Transcript Radio Telescopes Jansky`s Telescope

Radio Telescopes
Jansky’s Telescope
• Karl Jansky built a radio
antenna in 1931.
– Polarized array
– Study lightning noise
• Detected noise that shifted
4 minutes each day.
– Direction of Sagitarrius
– Consistent with
galactic source
Reber Telescope
• In 1937, Grote Reber built a 32foot-diameter parabolic dish
antenna in his backyard in
Wheaton, Illinois to seek
cosmic radio emissions.
Reber’s Map
In the spring of
1939, he was able
to detect cosmic
radio emissions
with his equipment.
In 1941, he made
the first survey of
the sky at radio
wavelengths
(160MHz).
Radio Spectrum
• The radio spectrum is divided
into three bands.
frequency
wavelength
Radio
100MHz - 100GHz
3m-3mm
Millimeter
100GHz-300GHz
3mm-1mm
Submillimeter
300GHz-1000GHz
1mm-0.3mm
Atmospheric Window
• The atmosphere is transparent from 50 m to 0.5 mm
– Long wavelengths reflected by ionosphere
– Short wavelength absorbed by O2, H2O; less at altitude
Radio Emission
2n 2
Fn  2 kT
c
Fn  n 
0.2    1.2
• Radio sources are measured in
Janskys (Jy).
– 1 Jy = 10-26 W m-2 Hz-1
– Differential flux Fn
• Thermal emission follows the
Rayleigh – Jeans law.
• Synchrotron radiation comes
from magnetic fields.
Antenna
• For MHz radio waves a
telescope is a half-wave dipole
antenna.
– Consists of two conductors
– Short separation
– Quarter wavelength each
• Cables at the center connect to
the receiver.
Jocelyn Bell and 81.5 MHz radio telescope (1967)
Radio Horns
• Horn antennas are used to
collect waves at GHz and
higher.
– Factor of 8 in bandwidth
– May be dielectric filled
– Waveguides to detector
~1 meter
~2cm
VLBA telescope
Sensors
• Radio sensors are matched
to the desired
wavelengths.
• High frequency sensors
are usually SIS.
– Superconductorinsulatorsuperconductor
– NiO – AlO
– Photon assisted
tunneling
VLBA telescope
Noise
• Small radio signals can be lost
in noise.
– Minimum detectable
brightness Bmin
– Integration time t
– Frequency bandwidth n
– Receiver-based constant K
• Noise temperature based on
multiamplifier stages with gain
Gn.
Bmin
2 Kn 2
 2
kTs
c tn
Tn
T2
Ts  T1    
G1
G1G2 Gn 1
Cryogenics
• One way to lower noise is to
operate the electronics at low
temperature.
– Reduces thermal noise
Vs  4kTs R
• Cooling to less than 20K is
accomplished with high
pressure helium gas.
– Compressor and pump
– Thermally isolated cryostat
Helium pump
Cryostat
Components
Horn
Purpose of the horn is to collect the radiation directed
to it from the antenna.
Amplifier
Increases the amplitude of the signal
Mixer
Used to change the frequency to a more easily used
frequency
Heterodyne
• The simplest receiver is a
heterodyne receiver similar to a
consumer radio.
– Preamp gain 10-1000
• A local oscillator mixes with
the input signal.
– Beat or intermediate
frequency
• Further amplification may be by
factors of 106 to 109.
• Heterodyne receivers have high
system temperatures.
– 10 K at radio
– 10 M at millimeter
• Phase sensitive techniques
reduce noise.
n sig  n osc n IF
Receivers
Radio
Horn
Preamp
Mixer
Convert to
desired form to
record/analyze
Store data
Millimeter and Submillimeter
Convert to
desired form to
record/analyze
Store data
Radio Receiver
Receivers are
inside
the dome
Aricibo
Millimeter Wave Receiver
Horns
~0.5 m
Amplifiers
Millimeter Wave Oscillator
Local oscillator
Millimeter Wave Electronics
Intermediate frequency (IF) plate
Submillimeter receiver
CSO, Mauna Kea
Submillimeter receiver
230 GHz mixer block
CSO, Mauna Kea
Antenna Dish
• A radio telescope is often noted
for the large dish.
• This is for the same optics as an
optical reflecting telescope.
– Rayleigh criterion applies
– Beam width at first nulls
– Dipole length x
1.22
BWFN  2 x
D
Steering
gear
Receiver
Antenna
mirror
Gain
• The maximum gain for a radio
telescope depends on the
effective area.
– Typically 1.6 for half-wave
dipole
• The effective area compares the
output power to the incoming
flux.
– Must be correctly polarized
4n 2
g  2 Aeff
c
Pn
Aeff 
Fn
Surface Errors
• The surface error of a dish must
be controlled.
– Less than 1/20 wavelength
– Losses to less than 30%
1/20 =0.15mm
VLBA (3mm-3m)
Radio
Mauna Kea
Very Smooth
• The surface error requirements are much stricter for submillimeter telescopes than for radio telescopes.
1/20 =0.05mm
Millimeter
NRAO 12 meter (1mm3mm)
Kitt Peak
1/20 =0.015mm
Sub-millimeter
JCMT (0.3mm-2.0mm)
Mauna Kea
Holographic Test
• This image was taken during
the SMT reflector's holographic
testing showing that the
deviations of the reflector.
• Deviations are nearing the
targeted 15 microns (about the
thickness of a human hair).