Matt Lebofsky and Stan Corroyer
Telescope Position Testing
In order to insure that we always know
which data match which location in the sky, we need to test
the telescope's positioning system. A good way to do this is
by finding a strong, naturally occurring source of noise in
the data.
We searched the SETI@home database for
Gaussians around two strong sources visible in our sky: the
Crab Nebula and the Virgo cluster. Between the two, we found
Gaussians closer to Virgo. Observe the following graph:
In the data we receive from Arecibo, we
collect a telescope string every five seconds, which
contains time and position information. A workunit lasts
about 107 seconds, so within the course of a workunit we see
about 21 telescope strings. The red dots above show us where
the telescope was pointing every five seconds. If you
connect these dots, you can see the path the telescope took
during the course of the workunit. We also show the real
location of the Virgo cluster which we nearly passed over
during the course of this workunit.
The coordinates for the Virgo cluster are:
RA: 12 30 49.85 (12.513847 hours)
Dec: +12 23 26.17 (12.390603 degrees)
Our closest telescope string to Virgo
contained the coordinates:
RA: 12.513 hours
Dec: 12.36 degrees
RA = Right Ascension
Dec = Declination
As the receiver slews past a noisy signal
like this, we'd expect to see several Gaussians, which we
did - 20 of them within 21 seconds, during which we were
within 2 arcmin of Virgo.
To reinforce these findings, we scoured
through SERENDIP data taken at the same time during the
above observation (which at 2451380.40240 in Julian date).
Both SERENDIP and SETI@home get their data and telescope
strings from the same feed, but SERENDIP data also contain
"engineering values" which tell us the system
stats of the SERENDIP instrument.
As we pass a strong source, we expect the
values pertaining to gain of the instrument to dip, since
less power is needed to observe the louder signals. In fact,
we do see such a dip:
Note these engineering values have integer
resolution, so they have a tendency to fluctuate between two
integer values. During the 130 seconds above, there are
about 20 seconds where the power dips from approximately
10.5 to approximately 9.5. As well, during the exact time
when the SETI@home observation was positionally closest to
Virgo, the SERENDIP engineering values remained solidly near
9, showing that our gain was at its lowest nearest to the
strong source.
This procedure was a successful end-to-end
test of all the data collection and analysis components of
SETI@home and SERENDIP: We are receiving the correct
time/coordinate information from Arecibo, and seeing
expected signals in the right places during our back-end
data analysis. We regularly do pointing tests such as this
to make sure our pointing data are as accurate as possible.
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