.. _E_Overview: 

********
Overview 
********


What DISCOS is
==============

DISCOS is the control software produced for the Italian radio telescopes. 
It is a distributed system based on ACS (ALMA Common Software), commanding all
the devices of the telescope and allowing the user to perform single-dish
observations in the most common modes. 
As of today, the code actually implemented for the telescopes (i.e. excluding
the huge ACS framework) amounts to > 600000 active lines. 

This guide is meant to help the observer in the use of DISCOS, without dealing
with the “behind-the-curtains” complex details of the system. 
This release focuses on **single-dish continuum and spectroscopy
observations**, as the fully integrated backends available on-site are the
analog total power one and the XARCOS spectrometer. Since both the hardware and
software implementations are continuously updated, this manual will forcibly
undergo frequent revisions. 
 
Here follows a simple schematization of the observing system, helpful to
visualize all the main devices DISCOS deals with and the most important
operations it performs. 
*Notice: the features not yet available in this release are shown in brackets*.


.. figure:: images/DISCOSTasks.png
   :scale: 100%
   :alt: DISCOS overall tasks
   :align: center



Glossary: terms and abbreviations
=================================

:>: this symbol precedes commands to be given in the operatorInput panel
:$: this symbol precedes commands to be given in a system shell
:Beam-parking: the antenna points to a fixed Azimuth-Elevation position 
:Bin: frequency channel
:DF: Dual-Feed
:FWHM: Full-Width Half-Maximum
:HPBW: Half-Power BeamWidth
:MS: Minor Servo system, which includes the subreflector 
:Nodding (aka “beam switching”): it involves two or more feeds. The source is 
 alternatively observed with each of the feeds, so that there is always one 
 feed “on source”
:OTF: On-The-Fly acquisition. The antenna moves according to user-defined
 parameters, scanning the sky at constant speed. Data acquisition is active
 during the scan
:Position switching (aka “on-off”): the beam is alternatively pointed to the
 source (on) and to a properly defined offset position (off)
:Preset: mount mode allowing only beam-parking observations, with no pointing
 model applied
:ProgramTrack: mount mode allowing tracking, OTF/raster scans, beam-parking 
:Raster: acquisition is performed via discrete pointings (in tracking or
 beam-parking mode), planned to sample a certain path/area on the sky 
:Section: acquisition stream (aka “logical channel”). E.g.: the 2 feeds of the
 K-band receiver used with the continuum backend produce 2 feeds x 2
 polarisations = 4 sections  
:SF: Single-Feed
:Slewing: motion of the antenna when it is going to the target position.
 Slewing is always performed at maximum speed, with no data acquisition taking
 place
:Temporised commands: user-input or schedule commands that are launched to a
 specific UT time by means of the “@” symbol
:Time-tagged commands: when commands are not temporised, the philosophy for
 their execution is asap (as soon as possible), but the system internally
 schedules them associating each operation to a properly computed time. This is
 particularly relevant for the positions commanded to the mount, so that the
 antenna motions take place along a fully controlled path (except in slewing)
:TPB: Total Power Backend
:Tracking: the antenna points to a target and, if it’s a celestial target,
 follows its sidereal motion