A Survey of Selected Radio Telescope Receiver Types

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A Survey of Selected Radio Telescope Receiver
Types
Dana Whitlow Microwave Receiver Specialist,
Arecibo Observatory Denis Urbain Microwave
Receiver Specialist, Arecibo Observatory
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• In this talk we will consider several types of
• receivers
• Single feed
• Focal plane arrays
• gt Traditional (Arecibo ALFA, Parkes MB20)
• gt Phased array (AO-40 upcoming at Arecibo)
• gt Incoherent detector array (USRA SOFIA, GBT
  Mustang)

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Single-beam versus Multi-beam

• Single beam (single pixel) operation seems like a
  waste of a perfectly good (well, almost) optical
  system. Its especially inefficient for survey
  work.
• Multiple beams permit considerably faster survey
  work, but having them is definitely an extra-cost
  (and extra-complication) option.

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G. Cortes-Medellin, K.F. Warnick, B. D. Jeffs,
G. Rajagopalan, P. Perillat, M. Elmer, D. Carter,
V. Asthana, T. Webb, A. Vishwas. Field of View
Characterization of Arecibo Radio Telescope with
a Phased Array Feed. IEEE Antennas and Prop
Symposium, Spokane, WA, Jul 2011
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ALFAs 7-ELEMENT CLOSE-PACKED FEED HORN ARRAY
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TRADITIONAL FOCAL PLANE ARRAY

• Receivers are independent, with no phase
  connection.
• Therefore each feed must take individual
  responsibility for matching its footprint to the
  main reflector, setting a minimum size
  requirement.
• Feeds of this size (always too large) cannot
  adequately spatially sample the electromagnetic
  field configuration at the focal plane to correct
  for off-axis aberrations and permit creation of a
  pattern of contiguous beams.

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Example of Off-axis Aberration (this is
primarily coma)
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FOCAL PLANE PHASED ARRAY

• Here the array comprises a grid of small antenna
  elements spaced by slightly less than l/2,
  thereby meeting the Nyquist criterion for full
  spatial sampling of the electric field
  configuration over the focal plane. The elements
  are often implemented as shortened half-wave
  dipoles.
• The outputs of the elements are vector summed
  with complex element- and beam-dependent
  weighting to produce the desired beam(s) on the
  sky.
• Assuming that sufficient processing capability is
  available, simultaneous production of many beams
  is possible.
• Beams can be well corrected for off-axis
  aberrations and (within reason) focus errors.
• Within limits, pattern notches can be formed to
  mitigate RFI.
• But theres a catch electrical interactions and
  noise coupling between the closely-packed
  elements seriously complicate the design process
  and tend to degrade overall noise performance.

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BYU 19-ELEMENT FOCAL PLANE PHASED ARRAY
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BYU 19-ELEMENT FOCAL PLANE PHASED ARRAY
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INCOHERENT DETECTOR ARRAYS

• Incredibly, heat detectors (such as bolometers
  and arrays thereof) can be made sensitive enough
  to be very useful for astronomy.
• Greatest usefulness (for radio astronomy) is in
  the mm-wave and sub-mm-wave regimes where
  fundamental quantum behavior places severe limits
  on the noise performance of coherent receivers.
• Incoherent detectors in general (including photon
  detectors as well as bolometers) extend astronomy
  upward in frequency all the way to the gamma ray
  regime.
• A variety of useful detection mechanisms are
  known and used all require cooling to
  sub-one-degree-Kelvin temperatures to work. In
  fact, usually well below one degree is required!

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SOME ADVANTAGES OF INCOHERENT DETECTION

• Extends upper frequency limits of
  high-sensitivity radio astronomy beyond current
  practical (and even theoretical) limits of
  conventional (coherent) radio telescope
  receivers.
• Uncouples the strict connection between beamwidth
  and effective aperture area that is
  characteristic of coherent receivers. This can
  sometimes be exploited to obtain a sensitivity
  advantage if diffraction-limited angular
  resolution is not required.
• Very wide pre-detection bandwidth (tens of GHz)
  is available, which is really great for continuum
  work.

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SOME ISSUES WITH INCOHERENT DETECTION

• No phase information is available from the
  detectors thus neither off-axis aberration
  correction nor participation in interferometry is
  possible.
• Sensors are inherently insensitive to
  polarization.
• Spectroscopy is usually considered impractical
  since nothing can be done post-detection, and
  versatile or tight pre-detection filtering is
  extremely hard to implement. Some attempts have
  been made.
• Extraordinary care is required in the design and
  implementation of the sensor (array) to keep out
  stray radiation everywhere in the electromagnetic
  spectrum, since the inherent bandwidth of a
  thermal sensor is essentially infinite.
  Accomplishing this adequately can be much more
  challenging than it looks at first glance.
• Great attention is also required in the sensors
  output signal handling circuitry to avoid
  microphonics, 1/f noise, etc.
• Cryogenic cooling is a challenge, especially in
  large arrays.