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{\Large\bf%
SETHI@Berkeley-- A Piggyback 21-cm Sky Survey at Arecibo}
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\begin{center}{\normalsize\vspace*{-5 pt}%
Eric Korpela, Paul Demorest, Eric Heien, Carl Heiles, Dan Werthimer\\
University of California, Berkeley
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\titlebox{Abstract}
{\newpar \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 sidereal 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.}}
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\titlebox{Observing Methodology}
{\newpar \LARGE{The UCB SETI searches use the 1420 MHz line feed on Carriage House 1 at the
  National Astronomy and Ionospheric Center's 305 meter radio telescope in
  Arecibo, Puerto Rico.  (See Figure 1) 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 sidereal rate.  If the primary observer's feed is tracking a position
  on the sky, the beam scans the sky at twice the sidereal rate.  At twice
  the sidereal rate, the beam width corresponds to a 12 second beam transit time.  \cite{cise}} Figures 2 and 3 show the path of the telescope beam over the course of 1 day, and the entire program respectively}
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 Figure 1.  A photo of the NAIC 305 meter telescope at Arecibo. The inset at
 right shows the details of the ``carriage house'' structure.  The feed used by
 this project is the line feed extending downward from the carriage house.
 (Photos courtesy of NAIC--Arecibo Observatory, a facility of the NSF.)}}
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 \centerline{Figure 2.  The path of the telescope beam on August 5, 2001.}}
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 \centerline{Figure 3.  Sky coverage for entire project.}}
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{\newpar \LARGE{The time domain data for the sky survey is recorded as follows:  first,
  a 30 MHz band from the receiver is converted to baseband using a pair
  of mixers and low pass filters.  The resulting complex signal is digitized and 
  then filtered to 2.5MHz using a pair of 192 tap FIR filters in the SERENDIP IV instrument. 
  \cite{serendip4}
  One bit samples are recorded on 35 GByte DLT tapes (one bit real and one bit imaginary per
  complex sample).   
  These tapes are shipped to Berkeley for use in the SETI@home program.  
  T$_{\rm sys}$ for the system is 75K.}}
{\newpar \LARGE{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.}}
\titlebox{Example Spectra}
{\newpar \LARGE{
Below we present an individual spectrum.  This
spectrum was taken along a line of sight near 3C192 (l,b)=(197.7,24.4).
The hydrogen column along this line of sight is $\sim4.2\times 10^{20}$
cm$^{-2}$
Figure 4 shows a single raw 5 second integration spectrum with 
1.22 kHz resolution.  
Figure 5 represents the sum of 5 adjacent spectra, and shows 
the typical
SNR that will be achieved in a 0.1 degree skymap pixel.
}}
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 \centerline{Figure 4. A 5 second integration spectrum}}
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 \centerline{Figure 5.  A 25 second integration along the same line of sight.}}
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{\newpar \LARGE 
Figure 6 shows 54 spectra accumulated over 272 seconds.  During the accumulation
of these spectra the telescope beam was moving at nearly the sidereal rate
($\sim$1.1 degrees over this duration).  Significant changes are seen
in the spectra are seen on scales approximating the beam width.
}
\centerline{\psfig{file=specsweep.eps}}
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 Figure 6.  Spectra generated from 272 seconds of data.  This sweep extends from 
(l,b)=(72.87,-22.99) to (l,b)=(73.96,-23.52).  Each spectrum represents
5 seconds of data and transit of about 1/5 the beam width.  Note the significant
changes in line shape and velocity on scales $\sim$ the beam width.}}
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\titlebox{Program Status}
{\newpar \LARGE {Data accumulation for SETI@home began in December 1998.  We currently have
  accumulated about 58 Msec of observation time, which will translate into
  11.5 million spectra.  
  The survey has covered 79.4\% of the accessible 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.}}
{\newpar \LARGE{ Generation of the spectral database is just beginning.  We anticipate analysis
  of the existing data to be complete in 12 months.}}
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% References
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{\vspace*{-0.2in}
\begin{thebibliography}{XXX}
\bibitem[Korpela et al. 2001]{cise}{Korpela et al. 2001 {\em Computing in
Science and Engineering}, {\bf 3}, 79.}
\bibitem[Werthimer et al. 1997]{serendip4}{Werthimer et al. 1997, in {\em
Astronomical and Biochemical Origins and the Search for Life in the Universe}, eds: Cosmovici, Bowyer, \& Werthimer, (Editrice Compositori:
Bologna), 711.}
\end{thebibliography}
}
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\titlebox{What is SETI@home?
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\centerline{\psfig{file=client.ps}}
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 Figure 8.  SETI@home screensaver client, version 3.03.}
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{\newpar \rm \Large{ SETI@home is currently the worlds largest distributed computing project,
  involving over three million volunteers in 226 countries.
  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.}}
{\newpar \rm \Large{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.}}
{\newpar \rm \Large{ SETI@home volunteers have collectively donated 762,000 CPU years for a total
  of $10^{21}$ floating point operations.  The results returned include
  2.7 billion potential signals which are being processed through data quality
  checkers and RFI removal algorithms.}}
{\newpar \rm \Large{ 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/}.}}}}
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\begin{table} \caption{\label{tab1}    }\end{table}
\begin{figure}\caption{\label{rawimage}}\end{figure}
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\end{document}
