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