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\title{SETHI@Berkeley-- A Piggyback 21-cm Sky Survey at Arecibo}
\author{Eric Korpela, Dan Werthimer, Eric Heien, Paul Demorest, Carl Heiles\\
University of California, Berkeley}
\date{ }
\begin{document}
\maketitle

\begin{abstract}
{\large 
SETI@home observes a 2.5 MHz bandwidth centered on 1420 MHz near the
21-cm line using a short line feed at Arecibo which provides a 6$^\prime$
beam.  This feed sits on Carriage House 1, which means that during normal
astronomical observations with the new Gregorian dome the feed scans across
the sky at twice the siderial rate.  We are using the SETI@home receiver to
obtain about $4.4\times 10^{6}$ spectra per year with integration time of 5 
seconds per spectrum.  We have accumulated 2.6 years of data covering 
most of the sky observable from Arecibo.  This survey has much better 
resolution than previous single disk surveys and much better sensitivity than
existing or planned interferometric surveys.}
\end{abstract}

\newpage
\ \\
\centerline{\LARGE Observing Methodology}
\ \\
{\large
The UCB SETI searches use the 1420 MHz line feed on Carriage House 1 at the
National Astronomy and Ionospeheric Center's 305 meter radio telescope in
Arecibo, Puerto Rico.  (See Figure XXX) This unique arrangement allows
observations to be conducted without interference with other uses
of the telescope.  This results in two main modes of observation.  If the
primary observers feed is stationary or stowed the beam scans across the 
sky at the siderial rate.  If the primary observer's feed is tracking a position
on the sky, the beam scans the sky at twice the siderial rate.  At twice
the siderial rate, the beam width corresponds to a 12 second beam transit time.

Figure XXX shows the path of the telescope over the course of one day, one
month, and two years.

The a 20 MHz band output of the receiver converted to digital samples and 
filtered to 2.5MHz using a 256 tap filter in the SERENDIP IV instrument. 
\cite{serendip4}
It is then converted to baseband
and, sampled at 2.5 MHz using 1 bit complex samples and recorded on 35 GByte 
DLT tapes.  These tapes are shipped to Berkeley for use in the SETI@home program.  T$_{\rm sys}$ for the system is 75K.\cite{cise}

The SETHI@Berkeley program analyzes these tapes to extract hydrogen spectra.
The 2.5 MHz time series data are converted to raw spectra using 2048 point 
FFTs ($\Delta\nu$=1220 Hz).  6144 FFTs are accumulated into a single 
power spectrum of 5.033 second
integration time.  The resulting power spectrum is corrected for 1 bit sampling
effects by using the Van Vleck correction.  The spectrum, its start and
and end coordinates, and its time are stored in a database for future use.
This database will be queried to develop spectral maps of the neutral hydrogen
distribution. 
}

\newpage
\ \\
\centerline{\LARGE Example Spectra}
\ \\
{\large
PAUL YOU SHOULD WRITE SOMETHING HERE ABOUT EACH OF THE SPECTRA.
Antenna temp at peak, estimate $\sigma$ in units of antenna temp.
X axis should be velocity (km/s) with zero at the frequency of the
21cm line.  Get this sign for positive and negative velocity right.
If you can recreate Carls image from the greenbank talk great.  If not
use it as is.
}
\newpage
\ \\
\centerline{\LARGE Program Status}
\ \\
{\large  
Data acumulation for SETI@home began in December 1998.  We currently have
acummulated about 58 Msec of observation time, which will translate into
11.5 million spectra.  
The survey has covered 79.4\% of the accesable sky.  If mapped
into pixels 1 beam width in size, the median exposure per pixel is $\sim$20 
seconds.  We anticipate that data collection will continue for at least
1 additional year.

Generation of the spectral database is just beginning.  We anticipate analysis
of the existing data to be complete in 12 months.
}
\newpage
\centerline{\psfig{file=client.ps}}
\vskip 0.1in
\centerline{\LARGE What is SETI@home?}
\ \\
{\large SETI@home is currently the worlds largest distributed computing project,
involving nearly 2.7 million volunteers in over 200 countries around the world.
Radio telescope data from the Arecibo radio observatory covering a 2.5 MHz wide
band centered on 1420 MHz is divided into work units of 10 kHz bandwidth and
107 second duration.  These work units are distributed over the Internet to 
the volunteers who run the SETI@home client program.  On Windows and Macintosh 
platforms this 
client presents itself as a screensaver (see above) which analyzes the data
when the computer is not otherwise in use.  A text based version is available
for most UNIX based platforms.  This client can be run in the background at
low priority.

The client searches for narrow band signals over 14 octaves of bandwidth
between 0.075 and 1221 Hz.  It performs this search at 24291 Doppler drift
rates between -50 Hz/s and +50 Hz/s.  It performs Gaussian fitting to detect
faint signals that match the profile expected as the telescope beam
moves past an object on the sky.  And it searches for pulsed signals of the
type described here.  The entire process typically requires 4 trillion
floating point operations to fully examine a single work unit.

SETI@home volunteers have collectively donated 523,000 CPU years for a total
of $6.4\times 10^{20}$ floating point operations.  The results returned include
1.56 billion potential signals which are being processed through data quality
checkers and RFI removal algorithms.

For more information about SETI@home see Korpela et al. (2001, {\em Computing 
in Science \& Engineering}, {\bf v3n1}, Scientific Programming) or visit\\
{\em http://setiathome.ssl.berkeley.edu/}.
}

\end{document}
