The Arecibo radio telescope

1
The Arecibo radio telescope

• 2009 teachers workshop
• Phil Perillat

2
Talk Outline

• Step from incoming signal through the telescope
  to the recorded data.
• Signals we receive
• A telescope with a spherical reflector.
• Tracking a source
• The front end receiver.
• From front end to recording device.
• The backend recording devices.

3
The signals we receive

• Optical vs AO Radio frequencies
• Optical and Radio waves are electromagnetic
  radiation that differ in frequency.
• The different frequencies measure different
  physical processes.
• Optical
• 61014 Hz , .5 microns wavelength
• Peak radiation from stars similar to our sun.
• Human eye adapted to this frequency.
• Radio
• 300 MHz to 10 GHz 1meter to 3 cm (AO frequency
  range).
• 1420 MHz line emission from a hydrogen spin flip
  (proton, electron spins aligned or unaligned).
• We can directly measure the Electric field (and
  its phase). This is hard to do in the optical.

4
The optics .. A parabolic reflector

• Properties of a parabolic reflector
• 1 axis of symmetry looking down from focus.
• Point focus has no spatial extent so high
  frequencies, and large bandwidths are possible.
• To track an object you must move the entire
  reflector.
• Arecibos dish is too large to move.
• An object moves through the A0 beam in 13 seconds
  (at 1400 MHz)
• High sensitivity observations require longer
  integration times.
• We need to be able to track a source for a longer
  time.

5
The optics .. A spherical dish

• Properties of a spherical reflector
• Looking along any radius is an axis of symmetry.
• Plane waves focus along a line.
• Focus spatially extended ? frequency dependent.
• Using a line feed, maximum frequency about 2 GHz
  (12cm)
• Maximum bandwidth of each linefeed about 40 MHz.
• To track an object, move the antenna at the line
  focus to follow the celestial object.
• Line feed must always point back at the center of
  curvature.
• Can track objects for up to 2hr 40 minutes
  (depending on how close to overhead the object
  transits)
• Longer integrations allow for more sensitive
  measurements.
• At higher zenith angle some of the beam falls off
  the dish.

6
The optics .. Gregorian reflectors

• Need spherical dish for tracking objects
• Want point focus to go to high frequencies and
  large bandwidths.
• Gregorian reflectors.. secondary tertiary.
• Let rays pass through line focus
• Use a secondary and tertiary shaped reflector to
  refocus the rays back to a point focus.
• Now have high frequency and wide bandwidth
  capabilities (just like a parabolic dish).

7
The optics ..beam size

• Angular units
• radian57.3 deg,
• 1 deg60 arcMinutes,
• 1 deg3600 arcSeconds.
• Beam size or spatial resolution.
• Depends on how many wavelengths fit into the
  reflector diameter (305 Meters for AO)
• BeamsizeKwavelength/dishDiameter (K close to 1)
• At 300 MHz (1 meter) beam15 arcminutes
• At 10 GHz (3 cm) beam27 arcSeconds
• Linear distance DradiusthetaRadians
• At the moon Radius384403(km),
  ThetaRadians27./(360057.3) so D50.3 kilometer
  spot size with the 10 GHz Beam.

8
Tracking a source

• Need to keep the telescope pointed at the source.
• Need to position feed to about 5 asecs accuracy
  (3 mm at the horn) in absolute space.
• Repeatable tracking errors
• Deflection in structure, incorrect curvature of
  rails.
• Make a model of these errors by tracking a number
  of sources rise to set and measuring their
  offsets.
• Create a model that adds these offsets to the
  requested positions when we track a normal
  source.

9
Tracking a source

• Non repeatable errors.
• Stretching of the main cables with temperature
  causes entire platform to move up and down.
• Use 6 distomats around the reflector rim to
  measure the position of the platform.
• Tiedown jacks and cables use the distomat
  information to pull or let up on the corners to
  keep the height of the platform fixed.
• With the distomats and tiedowns running every two
  minutes we can keep the average height of the
  platform to with 1 mm rms over a day.

10
The front end receiver.. horn

• The horn transitions the e/m wave from free space
  to a cable. Want to minimize the reflection at
  this interface.
• Free space impedance 376 ohms
• Cable impedance 50 ohms.
• The polarizer will separate the two orthogonal
  polarizations (linear or circular).
• The information in the two polarizations is
  independent.
• A polarizer can span about an octave bandwidth
  (maxFreq/2). This limits the frequency range of
  the front end and requires us to have multiple
  receivers.
• List of front end receivers at AO
• 327,430,1100-1800,1900-3100,3000-4000,4000-6000,60
  00-8000, and 8000-10000 MHz

11
The front end receiver..weak signals

• Astronomical signals are weak.
• Units, constants
• 1 Jansky(Jy) 1e-26 watts/m2/Hz (a strong
  source)
• Kb1.38e-23 Joules/Kelvin . Boltzmans constant
  converts between energy and temperature.
• How strong is a 1Jy source at AO?
• 40e3m2 1Jy 2.2.2e-22 watts/Hz
• Factor of two since two independent
  polarizations.
• Dividing by Kb and time averaging
• 2.2e-22/Kb 14.5 Kelvins/Jy.
• So a 1 Jy source will raise the noise temperature
  of the receiver system by 14.5 Kelvins (actually
  its more like 10 or 12 after efficiencies are
  included). .

12
The front end receiver .. cooled

• A dewar is used to cool the first set of
  amplifiers to about 15 Kelvins.
• A room temperature amplifier (temp300K) would
  add thermal noise gtgt than the 5 Kelvins from the
  sky or the few kelvins from the source.
• The dewar is first evacuated to reduce conduction
  and convection.
• Helium gas is used as the cooling medium
• The gas is cooled using free expansion in a
  compressor mounted on top of the dome. It is then
  circulated through the dewar.
• After 30 db (x1000) of gain in the dewar, the 25K
  dewar input signal temperature is now 25000K.
  Room temperature amps can then be used to
  increase the temperature without adding to the
  noise.

13
Mixing to an Intermediate freq

• We have about 10 dewars that cover 300 to 10 GHz
  (the rf RadioFrequency)
• After the dewar we translate (mix) the RF
  frequency to a common Intermediate Frequency
  (IF).
• Multiply the Rf signal by a fixed reference (LO)
  freq.
• cos(rf)cos(lo)1/2(cos(rflo) cos(rf-lo))
• Use a filter after the mixer to remove (rflo)
  frequencies. You are left with (rf-lo).
• We can then use common electronics for all the
  dewars.
• Example. Move 4.5 to 5.5 GHz down to 1-2 GHz
• IF1.5, RF5, LOIF RF6.5 GHz. The band is
  flipped
• 5.5 -gt 1.0 since 5.5 is 1.0 GHz from 6.5
• 5.0 -gt 1.5 since 5.0 is 1.5 GHz from 6.5
• 4.5 -gt 2.0 since 4.5 is 2.0 GHz from 6.5.

14
Processing and recording the signal

• Need to sample the analog voltage, process, and
  then record it.
• The processing done depends on the type of
  experiment we are doing.
• Square the voltage to get the total power.
• Used when looking at the continuum emission from
  objects.
• Measure the power versus frequency. Fourier
  transform the input voltages, then compute the
  power in each frequency bin.
• Used when measuring spectral lines and doppler
  shifts.
• Integrate to increase the signal to noise
• The noise goes down as 1/sqrt(bandwidthintegrat
  ionTime)

15
Spectral line emission from a galaxy

• Hydrogen radio emission at 1420.4058 MHz (21 cm).
• Most of the baryonic (normal) matter in the
  universe is hydrogen. Use the line emission to
  probe the universe.
• 1420 emission from a galaxy.
• Motion of galaxy away from us will doppler shift
  1420 to a lower freq.
• The larger the doppler shift the farther away the
  object is (expansion of the universe). 70Km/sec
  per megaParsec3.26e6 light years.
• The width of the line will measure the rotation
  of the galaxy
• One side of galaxy is approaching, the other side
  is moving away.
• The integral of all the energy under the profile
  measures the hydrogen mass of the galaxy..

16
ugc1291

• 1420 line doppler shifted to 1403.8 MHz
• df/f vel/c ? object vel3507 Km/sec
• Hubbles law 70Km/sec for each megaParsec (3.26e6
  light years).
• 3507/70 3.26E6163 Million light years away
  (time it took the light to get to us)
• The galaxy is about 1 MHz wide (about 210
  Km/sec). The galactic rotation velocity at the
  edge of the hydrogen disc is about 100 Km/second.

17
Summary

• Spherical dish so we can track
• Gregorian reflectors to get back to point focus
  with high freq and large bandwidths
• Cool the front end to improve system noise.
• Mix to intermediate freq for common analog
  processing
• Spectral analysis of spectral lines gives
  information on distance, rotation, and mass.