The Napkin Representation
Anita M. S. Richards
a.m.s.richards at manchester.ac.uk
Tue Jul 3 08:23:37 PDT 2007
Silvia, Roy et al.,
It seems to me that there are two approaches here - one from the point of
view of people interested mainly in the time domain behaviour of variable
or erruptive sources, or of data (e.g. Solar) where the viewpoiint changes
as a function of time. The other aspect is those of us who are venturing
into the time domain as high angular resolution or sensitivity reveals
variability or allows us to study it - alongside other multi-wavelength
properties. That's what I am more familiar with, and variability is
usually a different function of time at different wavelengths - the sort
of thing that the Characterisation model should be able to cope with for
>> 2. The simplest time series table consists of 2 columns only: one
>> giving the time and one giving the observable that is varying in time.
>> Is the new representation aimed only at this very simple time series?
> The motivation above would mean that the observable is a photon flux through
> a given transmission filter, expressed as magnitude or Jansky. So the current
> target is not about the general "time series", but rather about the much more
> specific "light curve".
Two issues - if we are allowing magnitudes, can we also allow X-ray
counts? (I would like to see all data calibrated in physical units, but it
ain't, and for the history of variable objects, when there are very few
X-ray telescopes, we ahve to take what we can get).
Secondly, typically I want to label data with the waveband (radio) and
with the observing wavelength or wavelength interval (or better still
frequency, but that's not important) - there might be several observations
at one wavelength and several at another. Would this have to be in
separate data sets with the wavelength information in a header-type
structure, or in a 3rd column? If in a header-type structure, i.e.
keepinf 2 columns only, then it is even more important to maintain a
header structure which existing VO tools of all kinds will recognise.
>> For those that were not at the HTN meeting it would be useful to know
>> what are the objections against using a VOtable representation.
> Many people prefer a more rigid schema than VOTable provides, a more pure XML
> model. But I do not see VOTable as a necessary part of a standard, it is just
> a representation. It is the names of quantities and what they mean, that is
> the important part.
I am very very wary of additional formats; this has been suggested be fore
but the beauty of VOTable is precisely that it is very simple and the
average astronomer in the wild can get to grips with it, whereas the full
complexity of XML scares us off. More imporantly, VOTable is so widely
used that there are many tools in both astronomy adn solar physics which
use it. For example, one of the students here has just written a package
to raid the MERLIN archive for all data on X-ray binaries, process the
data with parselTongue, and convert the output to a VOTable of variability
(flux density v. time at various frequencies) which can then be merged or
compared with variability data from other observatories. The point is,
that this is not someone starting with the intention of using VO tools, it
is someone who happens to find them more useful that the alternatives,
since VOTables are more likely than any other format to be produced by the
other data providers and recognised by the tools he wants to use... so he
is surely not the only one.
Any format can be used within an observatory, etc., but whilst Roy is
technically correct to say
> Think of translating base 2 numbers to base 10, it is
> easy to do, it is just representation.
in practice, - as he said earlier - every extra bit of complication is
another pain in the posterior and a deterent to the casual user to string
another command in their workflow - and more seriously, every format
conversion is another opportunity to lose metadata.
So, I would argue that any format in which data are published should be
intelligible to tools which handle VOTables, without any special extra
tools, and that all the essential metadata defined in IVOA standards is
preserved in the VOTables.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dr. Anita M. S. Richards, AstroGrid Astronomer
MERLIN/VLBI National Facility, University of Manchester,
Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).
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