science with a rover-based low-frequency dipole array a multi-configuration rover-borne dipole array...
TRANSCRIPT
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Science with a Rover-based Low-frequency
Dipole Array A multi-configuration rover-borne dipole array
for
Low-frequency RadioAstronomy from the Moon
Ettore Carretti
INAF-IRA Bologna
On behalf of the Radio WP group
in collaboration with CIRANO (PoliMi)E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Why =3-30 MHz astrophysics?
• Almost unexplored frequency band
– next years (2009+) the ground-based array LOFAR will explore the 30-80 and 120-240 MHz bands;
– < 30 MHz almost impossible from ground (ionosphere)
– Only strongest sources and very large scales maps (HPMW = 2°-10°)
TO OPEN A NEW ASTROPHYSICS WINDOW !TO OPEN A NEW ASTROPHYSICS WINDOW !
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Extragalactic astrophysics
• Cosmology
• Physics of Acceleration of high energy particles in the Universe
• Origin and amplification of magnetic fields in the Universe
• Physics of Black Holes and Duty cycles
• Most distant and oldest radio emitting plasma in the Universe
• New Physics … (coherent emission, serendipity)
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Galactic astrophysics
• Diffuse Emission from the Galaxy
• Magnetic field in the Galaxy
• Acceleration and diffusion of high energy particles in the Galaxy
• Mapping HII in the Galaxy
• Supernova Remnants in the Galaxy
• Physics of Micro-Quasars
• New Physics … (coherent emission, serendipity)
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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What is possible from ground?
2.1 MHz
Ellis 1982
3.7 MHz
Ellis 1982
8.3 MHz
Ellis 1982 Roger et al. 1999
22 MHz
= 2°-3°
LOW RESOLUTION !
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Resolution improvement by >10X...
2.1 MHz
Ellis 1982
Haslam et al. 1982
• subdegree resolution (at least) down to 3 MHz
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Low frequency: strong signal
• Low frequencies dominated by synchrotron
• Synch spectrum: negative slope => strong signal at low frequency
– Galactic diffuse emission >10000s K (30MHz) even at high lat.
– > 1 MK (2-3MHz)
• It is not required a huge collecting area to map the sky.
Haslam et al. 1982
408 MHz
Roger et al. 1999
22 MHz
= 2°-3°
2.1 MHz
Ellis 1982E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Instrument• Need for high resolution => large telescope aperture
• No need for high sensitivity (strong signal and first survey experiment)
• => interferometric small antenna array– many separated antenna, whose signals are 2-by-2 correlated– N antennas => N(N-1)/2 independent pairs– resolution = /Dmax (Dmax = maximum baseline – pair distance)
• 10 Km equivalent aperture: [MHz][m] [arcmin] ---------------------------------------------------- 100 3.3 1.0 30 10 3.5 10 33 10 3 100 35
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Array• each antenna pair is sensitive to the angular scale /D
• several baselines D are necessary to cover from largest to smallest angular scales
• D from “a few”-m to 10 Km (even 3 Km would be OK)
• Array “shape”
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Antennas• Which antenna?
• Mirrors not suitable:– need D >>
– 3 MHz => = 100m
– it’d be D = HUGE !?!
• Dipole antenna: “Natural” choice for a low frequency antenna– ./2 electric dipoles not suitable because of lengths and very narrow
frequency band
– magnetic dipoles: using coils, much higher efficiency with same length
– length: 10s-cm
– double dipoles: both polarizations
– Large F.o.V.: 90° => optimal for surveys, i.e. full sky mapping
– lightweight: just wireE. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Antenna (2)• Receiver: simple and well developed technologyReceiver: simple and well developed technology
• Dipole antennas
• Receiver: sky signal dominated conditions (10-kK to 1-MK)– amplifier sensitivity is not an issue (state-of-the-art technology not
necessary);
– commercial components can be used;
– standard Medium/Short-Wave telecommunication components (well tested in space-enviroment and space-qualified);
– standard electronics
• No “critical” technology to be developed
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Experiment• Main requirement:
• Mass < 60-70 Kg
10-15 antennas: not enough to realize all the baselines from 3-5m throughout 10km
• solution: reconfigurable array
• => Rover-Borne antennas (new approach w.r.t. ESA or EADS studies) PROs:
– it allows to deploy antennas from the lander to the wanted position with the wanted attitude.
– flexibility– it allows to re-deploy the antennas in different configurations of larger and
larger size
LIGHTWEIGHT!!!LIGHTWEIGHT!!!
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Rover-Borne Stations• Station = Station =
telescope (dipole + electronics) + rovertelescope (dipole + electronics) + rover
• Mass station: 5-6 kg
10 stations stay within mass requirements
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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A “walking” experiment
• reconfigurable array:
• first the 10 antennas realize the most compact core and observe the sky with this configuration;
• ... and so on for a number of steps until the 10km baselines are set so to realize the full array
• 6 steps:
Dmax = 30m, 100m, 300m, 1km, 3km, 10km
• then: antennas are moved to longer distances to realize a larger configuration and observe the sky;
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Where• Far side of the Moon:
Earth is shielded => RFI fully mitigated
• Lunar latitude:
within the range 30°- 60°
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Operations• observing frequencies:
– 3, 6, 12, 25, 50, 100 MHz
– 3, 4.5, 7, 10, 15, 22, 30 MHz
• lunar night observations – Sun is too strong !
• observations all lunar night long, for optimal angular frequency space coverage
• 1 array configuration a lunar night
• 1 lunar day (= 1 month) observing cycle: – 6 months to realize Dmax = 30m, 100m, 300m, 1km, 3km, 10km
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Operations (2)• Station data must be correlated for each station pair
• Data must be collected by one processing station. Two possible solutions:
– one master station (central one);– orbiter.
• Communications to Earth: only once a month (simple and cheap mission control)
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Science with WARP• Walking Array Radio Pathfinder (WARP):
– subdegree Galactic diffuse emission– relativistic electron age, confinement and diffusion processes, magnetic field
– New SNR: oldest visible at low freq only (high res to separate them from diffuse em.)
– Micro quasars (e.g. SS433): no lobes. Why? old, or p+?
– Extragalactic sources: all 3C catalogue:• spectrum, lowest energy e-, evolution history;• free-free absorption: W-IGM• synchrotron self-absorption: direct measurement of B, ne
– galaxy-cluster halos and relics
•... but ... most important.... it is an unexplored window: hunt for new sources and astrophysical (and physical) processes fully unknown so far.
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Conclusions• To open a NEW window in astrophysics.To open a NEW window in astrophysics.
• High interest of the scientific community (e.g. ESA and EADS studies for experiments in this frequency band).
• Instrument: a simple, easy, well developed technology: No “critical” developments requiredNo “critical” developments required
• New approach based on a Rover-borne experiment: FLEXIBILITYFLEXIBILITY
• Safe against failure of one/more stations
• Many-stations array realized with “few” stations
• Lightweight payload.E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007
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Conclusions (2)• ESA studies for a lunar experiment in such band
• WARP: unique opportunity for the italian community (astrophysics + space engineering) to earn experience and be candidate to lead the European mission.
• A rover colony getting around on the lunar surface: high impact for the Italian System
– italian space industry;– italian engineer research;– italian astrophysics research;– visibility;– italian space technology capabilities.
E. Carretti, Observation of the Universe from the Moon, Frascati, 07 May 2007