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\section{Overview}

The complete control circuit for the electron gun, including all power supplies, current measurement points, and part of the control and measurement circuitry from the Data Aquisition system is shown in Figure \ref{fig_electron_gun}.

\section{Electron Optics}

A diagram of the electron gun without attached electronics is shown in Figure \ref{fig_electron_gun_simple}, and a photograph of the gun is shown in Figure \ref{fig_electron_gun_photo}.



\subsection{Focusing the Gun}

The electron gun was been salvaged from an old Cathode Ray Oscilloscope (CRO). The original phosphor screen was also saved, which allowed for very rough focusing of the beam by viewing the illuminated region of the screen through a porthole in the vacuum chamber. Figure \ref{fig_electron_gun_screen_unfocused} and \ref{fig_electron_gun_screen_focused} are photographs of the illuminated spot under focusing conditions chosen to create the largest and sharpest possible spot sizes respectively.

The distance between the gun and target sample was different to the distance from the gun to its original screen. In addition, the original screen consisted of a cylinder with conductive coating at the same potential as the final electrode. These differences in configuration mean that the gun must be refocused for use in TCS.

The electron gun may be considered ``focussed'' if the beam is paraxial to the surface of the sample. 

By a process of trial and error

\section{Current Measurement}

Total Current Spectroscopy methods measure the slope of current through a sample with respect to the energy of electrons relative to the sample surface. Often lock-in amplifier techniques are used \cite{???}. These have the advantage of reducing system noise whilst determining the slope directly, but require more time to set up than measuring the current through the sample as a function of incident electron energy, and later taking the derivative.

To make 

\section{Data Aquisition and Automation}

\subsection{ADC Measurements}

\subsection{DAC Control of Initial Energy}

\section{Sources of Error}

\subsection{Accuracy of ADCs}


\subsection{Noise due to Instruments}

\subsubsection{
A plot of ADC counts vs time for several output voltages of 

\subsection{Noise due to Ground Loops}

\subsection{Reduction of Noise}


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