Inter processes Communication

Data flow Front End.

The front end of the data flow system is shown in the schematic diagram here. The output from the Phase 2 Proposal Preparation (P2PP) process is stored in a data base in Garching. The P2PP process can be run either on La Silla or in Garching on at the home institute of the observer. The alignment of the P2PP and the control system is performed by the usage of common Signature Files (tsf). The tsf files cannot be modified without violating the output of P2PP.

This is a critical point to realize. If a change in made in the tsf files in Garching or on the mountain then unless the tsf files held on the instrument workstation are also updated the system will not work.

The P2PP database can be accessed either via the P2PP tool (for modifications) or via the Observation Tool (OT) for execution. The observation tool passes the relevant parameters to BOB (Broker of Observation Blocks). The parameters are held in memory on BOB, which runs on the instrument workstation, unless you wish to save them on a local file which would be called an OBD file (OBservation Description).

VLT control system (VCS)

The high level control of the control system is provided by the Observation Soft ware (OS) running on the instrument workstation. The OS talks to three basic subsystems ICS (Instrument Control Software), DCS (Detector Control Software) and TCS (Telescope Co-ordination Software).

BOB sends SET-UP commands to OS which then distributes them to the subsystems using sequencer scripts (seq. files). BOB occasionally (and if one where to be strict illegally) reads the status of subsystems directly. This is a philosophical illegality since the system was designed spefically to permit this action. The underlying principle has been to minimize the possible interfaces and therefore whenever possible commands and status requests are routed via OS.

The diagram shown here,  has the names associated with running EMMI. For SUSI the names will be a little different and can be found in the rest of this document.The TIF interface to the TCS is done by linking the function directly into the OS as a library.

Telescope Co-ordination Software (TCS)

Here the level of abstraction is even higher (see diagram). A large number of lower level interactions exist that can be found in the VLT documentation. Here is the preset.

Auto guiding process command sequence.

When a preset is sent to the telescope either via TIF or directly the prsControl module does a catalogue search (based on the preset position - including the rotator angle) and populates the database. The preset command is forwarded to agwsControl and the database is scanned for guide stars. If none are found another search is performed. The agwsControl module calculates the position of the guide probe for each of the selected guide stars and rejects ones that are in positions not allowed (i.e. beyond the mechanical limits of the adapter).

The agwsControl module will pick the brightest star in the list that is accessible and issue a PRGS (probe to guide star) command which sends an explicit move to the guide probe.

Once the preset is completed and the telescope is tracking the user can issue a catalogue search command. This uses the current position and angle of the telescope to search for guide stars. This is used when the user has offset the telescope (in RA, Dec or Rotator Angle) following the completion of the preset. The new guide stars are selected. The user can then request an explicit PRGS  (see diagram). These actions are performed using the autoguider GUI described later.

Instrument exposure

The OS works by sending SET-UP commands to the instrument and detector. Except when commanded by BOB when it also sends SET-UP commands to the telescope. Additional commands are available on the OS from the links to TIF which allow the instrument to issue explicit commands, such as focus offsets and telescope offsets  (see diagram).

Data flow back-end (DFS)

Once the data have been written to the FITS file the data flow back-end takes over. The file creation on the instrument workstation creates a symbolic link to the file. A process called vcs-olac (VLT-Control-System-OnLine-ArChive) transfers the image from wemmi (the instrument workstation) to wg5arc (the archive workstation). The diagram shown here, has the files in the boxes and the processes in the arrows.

The transfer to the archive workstation is handled by a package called imp. The same system is used by the subscriber processes that can request the data from the archive workstation. Two subscriber processes run at the NTT. One is the doSubscribe (Data Organizer subscriber) which is run on the wg5pl (pipeline) workstation and another dhsSubscribe which is run from the nttastro account on wg5ops which transfers a copy of the data for the astronomer.  

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