### 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:

database@institution

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.

1http://www.nofs.navy.mil/