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### 6.2 Data Access Services

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
Table Access Protocol (TAP):
  make free-form queries to a remote database using an SQL-like query language

Detailed technical information about these protocols can be found at the IVOA web site (http://www.ivoa.net/) in the Cone Search, SIA, SSA and TAP documents, but these are by no means required reading for users of the services. These protocols (apart from Cone Search) are quite complex and have many specialised and optional features. The options offered for Cone Search and TAP are reasonably complete, but for SIA and SSA TOPCAT only provides a fairly basic level of interaction, and many features (for instance SSA queries by wavelength, or non-positional queries for theoretical data) are not accessible from it.

Cone Search, SIA and SSA are positional protocols meaning that they query a single region of the sky. 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.9.
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 Apparent Table and the remote table of object data, images or spectra. See the sections on Cone, SIA, SSA multiple query windows in Appendix A.9.

TAP is a much more powerful protocol not restricted to positional queries, and has its own interface. See the TAP load dialogue section in Appendix A.9.

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Next: Algebraic Expression Syntax
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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
Mailing list: topcat-user@bristol.ac.uk