9.1.3 Single dish radio telescopes, I-R, U-V, and X-ray telescopes

Radio Telescopes

• Purpose: Detect radio waves emitted by astronomical objects.
• Operation: Uses a parabolic dish to focus incoming radio waves onto a receiver.
• Atmosphere: Transparent to radio waves, so radio telescopes can be placed on the ground.
• Design: These telescopes are relatively simple in structure, often using a wire mesh instead of a solid mirror, as long as the mesh spacing is smaller than the radio wavelength.
• Challenges: Must be placed in isolated locations to avoid interference from human-made radio sources.

Comparison to Optical Telescopes:

Similarities	Differences
Basic Function: Both radio and optical telescopes work similarly by capturing and focusing incoming radiation to measure its intensity.	Wavelength and Size: Radio waves have much longer wavelengths than visible light. This requires radio telescopes to have larger diameters than optical telescopes to achieve comparable resolution.
Mobility and Tracking: Both types of telescopes can be moved to observe different regions of the sky or to follow moving celestial objects.	Construction Material: Radio telescopes often use a wire mesh instead of a solid mirror. As long as the mesh size is smaller than the wavelength of radio waves (typically less than λ/20), it can effectively reflect these waves. This makes radio telescope construction simpler and cheaper.
Parabolic Dish Design: The parabolic dish of a radio telescope serves a similar function to the objective mirror in a reflecting optical telescope, focusing incoming waves to a single focal point.	Image Formation: Unlike optical telescopes, radio telescopes must move across an area to build a complete image, as they do not capture a full image at once.
Location (Ground-Based): Both optical and radio telescopes can be positioned on the ground, as radio waves and visible light can pass through the Earth's atmosphere without significant interference.	Interference Sources: Radio telescopes are more affected by man-made interference from sources such as radio transmissions, mobile phones, and microwave ovens. Optical telescopes, in contrast, are more affected by natural interference, including weather conditions, light pollution, and stray radiation.

• Table Summary:
  • Similarities: Intercept radiation, ground-based, can track sources.
  • Differences: Radio telescopes are larger, simpler in design, and face more interference from human activity.

Infrared (IR) Telescopes

• Purpose: Detect cooler objects in space that emit infrared radiation.
• Design: Use large concave mirrors to focus IR radiation.
• Challenges: Must be cooled to near absolute zero with cryogenic fluids to prevent their own heat from interfering with observations.
• Placement: Must be placed in space as the atmosphere absorbs most infrared radiation.

Ultraviolet (UV) Telescopes

• Purpose: Detect objects emitting ultraviolet radiation.
• Challenges: Earth's ozone layer absorbs most UV radiation, so these telescopes must also be space-based.
• Design: Often use the Cassegrain configuration to focus UV rays.
• Uses: Observation of interstellar medium and star formation regions.

X-ray Telescopes

• Purpose: Detect high-energy X-rays emitted by sources like black holes and neutron stars.
• Challenges: Since X-rays would pass through normal mirrors, these telescopes use a combination of parabolic and hyperbolic mirrors to skim X-rays and focus them onto CCDs (Charge-Coupled Devices).
• Placement: Must be in space due to atmospheric absorption.
• Uses: Observing active galaxies and high-energy events.

Gamma-Ray Telescopes

• Purpose: Detect gamma radiation from high-energy cosmic events.
• Challenges: Gamma rays pass through mirrors, so detectors are composed of multiple layers of pixels that register gamma-ray photons as they pass through.
• Uses: Study of gamma-ray bursts (GRBs), quasars, black holes, and solar flares.

Gamma-Ray Bursts (GRBs)

• Types:
  • Short-lived GRBs: Lasting 0.01 to 1 second, associated with merging neutron stars or the formation of black holes.
  • Long-lived GRBs: Lasting from 10 to 1000 seconds, associated with Type II supernovae.