UCD use cases
Patricio F. Ortiz
pfo at star.le.ac.uk
Tue Oct 21 08:46:37 PDT 2003
here are my 2 cents.
On Tue, 21 Oct 2003, Roy Williams wrote:
> Please find below some possible use cases for UCD.
> -- How do the different proposed UCD/DM schemes help with these cases?
> -- What does the query look like in each case, and how would that query be
> -- Can you supply additional use cases? Please try to stay within the realm
> of the possible here!
> -- If you refer to a Data Model, please try to be concrete about
> who/how/example. Please do not simply invoke "the Future IVOA Data Model".
> Thank you
> (1) Cone search. How to decide which columns are the RA,Dec that was used in
> the search. What frame (B1950, J2000, ...) do these come from? If there are
> columns with ID, what sort of ID is it, and how do I resolve it?
pick RA and dec from UCDs: pos.eq.ra;main, pos.eq.dec;main
If the source is a VOTable, one can use the COOSYS element to figure out
If the source is plain metadata, we're missing the equinox element by using
the plain UCD. Within a DM, the equinox should be there and can be used to
solve the ambiguity. Now, if they are different, the question is whether
they should be precessed.
I assume that you mean src.object_id... Hmmm, invoke simbad or ned or any
name-solver with the appropriate output equinox. However, I would advocate
at this point in time to strongly avoid the existence of catalogues where
only the ID is provided. It would be an effort to solve those names for all
these catalogues, but once done, we don't have to worry about it anymore.
> (2) SIAP search. Find the column that contains the URLs where the images
> are. Find out if there are other columns that have RA,Dec of the image
I can see your point of differentiating RA/dec for sources as opposed to
observations in a log. RA/dec are coordinates regardless of what one
measure, so UCD1 didn't do anything about it. In the image catalogues,
pos.eq.[ra|dec];main represent the image pointing.
> (3) We have for example a crossmatch service that is clever enough to know
> about error ellipses. How does it get from a table the most sophisticated
> error info that is there: (a) position (b) circular error (c) ellipse error.
Thought about this one before :-)
Most complete case:
- semi-major axis error.pos.smaj
- semi-minor axis error.pos.smin
- ellipse orientation (PA) error.pos-ang
Alternatively, one could have ellipticity (error.ellipt) xor axis-ratio
(error.pos.ax-ratio) and major axis, in which case one has to compute
the semi-minor axis
Circular, simpler, we only need err.pos.smaj (or err.pos.rad)
Roy, please extend this case not just to errors, but to extended objects, as we
could be talking about overlap between galaxies or molecular clouds or
other extended structures/objects.
> (4) We want to compare photometry in two tables covering the same star
> cluster. How do I decide if they share measurements in the same filter? One
> has R band, the other has Halpha. What happens if fluxes are expressed
> differently -- eg number / energy /
> magnitude / luminosity density.
In a first approximation, I'd say stick to compare apples with apples and
oranges with oranges. UCDs should tell you which band is observed (oops,
that's UCD1s), therefore it should be clear whether the two tables contain
the same type of observations.
One interesting scenario here is to have a real Xmatch machine which
identifies the stars of one table with the ones with the other. I may want
to produce a diagram type R vs Halpha.
If more than two tables are involved, and one chooses one filter, we are
talking about forming light curves.
> (5) I want distances to stellar objects measured in meters, so I can make a
> 3D display for the children. How do I recognize a redshift (z) value, how do
> I recognize a radial velocity, how do I recognize an actual distance
Redshift, by its UCD: redshift.hc
radial velocity: veloc.hc (beware of radial velocities of expanding stars)
distance: phys.distance.true, drop anything measured in kpc or Mpc :-)
beware of things measured in km or au
Or, go to any catalogue measuring parallax and convert to distance.
convert your distances to meters (tool to be built)
> (6) I am looking for supernovae that have both optical and Xray
> measurements. Can I (should I) use UCD to help my search?
Optical and Xray UCDs appear not only in catalogues related to supernovae.
Look for catalogues with supernova in their title and then use the X-RAy
UCDs (SN are still discovered in optical, so they'll surely have optical
Look at a few catalogues with SNe for quantities which are proper of these
> (7) How do I find 21cm observations (that may be redshifted), which also
> have polarization information?
UCDs will help you with polarization. Try /pol./, 21cm for sure
frequency-wise: phot.flux.radio.1.4G (that's why we didn't use '.' to
separate the UCDs :-) but no assurance if the redshift is too high.
Scan the table title for /21/ && /cm/ if the radio UCD. I'd expect the
results to be nearly the same.
Note: I just used http://barbara.star.le.ac.uk/datoz/mykats.html to perform
the last search... I found a few cats with POL_ and RADIO_FLUX in 1.4Ghz
Sorry, no time to write the queries in a WLQL (wish list query
Patricio F. Ortiz pfo at star.le.ac.uk
Department of Physics and Astronomy
University of Leicester Tel: +44 (0)116 252 2015
LE1 7RH, UK
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