The help system for the TNG telescope and instruments control system is embedded in the User Interface of the WSS, whose preliminary structure has been defined in . Additional documents on the TNG WSS [2,3,4] are available for reference.
Back in 1992, a hypertext-based context-sensitive help system  (version 1.0) has been built. The help files were plain ASCII files where special characters (backslashes and double dots) defined the hypertext links: the links could be represented as branches of an information tree, which could be navigated in both directions. Pressing the HELP button on a window of the TNG TUI, a help window would pop on the screen. The help window was formed by three areas: the menu bar, the help text and the sub-topic list. The menu bar contained two buttons needed to navigate backwards in the help tree: the Index button took the user to the first screen of the help file (the main topic), the Back button took the user one level back in the help tree. The success of the HTML protocol since 1993 made this implementation of the TNG help system obsolete.
The current implementation  (version 2.0) of the help system for the TNG exploits a high level of integration between WSS (the software controlling the telescope and its instrumentation) and an HTML browser. Actually, the browser used for this purpose is NCSA Mosaic v2.4. This will certainly change in the near future while more capable and powerful browsers are made available. A method of communication between the TNG TUI and Netscape Corp's Netscape Navigator is being developed, and will be the method of choice in the next TUI release.
In the following the current method (TUI and NCSA Mosaic) is described. Clicking on the HELP button of any interactive panel will pop up Mosaic with the appropriate help page, since the WSS panels have a help button pointing to a specialized URL. This means that Mosaic can be also used to browse information stored wherever on the Internet (provided that this possibility is given to TNG observers); it will focus again on the TNG help system as soon as a HELP button is clicked again.
A representation of a workstation screen containing the telescope control window and a help window implemented with Mosaic is shown in Figure 1.
Figure 1: Representation of a workstation screen containing the main Galileo User Interface window, the telescope control window, and a help window implemented with Mosaic. The help window was obtained by clicking the help button on the main Galileo UIF window (upper left).
There is a high level of reciprocal control between HTML browser and
WSS: the Mosaic client uses scripts to control the WSS, while the
WSS controls Mosaic using the remote control mechanism of Mosaic
. This system allows Mosaic's signal handler to detect the
``extra" signal SIGUSR1, to determine the process id of the issuing
process, and to derive a config filename containing a directive (either a
goto to jump in a document or a newwin to open a new window
where to display a document) and the URL of the document to download and
Figure 2: Scheme of the archive system at the TNG