I. Radio Telescopes
A radio telescope is a fundamental device used to observe and study radio waves emitted by celestial bodies. It can measure the intensity, spectrum, and polarization of these radio waves. Components include directional antennas for collecting radio waves, high-sensitivity receivers for amplifying signals, and systems for recording, processing, and displaying information. In the 1960s, astronomy achieved four significant discoveries: pulsars, quasars, cosmic microwave background radiation, and interstellar organic molecules, collectively known as the ‘four discoveries’, all of which were made possible by radio telescopes.
II. Principle of Radio Telescopes
The principle of a radio telescope is akin to that of an optical reflecting telescope. Electromagnetic waves are reflected by a precise mirror surface to converge at a common focal point in phase. Achieving in-phase focusing is straightforward using a rotating paraboloid as the mirror surface, hence most radio telescope antennas are paraboloids. The mean square error rate of the radio telescope’s mirror surface compared to an ideal paraboloid is typically no greater than λ/16 to λ/10, allowing the telescope to effectively operate in radio wavebands with wavelengths greater than λ.
For meter or long decimeter wave observations, a metal mesh can serve as the mirror; for centimeter and millimeter wave observations, a smooth, precise metal plate or coating is required. Radio waves from celestial bodies are collected at the telescope’s focal point and must reach a sufficient power level before detection by the receiver.
Current detection technology requires a minimum of 10-20 watts. The radio frequency (RF) signal is initially amplified 10-1000 times at the focal point, converted to a lower frequency (intermediate frequency), and transmitted to the control room via cable. There, it undergoes further amplification, detection, and processing tailored to the specific study requirements.
III. Principle of Optical Telescopes
Optical telescopes operate based on the principle of light refraction. They rely on two lenses: a large diameter, long focal length convex lens at the front (objective lens) and a small diameter, short focal length lens at the rear (eyepiece). The objective lens converges light from distant scenes into an inverted, reduced real image behind it. This image, inverted, falls precisely on the front focus of the eyepiece, magnifying the scene as viewed through the eyepiece.
IV. Differences Between Optical and Radio Telescopes
Both optical and radio telescopes observe electromagnetic waves emitted by celestial objects, but they differ in the wavelengths they receive. Optical telescopes receive visible light, while radio telescopes receive radio waves, capturing light invisible to the naked eye. Radio telescopes have higher resolution capabilities than optical telescopes, as they can detect light with wavelengths much longer than those visible to optical telescopes. However, images produced by radio telescopes require computer processing and do not represent the true visual appearance of celestial bodies with the same accuracy as optical telescopes.
The farthest celestial objects observable are viewed by optical telescopes such as the Hubble Telescope, which, despite its small 2.4-meter diameter, avoids atmospheric interference and can view objects up to 14 billion light-years away. In contrast, ground-based telescopes like the 5-meter Haier telescope can only view objects up to 2 billion light-years away, as atmospheric conditions attenuate starlight by a factor of 13.