coneskymatch
: Crossmatches table on sky position against Cone Search service
coneskymatch
is a utility which performs a
cone search
(cone search is an IVOA standard protocol)
query for each row of an input table and concatenates the results
of all these queries together into one big output table.
You give the service URL
for the cone search server you wish
to use, and expressions (usually column names)
defining how to get the search parameters
(sky position and search radius) for each row of the input table.
The program then goes through the input table and dispatches
a cone search query to the server for each row.
For each of these queries the service should respond with a
VOTable containing the objects it knows about in the specified region;
hopefully the columns will be the same or very similar for
all the different queries since they are using the same service.
The response tables are stitched together top-to-bottom
(in the same way as tcat
)
and the result is output.
The effect of this is like doing a positional crossmatch where one of the catalogues is local and the other is remote and provides a cone search service. Because of both the network communication and the naive algorithm however, it is only suitable if the local catalogue has a rather small number of rows.
The 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 cone search 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.