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

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.