have a wild mixture of policies (prediction: most of them will have
an empty string in that "mandatory" field), and we'd have all the
disadvantages and none of the advantages, and that'd be a shame.

Cheers,

       Markus