\parindent 0pt
\parskip 0.3cm
%\section{User-Supplied Instruments}
\section{USER-SUPPLIED INSTRUMENTS} \label{ch16:usi}

The Acquisition and Guidance Unit at Cassegrain includes an instrument 
mounting plate to which user-supplied instruments can be attached. The
overall dimensions of the A\&G Unit are 
2.26{\tt m}$\times$1.38{\tt m}$\times$0.50{\tt m}; the maximum loading is
500~{\tt kg} on the outer columns and 200~{\tt kg} on the circular central
pattern of holes (see figure \ref{ch16:fig1}). Test carried out with a
500~{\tt kg} load showed that the maximum shift (flexure)
produced at the Cassegrain
focus when moving the telescope through 90\degs, from a horizontal to a
vertical position, is 20~$\mu${\tt m}.

Figure \ref{ch16:fig2} shows the maximum dimensions possible for instruments
to be attached to the A\&G Unit.

The optimum focal position is 150~{\tt mm} below the back surface of the
instrument mounting plate. The total movement of the focus about this position,
determined by the the maximum travel of the telescope top end, is 
$\pm$729~{\tt mm}. The aberrations in the image resulting from a focal shift
are shown in figure \ref{ch2:fig7} (chapter 2). Note, however, that the
A\&G Unit TV camera and autoguider are set to be in focus at the
optimum focal position. Instruments requiring a different focal distance can
be accomodated by inserting an appropriate lens in the holder of the TV probe
(where no lens is required at the nominal focus) and by substituting the
reducing lens in the autoguider probe. Observers requiring this flexibility
must provide their own lenses and arrange for their use well in advance of an
observing run.

User-supplied instruments may be controlled from the PE~3210 Instrument
Computer (see section \ref{ch9:PE3210}).

\begin{figure}
\vspace{21.0cm}
\caption{The Cassegrain A\&G Unit: Instrument Mounting Holes}
\label{ch16:fig1}
\end{figure}
\begin{figure}
\vspace{21.0cm}
\label{ch16:fig2}
\caption{The Cassegrain A\&G Unit: Space Restrictions for Instruments}
\end{figure}
