On wt5tcs a number of high level operations processes are running. These provide for the co-ordination of the underlying software and monitor the health of the system.

Mode Switching (mswControl)

This the supervisory task for the whole telescope. Mode switching keeps track of the state of modules which are required to run the TCS. The global state of the system is determined by the mode switching module and is reflected on the main TCS panel.

Each subsystem of the TCS has a state and a sub state. The basic states are:  LOADED, STANDBY and ONLINE. The sub states depend on the module but for example the tracking LCUs can have sub states TRACKING, PRESETING, ERROR and others.

Starting and stooping the telescope is done via commands to mode switching, which in turn co-ordinates the activities such that they occur in a safe manner. For example, it is dangerous to switch the building "off" before switching azimuth "off" since the building can then be hit by the telescope. Mode switching ensures the correct sequence.

Preseting (prsControl)

This is the main interface to the telescope functions in normal operations. Preseting can be performed either by direct commands to the prsControl task or via setup files. All commands that determine the initial position of the telescope at the time the preset will be completed are sent directly to prsControl. The NTT can preset to a position determined by RA, Dec, Equinox, Epoch, Proper Motions, offset from RA, Dec, wavelength of observation, parallax of target, radial velocity of target, rotator angle offset and additional velocity of target in RA and Dec.

In addition, prsControl instructs the auto guiding and image analysis software to start automatically (or not) when the preset is completed.

Tracking workstation (trkwsControl)

Tracking workstation acts as a conduit to the tracking LCUs namely azimuth, altitude and rotators. The preset command is passed directly to the LCUs which actually perform all the arithmetic to convert the parameters specified above into an apparent Altitude and Azimuth position.

Any commands that should be performed while the telescope is tracking a target should be issued to the trkwsControl module. This module works with direct commands only and not via setup files.

Additional velocity changes, offsetting the rotator and offsetting the telescope are handled by trkwsControl.

Auto guiding workstation (agwsControl)

Auto guiding workstation co-ordinates the activities associated with auto guiding. It receives the set-up file from the prsControl module and determines where to position the guide probe so as to find the guide star selected. If no guide star is present in the setup file, then agwsControl will perform an "on the fly" search for a suitable guide star.

From mode switching, agwsControl receives the information about the active focus and therefore can determine which guide camera to instruct and which guide probe to move.

Note, that although in principle it is possible to drive the NTT directly from the LCU level, by doing so and bypassing mode switching certain functions such as auto guiding become extremely complex to configure correctly.

Image analysis workstation (actconControl)

The master process for image analysis, provides the means by which the calculated corrections for M1 and M2 can be passed to the appropriate LCUs for execution. The active optics system uses the mode switching module to determine which TCCD to listen to for data. In addition, the focus being used determines which adapter functions are to be controlled. Note that the switching of the pinhole, dichroic and reference light are under the control of the active optics software and are automatically switched as necessary. On the LCU one set of calculations is per formed and then send to the workstation. The calibrations applicable are FOCUS dependent and are resident in the LCU database. The same database is present on both LCUs (lt5iaa & lt5iab) and the software chooses the calibrations according to the focus. Another control process on the workstation actcalControl performs averaging of the data depending on the parameters of the active optics loop. In principle the active optics can reject a correction that is outside specified limits.

actconControl also provides the interaction with the preseting module to determine whether active optics is to be initiated immediately after the preset is finished and also prohibit a preset of the telescope while a force setting on M1 is taking place.

The force setting parameters can either be calculated by the active optics or by the M1 module which resides within the M1 LCU. This permits then default force set tings without the need for active optics to be functioning.

Note that, image analysis in closed loop is the default mode of operation for the NTT.

Common software.

The VLT control system is built on a large layer of common control software (CCS) which in turns lives on top of the HP RTAP software. Another common module is provided by LCC, the CCS equivalent running on the LCUs.

Communication between the LCUs and the workstations is done either via messages or via database reads and writes. To this purpose, the database on the LCUs (or parts thereof) is SCANned into the RTAP database on the workstation.

Scans can be triggered on change or by polling at a fixed frequency. On the NTT system we have chosen, as much as possible, to poll the database for information since this provides for a fixed system load. As a result, some values such as the sidereal time on the workstation are not updated at very high frequencies.

LCU processes

LCUs start a number of processes at boot time and certain others when the software initializes. Common process to all LCUs are lccServer, msgServer and rdbServer. The actual application code is then LCU dependent.

On the tracking LCUs you may find two extra processes, the axis task and the trk task. So for example on the azimuth LCU (lt5az) you would find the task azServer and aztrkServer. The azServer actually controls the axis, reads the encoders etc. etc., while the aztrkServer performs all the calculations necessary to deliver an angle to which the azServer needs to position the telescope. The trkServer processes are identical on all tracking LCUs. That is to say that there is no distinction as far as he tracking software on the LCU is concerned whether it is connected to one of the rotators or the azimuth axis.

On the LCUs connected to the CCD cameras application software also runs. The autoguider error vector conversion to RA and Dec is done in the LCU as are ce