coneskymatch: Crossmatches table on sky position against remote cone service
Note: this command is very inefficient for large tables,
and in most cases
provide better alternatives.
coneskymatch is a utility which performs a
query to a remote server for each row of an input table.
Each of these queries returns a table with one row for each
item held by the server in the region of sky represented by
the input row.
The results of all the queries are then concatenated into one big
output table which is the output of this command.
The type of virtual observatory service queried is determined by the
Typically it will be a
Cone Search service,
which queries a remote catalogue for astronomical objects or sources
in a particular region.
However, you can also query
Simple Image Access and
Simple Spectral Access services
in just the same way, to return tables of available
image and spectral resources in the relevant regions.
The identity of the server to query is given by the
Some advice about how to locate URLs for suitable services is given
in Appendix B.3.3.
The effect of this command is like doing a positional crossmatch where one of the catalogues is local and the other is remote and exposes its data via a cone search/SIA/SSA service. Because of both the network communication and the necessarily naive crossmatching algorithm (which scales linearly with the size of the local catalogue) however, it is only suitable if the local catalogue has a reasonably small number of rows, unless you are prepared to wait a long time.
parallel parameter allows you to perform multiple
cone searches concurrently, so that instead of completing the
first cone search, then the second, then the third,
the program can be executing a number of them at once.
This can speed up operation considerably, especially
in the face of network latency, but beware that submitting a very large
number of queries simultaneously to the same server may overload it,
resulting in some combination of failed queries, ultimately slower runtimes,
and unpopularity with server admins.
Best to start with a low parallelism and cautiously increase it to
see whether there are gains in performance.
Note that when running,
coneskymatch can generate a lot
of WARNING messages. Most of these are complaining about badly formed
VOTables being returned from the cone search services. STILTS does its
best to work out what the service responses mean in this case,
and usually makes a good enough job of it.
Note: this task was known as
multicone in its experimental
form in STILTS v1.2 and v1.3.