Comments on Spectrum Data Model 0.9
amsr at jb.man.ac.uk
Tue Sep 21 05:19:56 PDT 2004
It looks really good and I was pleased to see that this relates to
other Data Models, I have not done a full check for consistency but
that sort of thing gives me conficence that it is designed to be
used. I am also impressed by the coverage of errors etc.
I am still having trouble figuring out what parts of the model are
intended to apply per data set and which parts are per data point. By
analogy, I would expect that the Obs. data model could describe a data
cube of images with a single set of metadata, if each plane of the
image was the same apart from its content. Of couse, the local
statistical noise might be different in each plane but then in a
single image it might be differnt in diffeernt regions and we don;t
use different metadata for each pixel.
As I understand it, the Spectrum model suggests that each spectral
point, has centre, max and min of bin in spectral units (and other
metadata) and that is indeed required for some SEDs e.g. as provided
But in fact some IVOA examples given just give centre of bin e.g. for
many samples which are equally spaced in the specified units. In that
case it would be useful to have the (constant) bin width in the header
to aid extracting metadata for the Obs DM, Registry etc. Will this be
covered under Resolution? Moreover, other parameters such as the
available errors may be constant for the whole data set (or each point
may have its own errors).
i.e., I want to be able to get at information which says "Spectrum of
BigSupernova from 1719 to 1721 MHz in 2048 equal-sized channels"
which may then be a FITS file or ascii etc. with a header which gives
the metadata which are common to every spectral point but the data
itself is just a series of pairs of frequencies and flux densities.
If we are talking about e.g. 50 separate monitoring observations in
512 channels x 4 polarizations, all repeated for 10 surces, the
difference between using 2 numbers for each spectral point and about 6
starts to become significant when constructing spectra on the fly,
returning lots of spectra, trawling metadata or just intimidating the
There are more difficult situations - I am not arguing that they
should all be covered now, but which are and which are we deferring?
I am assuming in the above that the data have been fully calibrated
and there is not a separate bandpass function (i.e. sensitivity
changes with frequency) but that is not always the case. In any event
the user might want access to the bandpass correction function which
had been applied which might be a mathematical function or a look-up
table. Data may be stored in other formats e.g. all the spectral
coordinate information in the header and just a table with pairs of
values for channel No and flux density, or even a single vector of
It seems to me that, at present, a lot of data providers will have
some hard work to provide conformant data. That is not necessarily an
argument against the model (apart from the unneccesary bloat with I
will continue to heckle about!) but it does suggest that it might be
useful to define a very few aspects as "must" and offer help in
sorting out the rest. I would rather see that, than use alternative
`simplified' models like the Dobos-Budvari schema (Sect. 8.5) -
although some such simplification might be useful if in fact some
aspects of e.g. Coverage do actually belong in the Obs data model and
should be provided by a link rather than duplicated. There are
problems with the Dobos-Budvari schema as it stands, e.g. it is a
nightmare trying to manipulate a spectrum equally spaced in frequency
if you can only use wavelength units, although the latter might be
adequate for the Registry.
For the most recent/future data sets, I see that it model is applied
to SDSS; what do other people think, e.g. ISO, ALMA? I would also
like to know what Ivo Busko (SpecView designer) makes of it.
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Dr. Anita M. S. Richards, AVO 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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