\section{Equipment}

In the experiment, we used a $2.5{\rm\,m}$ diameter parabolic dish
antenna.  The dish is set on an equatorial mount that can be
controlled remotely by a computer.  The control program will convert
the position from celestial or galactic coordinates and track the
target position while collecting data.

The dish focuses the signal onto a simple feed connected to a
preamplifier mounted together with the feed.  The preamplifier
includes a bandpass filter to reduce noise, and a noise source that
was useful for calibration of the output signal.

\begin{figure}[ht]
\begin{center}
\leavevmode
\epsfxsize=0.8\hsize
\epsfbox{/mit/bert/Class/JuniorLab/paper/equipment.eps}
\end{center}
\caption{Diagram of equipment used in the experiment}
\end{figure}

The signal then passes through a series of heterodyne mixers,
amplifiers and bandpass filters that reduces its frequency to the
range of 0--3~MHz.  This range will correspond to a 3~MHz frequency
band centered on 1420~MHz (the frequency corresponding to the 21cm
hydrogen line).

This signal is then converted to digital data using a 2-bit A/D
converter.  An autocorrelator board containing two 16-channel digital
correlator chips serves to compute the autocorrelation function of the
data and pass it on to an IBM PC compatible computer.




