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A number of different service types are defined and listed
in the registry; the ones that
TOPCAT currently knows how to access are the following:
-
Cone Search:
- retrieve entries in a certain region of the sky
from a remote catalogue
-
Simple Image Access (SIA):
- find image data (often in FITS format) in a certain region of the sky
from a remote image archive
-
Simple Spectral Access (SSA):
- find spectral data, usually in a certain region of the sky,
from a remote archive of spectral observations or theoretical models
Detailed technical information about these protocols can be found
at the IVOA web site (http://www.ivoa.net/) in the
Cone Search,
SIA and
SSA
documents, but these are by no means required reading for users of the
services.
These protocols, especially SSA and SIA, are quite complex,
and have many specialised and optional features.
Note that TOPCAT provides only a fairly basic level of interaction with
them, and many features (for instance SSA queries by wavelength,
or non-positional queries for theoretical data) are not accessible from it.
TOPCAT provides access to these service types in two main ways:
-
Single positional query
- If you enter by hand a sky position (RA, Dec) and radius,
you can download a table containing the results of a search for a
single (usually, small) region on the sky.
See the sections on
Cone,
SIA,
SSA
load dialogues in Appendix A.7.
-
Multiple positional query
- You can define how each row of an input table selects a region on
the sky. This will usually correspond to selecting a column for RA
and a column for Dec, and either a fixed radius or a column giving
the radius. Then a positional query can be made for each row of
the input table.
The result is effectively a join between the input table and the
remote table of object data, images or spectra.
See the sections on
Cone,
SIA,
SSA
multiple query windows in Appendix A.7.
Next Previous Up Contents
Next: Algebraic Expression Syntax
Up: Virtual Observatory Access
Previous: The Registry
TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page:
http://www.starlink.ac.uk/topcat/
Author email:
m.b.taylor@bristol.ac.uk