Astro 2223 Fall 2008 Lect 3 Sept 2hosting.astro.cornell.edu/academics/courses/astro233/A... ·...
Transcript of Astro 2223 Fall 2008 Lect 3 Sept 2hosting.astro.cornell.edu/academics/courses/astro233/A... ·...
1
ASTRO 2233
Fall 2008
Lecture 3
Radio Telescopes and the Arecibo Telescope
Don Campbell
Ionospheric absorption
Interstellar absorption Atmospheric absorption
RADIO TELESCOPES
Traditional frequencies for radio astronomical observations:0.327 GHz (λ = 90 cm) Deuterium line (protected)
0.408 GHz (λ = 70 cm) Protected in some countries
1.420 GHz (λ = 21 cm) Atomic hydrogen (1.40 to 1.42 GHz protected)
1.665/1.667 GHz (λ = 18 cm) OH line (protected)
5.0 GHz (λ = 6 cm)
10.0 GHz (λ = 3 cm)
22 GHz (λ = 1.3 cm) Water vapor
100 GHz (λ = 3 mm)
380 GHz (λ = 0.8 mm)
850 GHz (λ = 0.35 mm)
NB: Even for the protected lines red-shifted observations can be seriously affected by RFI (interference)
u mm Band300 - 3000 GHzmm Band 110 - 300 GHz W Band 75 - 110 GHzV Band 40 - 75 GHzKa Band 11.1 - 7.5 mm 27 - 40 GHz K Band 1.67 - 1.11 cm18 - 27 GHz Ku Band 2.5 - 1.67 cm12 - 18 GHz X Band 3.75 - 2.50 cm8 - 12 GHz C Band 7.5 - 3.75 cm4 - 8 GHz S Band 15 - 7.5 cm2 - 4 GHz L Band 30 - 15 cm 1 - 2 GHz
Band designation WavelengthFrequency
Traditional radio band nomenclature:
2
First detection of cosmic radio waves by Karl Jansky in 1932 using antenna at 20 MHz (15 m wavelength). Antenna is 3 wavelengths long.
SOURCES OF RADIATION
Thermal emission
– continuum - Radiation from hot surfaces or gases
- line emission – transition between energy levels in atoms or molecules
Non – thermal - Due to high energy electrons moving in magnetic fields
called synchrotron emission (lower energy – cyclotron emission)
Radio, IR, Optical - Primarily thermal, some non-thermal
Gamma rays and X-rays - primarily non-thermal
Normally characterize radiation by its intensity:
I = energy/unit area/sec/steradian ( 4 π steradians in a sphere)
e.g. (joules/sec) m-2 str-1 = watts m-2 str-1
Intensity per “color” interval - frequency spectrum
Iν = I / Δν or Iλ = I / λ Optical - use prism or grating
Radio - use filter bank, digital spectrometer
A millimeter-wavelength emission spectrum of the core of the Orion giant molecular cloud, made at the Owens Valley Radio Observatory. Thespectrum covers a small interval in the atmospheric window at 1.3 mm. Over 800 spectral features are seen!
3
Fraunhofer absorption lines in the solar spectrum.
watts m-2 Hz-1 str-1
ν = frequency
h = Planck’s const
k = Boltzmann’s const
T = temperature (in Kelvin)
λ = wavelength
Planck’s Law
In wavelength (c = ν λ)
I(λ) = 2 h c2/λ5 x 1/ (ehc/kTλ – 1) watts m-3 str-1
TWO APPROXIMATIONS TO PLANCK’S LAW
Wien’s law: At short (optical) wavelengths (high frequencies) hν >> kT
Then: I(ν) = 2 h ν3 / c2 e-hν/kT watts m-2 Hz-1 str-1
Raleigh-Jeans approximation: At long wavelengths (radio) hν << kT
Then: I(ν) = 2 k T/λ2 watts m-2 Hz-1 str-1
N.B. Very simple expression, I(ν) is proportional to T and inversely proportional to λ2
WIEN’S DISPLACEMENT LAW
λmax T = const = 0.0051 mK for I(ν) dependence of Planck’s law
= 0.0029 mK for I(λ) dependence of Planck’s law
e.g. Sun’s temperature is ~ 6,000 K; peak emission λ is ~0.5 μm
Cosmic background is ~ 2.7 K; can use Wien’s law to get peak emission wavelength ( = [λmaxT]sun / 2.7 = ~ 1.1 mm)
TOTAL ENERGY EMITTED PER SECOND FROM A HOT SURFACE
Integrate Planck’s function over frequency (ν) and solid angle
Total radiated power F = ∫2π dφ ∫π/2 cos(θ) dθ ∫ I(ν) dν watts m-2
Can show that: F = σ T4 watts m-2 Stefan-Boltzmann law
where σ = Stefan-Boltzmann constant = 5.67 10-8 J m-2 s-1 K4
e.g. Total power radiated by the Sun = surface area of the sun x σ T4
~
4
Reber’s chart recording and maps of the radio emission from our galaxy at 160 MHz and 480 MHz
θPower response function or “Beam Pattern” P(θ)
P(0) = 1.0
Half power beam width (HPBW) ~ λ/D radians
Point source (angular size << HPBW)
Power received by the antenna is
Prec = ½ Ae I watts Hz-1
Ae = Effective area of the telescope in m2
I = intensity of the radiation from the point source at the earth in watts m-2 Hz -1
Extended source (ang size > HPBW)
( ) ( )1/ 2rec eP A I P dθ θ θ= ∫
Units of “cosmic source” intensity at the surface of the Earth is the Jansky
One Jansky = 10-26 watts m-2 Hz-1Extended source
side lobes
5
POLARIZATION
Linear Polarization: Direction of the electric vector perpendicular to the direction of propagation.
Can propagate two orthogonal (perpendicular)
linear polarizations
Circular Polarization: If the two orthogonal linear polarizations are 90 degrees “out of phase” with each other (i.e. sine and cosine waves) then the electric vector resulting from their sum will stay at a constant amplitude but rotate with the frequency of the wave
What do you want for a (radio) telescope?
High sensitivity => large area
Good resolution => large size/wavelength i.e small λ/D
Wide frequency coverage
Low cost
University of Illinois
Cylindrical parabola ~ 600 ft x 400 ft 600 MHz (late 1950s)
Australian CSIRO fixed parabola – can be steered over small angular range, about 10 beamwidths, by moving focal point receiver (1950s).
Early telescope designs
6
Jodrell Bank 250 ft telescope built 1957 210 ft (64 m) Parkes, Australia telescope built 1963
Apollo landing TV – DISH movie
Nancy, France – Kraus type telescope ( original built at U. Ohio)
Spherical x parabolic main reflector illuminated by a tiltable flat reflector.
300 ft NRAO Green Bank transit telescope ~1962 - 1987
7
100 m Green Bank Telescope
Arecibo 1000 ft spherical telescope built 1963
November 1959
8
ARECIBO 305m TELESCOPE
Spherical reflector optics
Second 430 MHz line feed built in 1971 – circular slotted tapered waveguide
Illuminated entire 305 m aperture at zenith - ~18 K/Jy
Performance degrades with zenith angle
- gain drops due to spillover
- system temperature rises due
spillover of feed illumination
on to 300 K “hot” ground
All line feeds suffer from this problem
9
1973 to 1974 – Wire mesh surface was replaced with 38778 aluminum panels ~ 1 m x 2 m. Perforated aluminum sheet over curved frame. 340 different curvatures. New line feeds at 21 cm and 13 cm illuminate ~50% of surface => 8 K/Jy. Gregorian
Optics
Dome diameter 25 m
Weight with equipment ~80 tons
Arecibo’s secondary and tertiary reflectors
10
Horns
7 – horn ALFA 21 cm receiver system(Constructed by ATNF receiver group, Australia)
Higher telescope load required additional cables – two more in every location
11
Computer controlled mechanical jacks at three tie-down locations maintain height and attitude of the suspended platform.
Maximum load on each jack is 60 tons - concrete block weighs 60 tons and is free to lift.
The “illuminated” (red) area by the Gregorian system on the surface of the Arecibo reflector. The illuminated area is ~ 213 m x 235 m. It moves radially as the zenith angle (900 – the elevation angle) changes from o0 to 200 (the maximum value). The location shown is for ZA ~ 150.
12