-------- Original Message --------
Subject: UCDs and VOEvents
Date: Wed, 27 Apr 2005 18:24:56 +0200
From: Andrea Preite Martinez <andrea.preitemartinez at rm.iasf.cnr.it>
To: roy at cacr.caltech.edu
Roy, can you please distribute this message to the members of the VOEvent
forum?
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Dear Roy, dear members of the VOE group,
> Dear Andrea and Sebastien
> This note is to ask the UCD working group for help in building a new
section of the UCD
> vocabulary in support of the VOEvent working group.
> ...
thank you for your very interesting and challenging proposal!
We think the UCDwg can be the right place to discuss all semantic aspects of
the VO,
not only those related to "bona fide" UCDs.
Of course, we should clearly keep on the right track, as already defined
in the last years, for what concerns the UCDs, their definition,
their semantics, their syntax, their usage.
For the main document on UCDs we have to go on from the stage of
Proposed Recommendation to the formal stage of IVOA Recommendation.
We also have to upgrade the (new!) list of ucd-words from the level of
Draft to that of PR, through a formal RFC.
We can do this in Kyoto.
Your proposal of opening a discussion in the UCDwg
> with the objective of
> describing "immediate astronomical events" in a semantically-meaningful
way
> ...
> covering astronomical event types
is, as we said, both interesting and challenging.
Interesting, because
- it is a problem that has been tackled many times giving rise to a
number of "subjective" solutions (each one valid/sound in its own
context):
it is time to discuss it starting from the work already done,
but trying to do it in a "semantically-meaningful way"
- it has to do with a semantic problem that is hard to locate in/assign to
other IVOA WGs;
- the UCDwg has the right competences to do the job.
Challenging, because
- definitely hard to do
- not to be mixed up with UCDs, that are ment to serve other purposes.
The "mail storm" produced by your proposal is a clear indication of
interest,
but it also tells us that the discussion has to be kept within clear and
firm boundaries, having in mind the distinction between UCDs and
their role on one hand, and the "event vocabulary" on the other.
In conclusion, the answer to your questions:
> Could you help us with doing this?
> Perhaps a meeting at the Kyoto IVOA would be appropriate?
is yes, we can help,
yes, the final part of the UCD session in Kyoto will be devoted
to the "event vocabulary".
All VOEvent people are of course invited to this session.
A tentative agenda could be:
- A short presentation of the problem (somebody from the VOEwg has to
volunteer for that!
Please, let me (=>Andrea) know.)
- Plenary discussion, actions.
In order not to diverge too much before the start of the discussion
in Kyoto, here you can find a few remarks/comments on
http://monet.uni-sw.gwdg.de/twiki/bin/view/VOEvent/UnifiedContentDescriptors
> Unfortunately, the IVOA/UCD list is incomplete: there's no complete
> wavelength/frequency coverage in "em.*", for instance (e.g. no
em.microwave).
The UCD coverage of the electromagnetic spectrum *is* complete. The
top-level wavelength/frequency division simply follows the one adopted for
the
Registry description for more compatibility, that is 7 large bands:
radio, mm, IR, optical, UV, X-ray, gamma-ray.
See "Spectral Coverage" in
http://www.ivoa.net/Documents/REC/ResMetadata/RM-20040426.html
Therefore, there is no top level like EUV, sub-mm or microwave, but
I guess microwave is covered by the following UCD :
em.radio.1500-3000MHz, em.radio.3-6GHz, em.radio.6-12GHz, ...
It is important to first notice that the description of the
electromagnetic division in the em.* words of the UCD is a special case:
it can be seen as an enumeration of possible values for the
observation passband.
Take for example the description of an IRAS 12 micron flux in VOTable.
We can write it:
<FIELD name="flux" ucd="phot.flux;em.IR.8-15um">
But we could have written:
<FIELD name="flux" ID="col4" ucd="phot.flux"/>
<GROUP>
<PARAM name="band" ucd="instr.passband" value="IRAS 12 micron"/>
<FIELDref ref="col4"/>
</GROUP>
The introduction of all the em.* terms in the UCD vocabulary was done
to allow the first description, instead of the 2nd that is less standard,
because different people would use different values for the "bandpass"
description (12um, IRAS 12 micron, 8.5-15micron, ...)
The em.* words can be seen as named instances of the generic
"instr.bandpass" UCD.
These words were introduced into the vocabulary for two reasons:
- they are absolutely needed: most astronomical information comes from
photons, and we need to expres in which part of the spectrum a flux
or magnitude was measured.
- the problem is relatively simple, as it roughly only implies defining
intervals on a one-dimensional axis.
The astronomical object types haven't been so far introduced in the UCD
vocabulary for several reasons:
- the main use of UCDs is to describe table columns. The existing
"src.class" and "meta.code.class" UCDs were sufficient for this purpose,
allowing to describe that a catalogue's column contains an astronomical
source classification. This classification
(often a numeric code for UCD meta.code.class) is what the author
of the catalogue writes, and is not standardized.
- Even for the description of a single measurement, we should avoid
putting all the metadata into a UCD.
We'd hate to see that value 5.23mag is described by
ucd="phot.mag;em.opt.V;object.star.supernova.individual.SN1987A;time.epoch.1990"
Instead, we'd rather say (in VOTable-like syntax):
<GROUP>
<PARAM ucd="phot.mag;em.opt.V" value="5.23" unit="mag"/>
<PARAM ucd="time.epoch" value="1990-02-23"/>
<PARAM ucd="meta.id" value="SN 1987A"/>
<PARAM ucd="src.class" value="Supernova" />
</GROUP>
- The precise definition of object types is very complex. There are
examples in the journal keywords, but also in SIMBAD:
http://simbad.u-strasbg.fr/guide/chF.htx
or the IAU thesaurus
http://msowww.anu.edu.au/library/thesaurus/english/
...
The object type definition is difficult, because we often have to deal
with hierarchical structure/classification, and an object can belong
to different classes: a galaxy can be at the same time:
- galaxy
- galaxy in cluster
- IR source
- radio source
- X-ray source
- emission-line galaxy
- galaxy with active nucleus (AGN)
- ...
The classification can depend on the background of the astronomer
(e.g. radio or optical astronomer), and in some cases the boundaries between
different classes are fuzzy...
(Also !) for these reasons, the definition of standard words for object
types
was deliberately left out of the UCD vocabulary.
But it might be useful to create a new "object" branch,
to enumerate a list of possible standardized object types.
A careful explanation of each term should be provided, so there is no
ambiguity:
e.g. does Planetary_Nebula mean the central star, the nebula or both?
This list of terms would correspond to an enumeration of possible
values corresponding to ucd="src.class".
In the same spirit, the "events" are poorly described with the current
ucd words. There are a few ones, like pos.lunar.occult, phys.mass.loss
or src.var.pulse, but they can certainly be better described by domain
specialists.
Andrea
Sebastien
==============================================================================
Andrea Preite Martinez andrea at rm.iasf.cnr.it
Istituto di Astrofisica Spaziale Tel.:+39.06.4993.4641
Area di Ricerca di Tor Vergata Fax.:+39.06.2066.0188
Via del Fosso del Cavaliere 100 Cell:+39.339.3817355
00133 Roma
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