ImageDM - ObsCore Convergence

[Douglas Tody]

It was agreed in the fall 2013 interop to do whatever is necessary to
bring the Image data model into convergence with ObsCore (e.g., ObsTAP),
in particular making data models such as ImageDM and SpectralDM
extensions of ObsCore, all sharing a common core data model.  We are
fast-tracking an updated version of both the ImageDM and SIAV2 drafts
that will make a first cut at this convergence, sufficient to support
high-priority ongoing development this fall for the Cube initiative,
while further development of the working drafts goes forward in parallel
leading to the spring 2014 interop.

An initial analysis of what is required has been performed by a subset
of the ImageDM author group (FB, ML, MCD, DT, including all directly
involved in the related data models) and the results are summarized on
the ImageDM TWiki page:

     http://wiki.ivoa.net/twiki/bin/view/IVOA/ImageDM

In particular we generated the following as part of the analysis:

     Initial analysis of ImageDM-ObsCore convergence (all)
     Spreadsheet summarizing the ImageDM and ObsCore differences (DT)
     ImageDM and ObsCore Utypes vs Char2 (FB)

The consensus at this point appears to be that we finalize a set of
Characterisation V2 Utypes, use these for both ImageDM and SpectralDM,
and update the older ObsCore/ObsTAP document to comply. This will take a
while, but all we need for ongoing Cube development this fall is the
updated ImageDM and SIAV2 drafts plus a summary of the revised Utypes
planned for the updated version of ObsCore.  Our prototypes will use the
updated versions of all of these.

The next step will be to finalize the affected set of Utypes (about 20
or so Utypes differ between the current ImageDM, ObsCore, and Char2).
After that we will have what is needed to update the current ImageDM and
SIAV2 drafts.   A related effort is already underway within DAL to
update other aspects of SIAV2 and a new generic AccessData protocol.