2 The ACL Format

catremote uses the ACL format developed by Allan Brighton and colleagues at ESO to access remote catalogues and databases. The ACL format is fully documented in SSN/75[3]. The following brief description merely gives sufficient details to allow the operation of catremote to be understood.

The ACL format is implemented using the Hyper-Text Transfer Protocol (HTTP) developed as part of the World Wide Web. catremote querying a remote database is an example of a ‘client-server architecture’, with catremote acting as the client and the remote database, or more exactly the program which accesses it, as the server. In the ACL format a so-called ‘configuration file’ mediates the interaction between client and server. This configuration file comprises a list of one or more databases, giving details for each. Usually each ‘database’ will be a simple astronomical catalogue. However, other alternatives are possible: archives, name servers, etc (Table 4, below, lists all the possibilities). Consequently, in this document the generic term ‘database’ is used to denote each entry.

catremote accesses a given configuration file and the databases which this file contains are the ones that catremote currently knows about.

The configuration file lists various details for each database, such as: the URL to access it, the type of database it is (its so-called ‘server type’: catalogue, image server, name server etc), the type of queries supported, etc. Most of these details are not germane here. However, one item which is important is the so-called ‘short_name’ or simply ‘name’ of the database. This quantity is used to identify the database, for example you would supply it in response to a prompt from catremote. It is a short character string (without embedded spaces) and conventionally it has the form:

where database is an abbreviation for the database and institution a standardised abbreviation for the institution where the on-line version is located. By convention institution has three or four characters. For example, the usual name of the version of the USNO¹ PMM astrometric catalogue maintained by ESO is usno@eso.