\newpage
\section{\NCS\ Explained}
\label{sec-explained}

In this section we explain in more detail 
some general aspects of the NCS.

%%%%%%%%%%%
\subsection{Coordinate Systems, Map Projections, and Position Offsets}
\label{sec-explainSysProOff}

\input{inc/pakoIncSysProOff.tex}

\input{inc/pakoIncSysProOffMore.tex}

\notes.
More explanations about coordinate systems, projections, and how to define
descriptive coordinate sytems will be added as they become available
for general use.

\input{inc/pakoIncRemarkOnOffsets.tex}

%%%%%%%%%%%
\subsection{Azimuth Topology}
\label{sec-azimuthTopology}

\input{inc/pakoIncTopology.tex}

Note that \command{SET TOPOLOGY} only has an effect for sources with an
azimuth in the overlap range; when a source is in the azimuth range
100 to 420 degrees \command{SET TOPOLOGY} does not matter.

Compare Figure \ref{fig-azimuthtopology}.

\begin{figure}
  \includegraphics[angle=0,width=16cm]{azimuthTopology.pdf}
\caption[azimuthTopology]
{
Azimuth Topology. The left side shows in blue the range for 
\command{SET TOPOLOGY LOW}, 
the right side shows in red the range for 
\command{SET TOPOLOGY HIGH}.  
(Compare Section \ref{sec-azimuthTopology}; figure prepared by Joaqu\'in Santiago)
}
\label{fig-azimuthtopology}
\end{figure}


%%%%%%%%%%%
\subsection{Switching Modes}

\input{inc/pakoIncSwitching.tex}

\input{incDemo/pakoDemoSwitching.tex}

%%%%%%%%%%%%%%
\subsubsection{\swb}

\swb\ is realized through a rotating chopper wheel in the receiver cabin,
which during each rotatation (= switching cycle) moves 2 reflecting (!)
blades into the beam path in front of the 4th mirror, for a total of 4 phases:
direct beam path to the source (same as in \total!),
beam path offset by one blade of the chopper wheel,
direct beam path to the source (same as in \total!),
beam path offset by the other blade of the chopper wheel.
The offset, rotation period, and blanking times are fixed.
\swb\ is normally only used for \pointing\ and \focus.

The source signal is calculated as difference between the
direct and offset phases.

(\notes. This ``beam-switching'' chopper wheel, should not
be confused with a ``calibration chopper wheel'' as it is used
at some other mm-wave observatories.  
Calibration at the \thirtyM\ is done with different hardware.)


%%%%%%%%%%%%%%
\subsubsection{\swf}

\swf\ switches between 2 different frequencies, so that 
there are 2 phases.  The (source) signal is calculated
as the difference between these 2 phases.

\swf\ is normally used with \track\ or \otfmap.

\input{inc/pakoIncSwf.tex}


%%%%%%%%%%%%%%
\subsubsection{\sww}

During \sww\ the wobbling secondary mirror is switched between 2 positions, 
which are offset from the telescope axis by $\pm$ a fixed amount. 

Thus there are 2 phases, and the signal is calculated as the difference between
these 2 phases.  

As the positions in both phases are offset from the telescope axis,
in some observing modes, e.g, \pointing, \focus, \onoff,
the telescope position needs to be adjusted 
to compensate --- this is done automatically.

\sww\ is normally used with \onoff\ or, for bolometer continuum mapping,
with \otfmap.

Note that for \otfmap\ with \sww, special restoration
algorithms are needed to recover an image of the source brightness
distribution. These are available, e. g., in the MOPSIC software.

If \sww\ and \onoff\ are combined in the standard way, we
effectively take data at 3 positions:
1. the source position;
2. the \command{source position + throw}  (offset in the ``true-angle'' horizontal system);
3. the \command{source position - throw}  (offset in the ``true-angle'' horizontal system),
with \command{throw = ABS(wOffset2-wOffset1)}.
Data from 1. are treated as source signal, data from 2. and 3. as
off-source reference signal.  Note that in the astronomical
coordinates, positions 2. and 3. will rotate around the source position
1.  Therefore one must normally be sure that the extent of the source
is less than \command{throw-beamWidth (/2)}.


%%%%%%%%%%%%%%
\subsubsection{\total}

\input{inc/pakoIncSwitchingTotal.tex}

Normally, when using \total\, one or several positions ``ON'' the source
are observed alternating with one or several ``OFF''-source reference positions,
and the signal is calculated as the difference between ``ON'' and ``OFF''.


%%%%%%%%%%%
\subsection{Observing Modes}

\input{inc/pakoIncOM.tex}


%%%%%%%%%%%%%%
\subsubsection{CALIBRATE}

\input{cook/pakoCookCalibrate.tex}

The special calibration subscans are done by switching the
beam optics so that the receivers see a special calibration unit
in the receiver cabin.


%%%%%%%%%%%%%%
\subsubsection{POINTING}

\input{cook/pakoCookPointing.tex}


%%%%%%%%%%%%%%
\subsubsection{FOCUS}

\input{cook/pakoCookFocus.tex}


%%%%%%%%%%%%%%
\subsubsection{TIP}

\input{cook/pakoCookBolometerTip.tex}


%%%%%%%%%%%%%%
\subsubsection{ONOFF}

\input{cook/pakoCookOnoff.tex}
\input{cook/pakoCookOnoffSwWobbler.tex}
\input{cook/pakoCookOnoffSwWobblerTwo.tex}


%%%%%%%%%%%%%%
\subsubsection{OTFMAP}

\input{cook/pakoCookOtf.tex}


%%%%%%%%%%%%%%
\subsubsection{RASTER}

\texttt{<<}
\command{RASTER} (not yet implemented),
will be similar to \command{ONOFF} and \command{TRACK} but allow to observe several 
``ON'' positions. 
To be used with \total\ or (maybe) \swf.
In any case it is recommended to use \onoff\ instead of \raster\
with several ON per OFF.
\texttt{>>}


%%%%%%%%%%%%%%
\subsubsection{TRACK}

\input{cook/pakoCookTrack.tex}


%%%%%%%%%%%%%%
\subsubsection{VLBI}

\command{VLBI}.
a special observing mode to track the source position specified with
\command{SOURCE} during VLBI scans. 
It should only be used for VLBI and is always used with \total.



%%%%%%%%%%%%
\subsection{Receivers}
This section remains to be written.  For the time being please refer to the
information available on the web pages about the \thirtyM.


%%%%%%%%%%%%
\subsection{Backends}
This section remains to be written.  For the time being please refer to the
information available on the web pages about the \thirtyM.







