6.4 Radio Telescopes

1. Introduction to Radio Astronomy and Its Beginnings

  • Discovery of Cosmic Radio Waves

    • In the early 1930s, Karl G. Jansky, a Bell Telephone Laboratories engineer, detected mysterious radio static while experimenting with long-range radio communication antennas.

    • He observed this radiation was strongest about four minutes earlier each successive day, concluding it originated from a region fixed on the celestial sphere, aligning with Earth's sidereal rotation period being four minutes shorter than a solar day.

    • Subsequent investigation confirmed the source was part of the Milky Way Galaxy, marking Jansky's discovery of the first cosmic radio waves.

    • In 1936, amateur astronomer Grote Reber built the first antenna specifically for cosmic radio waves and conducted pioneering sky surveys, working largely alone for a decade before professional astronomers recognized the field's potential.

  • Nature of Radio Waves from Space

    • Radio waves are a form of electromagnetic radiation, similar to light, and are not sound waves.

    • Unlike light, they cannot be detected by our senses and require electronic equipment.

    • Cosmic radio signals do not contain encoded information like commercial radio but carry data about the chemistry and physical conditions of their sources.

2. Detection of Radio Energy from Space

  • How Radio Waves are Detected

    • Principle: Electromagnetic waves can cause charged particles to move. Radio waves induce a feeble current in electrical conductors like antennas.

    • Antenna Function: An antenna intercepts radio waves, generating a current.

    • Amplification: This current is amplified in a radio receiver until it is strong enough to be measured or recorded.

    • Frequency Tuning: Receivers can be tuned to select specific frequencies, although astronomical receivers often use sophisticated techniques to detect thousands of frequency bands simultaneously.

    • Data Processing: The receiver acts like a spectrometer, providing information on radiation strength at each wavelength/frequency. Signals are processed by computers and recorded for analysis.

  • Radio Telescope Design

    • Similar to how light reflects from shiny surfaces, radio waves are reflected by conducting surfaces.

    • A radio-reflecting telescope uses a concave metal reflector (a "dish") to collect radio waves.

    • The collected waves are reflected to a focal point, where they are directed to a receiver for analysis.

    • Radio astronomers often create pictorial representations (radio images) of observed sources, revealing details invisible in visible-light photographs (e.g., vast jets and complex emission regions in galaxies like Cygnus A).

3. World’s Largest Radio Telescopes

  • Individual Dish Telescopes

    • Five-hundred-meter Aperture Spherical Telescope (FAST) (Guizhou, China): The world's largest single-dish radio telescope with a 500-meter fixed dish.

    • Robert C. Byrd Green Bank Telescope (GBT) (Green Bank, WV, USA): A fully steerable dish, approximately 100 \text{ meters} in aperture.

    • Effelsberg 100-m Telescope (Bonn, Germany): Another 100-meter steerable dish.

    • The Arecibo Observatory in Puerto Rico, a 305-meter fixed dish, was historically a leading radar astronomy facility but was decommissioned in 2020 after severe damage.

  • Arrays of Radio Dishes (Interferometers)

    • These instruments combine multiple dishes to achieve higher resolution (discussed in the next section).

    • Square Kilometre Array (SKA) (South Africa and Western Australia): A future instrument with thousands of dishes and a square kilometer collecting area, partially operational by 2020.

    • Atacama Large Millimeter/submillimeter Array (ALMA) (Atacama Desert, Northern Chile): Consists of 66 7-meter and 12-meter dishes.

    • Jansky Very Large Array (VLA) (Socorro, New Mexico, USA): Composed of 27 movable 25-meter dishes spread over about 36 \text{ kilometers}.

    • Very Long Baseline Array (VLBA) (Ten US sites, HI to Virgin Islands): A 10-element array of 25-meter dishes with baselines up to 9000 \text{ km}.

4. Radio Interferometry

  • Resolution Challenge for Radio Waves

    • A telescope's resolution (ability to show fine detail) depends on its aperture and the wavelength of radiation.

    • Due to their long wavelengths, radio waves present significant challenges for achieving high resolution; even the largest single radio dishes cannot resolve as much detail as a small visible-light telescope.

  • Definition of Interferometry

    • An interferometer is a technique where two or more telescopes are linked together electronically to sharpen images.

    • The term "interference" refers to the technical way multiple waves interact when arriving at instruments, allowing for greater detail extraction.

  • Benefits of Interferometers Over Single-Dish Telescopes

    • Enhanced Resolution: The resolution of an interferometer depends on the separation between the telescopes (the baseline), not on their individual apertures.

      • Two telescopes separated by 1 \text{ kilometer} can provide the same resolution as a single dish 1 \text{ kilometer} across.

    • Formation of Arrays: Combining a large number of radio dishes into an interferometer array (e.g., VLA, ALMA) further improves resolution by effectively working as many two-dish interferometers.

    • High-Resolution Imaging: Computer processing of data from arrays allows for the reconstruction of high-resolution radio images, comparable to visible-light telescopes.

      • The VLA achieves a resolution of about 1 \text{ arcsecond}.

      • ALMA has reached resolutions down to 6 \text{ milliarcseconds} (0.006 \text{ arcseconds}), a remarkable achievement for radio astronomy.

    • Overcoming Physical Connection Limits: Initially, interferometer size was limited by the need for physical wiring.

      • Modern technology allows precise timing of electromagnetic waves at widely separated telescopes, combining data later without physical connection.

      • The Very Long Baseline Array (VLBA), with telescopes stretching from the Virgin Islands to Hawaii, achieves resolutions of 0.0001 \text{ arcseconds}, distinguishing features as small as 10 \text{ astronomical units (AU)} at the Galactic center.

  • Visible-Light Interferometers

    • Advances in technology have also enabled interferometry at visible-light and infrared wavelengths.

    • Observatories like the CHARA Array, Very Large Telescope, and Keck telescopes now combine light from multiple dishes as interferometers to achieve much greater resolution than single telescopes.

5. Radar Astronomy

  • Technique and Application

    • Radar involves transmitting radio waves to an object in the solar system and detecting the reflected radiation.

    • By precisely measuring the round-trip time of the radio waves (knowing the speed of light), the distance to the object or features on its surface can be determined.

    • Radar observations are used to:

      • Determine distances to planets.

      • Measure the speed of objects using the Doppler effect.

      • Aid in spacecraft navigation.

      • Determine rotation periods of Venus and Mercury.

      • Probe Earth-approaching asteroids and investigate surface features (mountains, valleys) on Mercury, Venus, Mars, and Jupiter's large moons.

  • Radar Telescopes

    • Any radio dish can serve as a radar telescope if equipped with both a powerful transmitter and a receiver.

    • The Arecibo telescope (305-meter fixed dish) was a leading radar astronomy facility before its decommissioning.

    • The Five-hundred-meter Aperture Spherical Telescope (FAST) in China is an even larger facility that can potentially be used for radar astronomy.