\subsection{A-Frame Detector Motor}

This is an MX motor driver for the pseudomotors used by Gerd Rosenbaum's
A-frame CCD detector mount.  The geometry of this detector mount is shown
in the following figure:
\begin{figure}
\MxPicture{aframe}
\caption{A-frame CCD detector mount designed by Gerd Rosenbaum}
\end{figure}

The pseudomotors available are:
\begin{itemize}
  \item \textit{detector\_distance} - This is the length of the line
  	perpendicular to the plane containing the front face of the detector
	which passed through the center of rotation of the goniometer head.
%
  \item \textit{detector\_horizontal\_angle} - This is the angle between
 	the line used by the detector distance and the horizonal plane. 
%
  \item \textit{detector\_offset} - This is the distance between the centerline
  	of the detector and the line used to define the detector distance above.
\end{itemize}

There are three constants that describe the system:
\begin{itemize}
  \item \textit{A} - This is the perpendicular distance between the two
  	vertical supports that hold up the detector.
%
  \item \textit{B} - This is the distance along the centerline of the
  	detector from the front face of the detector to the point where
	a perpendicular from the downstream detector pivot intersects
	this line.
%
  \item \textit{C} - This is the separation between the centerline of the
  	detector and the line defining the detector distance above.
\end{itemize}

The pseudomotors depend on the positions of three real motors.  These are:
\begin{itemize}
  \item \textit{dv\_upstream} - This motor controls the height of the upstream
  	vertical detector support.
%
  \item \textit{dv\_downstream} - This motor controls the height of the
  	downstream vertical detector support.
%
  \item \textit{dh} - This motor controls the horizontal position of the
  	vertical detector supports.
\end{itemize}

Confused?  I am planning to write a short document that describes the
definitions of these parameters in more detail and derives the formulas
describing them.  If you are reading this text and I have not yet written
that document, then pester me until I do write it.

\textbf{Warning:}  The detector horizonal angle is expressed internally
in radians.  If you want to display the angle in degrees, use the scale field
of the angle pseudomotor to do the conversion.

