The Crab Nebula: An example of
telescope position testing
Eric Person, Paul
Demorest |
SETI@home never sleeps. Our receiver collects signals
from Arecibo around
the clock while other projects point the telescope at celestial
targets. Because we're not the ones aiming the telescope, SETI@home
takes great care to document the celestial positions of the data it
collects. The SETI@home receiver logs the position of the telescope
every five seconds, letting us know exactly where in the sky the
telescope is pointing.
| Occasionally we test the accuracy of this
position data by observing known sources of noise (such as
supernovas) and making sure our data correctly identifies the
locations of these sources. One example of such a known source
is the Crab Nebula. The Crab Nebula is the remnant of a
supernova and is located in the constellation Taurus. Embedded
in the nebula is a rapidly spinning pulsar that emits pulses
of radiation 33 times a second. When the telescope passes over
the Crab, our receiver should always receive significant
interference. |
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The Crab
Nebula
(image courtesy of the Palomar
Observatory) | The Crab Nebula's
celestial coordinates are: RA 05h 34m 32s
Dec 22º 00' 52" (J2000 coordinates)
(Note: For more information about RA and Dec,
see our reference on key
features of skymaps.)
By comparing SETI@home's data at this location to the Crab
Nebula's expected noise pattern, we can confirm our position
records. Below are two graphs showing noise levels over time as the
telescope passes over the Crab. The one on the left represents
SETI@home data from March 17, 2002; the one on the right is from
July 19, 2000.
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An explanation of the graphsThese graphs are part of a hydrogen
survey currently underway at SETI@home. Each graph contains 17
vertically stacked plots—these are snapshots of the noise
distribution across SETI@home's 2.5 MHz frequency band, and there
are 5 seconds of elapsed time between each plot. The x-axis
represents velocity relative to our solar system; negative
velocities represent hydrogen moving away from us, and positive
velocities represent movement toward us. The large humps appearing
in the middle of each plot means that we detect more hydrogen at
slower speeds relative to our solar system.
Mind the dipNow, back to the Crab Nebula. Remember that
these are plots taken as the telesope passed over the Crab; the
plots at the bottom of the vertical stack show hydrogen levels
before hitting the Crab, and the plots at the top show levels after
the telescope passed. Note that in the middle plots there is a dip
that appears, just after the large middle hump. The dip occurs when
the background noise is great enough to drown out hydrogen
detection—interference from the Crab Nebula significantly jacks up
the threshold to detect signals above noise. The influence of the
Crab in this data provides concrete evidence that the data stored in
SETI@home's database is accurate.
SERENDIP Engineering ValuesThe influence of the Crab Nebula
can also be seen by viewing plots of data from the SERENDIP
spectrometer. SERENDIP and SETI@home share the same receiver at
Arecibo, but SERENDIP also collects engineering values (statistics
tracking the performance of the collection system). Since less
amplification is needed to measure stronger signals, we expect the
total amplification of the system to dip when the telescope crosses
noisy areas of the sky. The two graphs below (again from data
collected on March 17, 2002 and July 19, 2000) plot volume vs. time.
The volume levels are output by a AGC circuit which is trying to
keep its output power constant. As we pass over the Crab, more power
is coming in, so the AGC turns its "volume knob" down to compensate.
We can clearly see the dip as we pass over the source on these two
occasions:
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Gaussian center = 0.40256358 days
Gaussian width =
30.71 seconds |
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Gaussian center = 0.06263507 days
Gaussian width =
37.65 seconds |
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