New Spectrum DM document

[Arnold Rots]

Since I was supposed to be working on the STC document, I had not paid
much attention to the spectral model, yet, but it seems that it is
overdue for some comments regarding STC matters.  Here are some quick
comments.

Defaults:
I maintain that defaults are a very bad idea for the VO:
subcommunities have their own ideas about what are sensible defaults
and it will cause confusion if defaults are counter-intuitive.
The only "default" that is universally applicable and intuitive is
"UNKNOWN" and I would strongly recommend adopting this in the SDM.
It holds for, say, accuracy (if it's missing it's UNKNOWN) as well as
for coordinate systems.
The specification in STC says that UNKNOWN means that it is up to the
client to assign a default value - if the client so desires.
Adding something as simple as the four characters "ICRS" is hardly a
burden.

Adding "J2000" to ICRS is incorrect (or redundant, if you like)

On 4.4 and 4.5:
Why not use the STC formulation for these coordinates; after all, it's
space and time...

5.1.2
This discussion is not about the time frame, it is about how to
express a time coordinate.  It would seem to me that one could just
use STC's astronTimeType.
With all due respect, the argument about why ISO-8601 cannot be used
is bogus.  MJD starts well after 1 AD/CE, so is even more restrictive
than ISO-8601.

5.1.3/4
Why not use the RedshiftFrame as defined in STC?

5.1.5
Bullet 1: not correct - Cartesian is included in the Frames and MJD is
a format issue.
Bullet 2: I don't undestand this statement.  STC's RedshiftFrame does
not requre a CustomReferencePosition.
Bullet 3: What is the advantage of doing this differently?
Bullet 4: Why not?
Optional: I already stated my objections to specific defaults (rather
than UNKNOWN); I suspect the third bullet will especially get you into
hot water in this respect.

5.2.1 Coverage
Is also properly available in STC

5.2.2 Regions
This is a subset of STC-S


  - Arnold

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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 head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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