IVOA - Spectrum Data Model change request

[CresitelloDittmar, Mark]

Vandana,  (cc-ing the DM working group to document the discussion)

I've been looking into the changes you have suggested for the Spectrum_1.1
model.
  * May 2021 interop presentation
<https://wiki.ivoa.net/internal/IVOA/InterOpMay2021DAL/Desai_IVOA_SpectralModel_2021.pdf>

My immediate goal is to review the requested changes and the model
doc/schema, to put together a plan which I will present at the upcoming
interop.

As I understand it, there are 2 main changes:
  1) add 'order' to the SpectralAxis data content
      The current model supports a single 1-D spectrum, and has no concept
of spectral order.
      So, even if the data were separated into separate Spectrum instances,
the 'order' information would be lost.
      This request would add a hook which would allow data containing
multiple spectral orders to be separated in application.

  2) explicit support for Upper and Lower limits.
      The current model 'supports' upper limits via a specialized usage of
the Accuracy.StatErrHigh/Low elements.
      This is both cumbersome and technically incorrect.
      The request is to add the Upper Limit and Lower Limit concepts to the
Accuracy sub-model.
         * There was a brief discussion about this during the Spectral_2.0
work (I think).  If I remember
            correctly, these can better be considered specialized Measure
types, rather than a form of
            uncertainty (Accuracy/Error).
            In other words:  the FluxAxis.Value IS an upper limit.
         * I assume there is additional associated information which goes
along with the upper/lower limit
            but I'd consider that out of the scope of this request.
         * as an FYI: this is something I need to work for the Cube model
anway!

      I don't think we should spend much time now debating how upper/lower
limits should be modeled, but
      I'd appreciate some more information on how you'd expect these
elements to be populated.
         1) FluxAxis.Value: is this NULL?, 0?, or the determined
upper/lower limit?
         2) FluxAxis.Accuracy.UpperLimit: is this repeating the value? or
an independent value/column?
         3) FluxAxis.Accuracy.LowerLimit: same

And 1 less-critical request:
   3) move to VOUnit-1.0 rather than specify OGIP units compliance.

Please let me know your thoughts on the above, while I continue working out
the options.

As we move forward with this process, there will be 2 critical pieces for
which we will need support:
  1) some minimal iterating to refine the definitions of the new elements
  2) implementations!  It looks like you are in a good position to test out
any proposed changes in action.
Please let me know if you are willing/able to help out in this regard.

Mark
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