Non-sky MOCs

[Mark Taylor]

Pierre, Ada and MOC authors,

the VODataService 1.2 PR
(https://wiki.ivoa.net/twiki/bin/view/IVOA/VODataServiceV12RFC)
is planning to permit use of MOCs for coverage information,
and currently contains the text:

   "By default, the MOCs are to be interpreted in the ICRS. 
    Future extensions to non-celestial frames (e.g., on planet surfaces) 
    will use the frame attribute. However, the only celestial reference 
    system allowed is ICRS."

I don't think that this conflicts with the spirit of the MOC text
(no galactic/ecliptic sky coordinate systems allowed), but the 
existing MOC 2.0 draft currently has language like:

    Consequently, the SMOC MUST be based on the HEALPix tessellation
    of the sphere, expressed in the ICRS coordinate reference system.

which appears to exclude the possibility of using MOC with any non-sky
coordinate system.

Could we consider relaxing the language to make it clear that MOC
use on the sky is required to use ICRS, but future applications to
non-sky domains are not prohibited?

Mark

--
Mark Taylor  Astronomical Programmer  Physics, Bristol University, UK
m.b.taylor at bristol.ac.uk          http://www.star.bristol.ac.uk/~mbt/