Ⅱ. Telescopes & Detectors

The Optical Telescope

The purpose of the telescope is to enhance light gathering power and to increase resolving power. Light gathering power is proportionate to the square of the aperture, and resolving power, which results from a diffraction phenomenon, is proportionate to λ/D (wavelength/ the diameter of the telescope. Structurally, optical telescopes can be broadly distinguished into refracting telescopes and reflecting telescopes. Since reflecting telescopes have various advantages, such as the ability to eliminate spherical aberration and chromatic aberration, it is the telescope used for the majority of research. Other examples include the meridian transit instrument used to accurately measure transit time, the Schmidt telescope which offers a wide scope, and the Coude telescope used to photograph a high dispersive spectrum. Optical telescopes are installed in locations that offer ideal views and many feasible days for observation. In the northern hemisphere, the island of Mauna Kea in Hawaii is known to be the optimal location.

Currently, advanced nations are competing to lead the way in developing large-scale telescopes on land and observatory satellites equipped with the latest technology in outer space. One representative case is the VLT (Very Large Telescope) project being pursued by the European Union. In Korea, Bohyunsan Optical Astronomy Observatory operates a 1.8 m telescope and the Taeduk Radio Astronomy Observatory operates a 10m radio telescope. Even just around a decade ago, telescopes that measured around 3 m were considered to be large scale, but recently, the size must reach around 8m or 10m to be considered to belong within the category of a large-scale telescope.

Radio Telescope

Radio telescopes are used for radio observations, and were first developed by Jensky. Due to their inferior resolving power, the size of radio telescopes is significantly larger than the size of optical telescopes. Radio interferometry was developed to improve the resolving power of radio telescopes, and techniques such as VLA (Very Long Array) and VLBI (Very Long Baseline Interferometry) are currently applied for operation. The spectrometer was developed to record spectrums and it is categorized into prism spectrometers and diffraction spectrometers.

Space Telescope

When it is difficult to observe stars from land due to the absorption of the light from the stars by the earth’s atmospheric layer, we rely on other instruments or observation satellites to conduct observations. Absorption occurs frequently in the wavelength bands of the gamma rays, x-rays, ultraviolet rays and infrared rays. Numerous observation satellites have been launched up to the present and have contributed significant developments in modern astronomy. Successful examples include ROSAT, IRAS, etc.

The Hubble Space Telescope is a 2.4m telescope positioned in an orbit with an altitude of 500 km to overcome the limitations of terrestrial telescopes hampered by the inevitable presence of the earth’s atmospheric layer. The Hubble Telescope is capable of observing astronomical objects that are 50 times darker than those observable by any terrestrial telescope, and for this reason it has enabled revolutionary developments in the field of optical astronomy.


Detectors are no less important than telescopes for the purpose of observing celestial bodies. The human eye itself is a light detector and in photographic observation we use photographic plates and in photoelectric observation we use the photomultiplier tube utilizing the photoelectric effect as detectors. More recently, the Charge Coupled Device (CCD) has been developed and enabled a revolution in astronomical observation, and this detector offering various advantages including its advanced quantum efficiency across a broad range of wavelengths.

Types of Astronomical Observation

Astronomical observation can be broadly categorized into photographic observation, photoelectric observation, and spectroscopic observation. In photographic observation, a photograph is taken of a relatively broad region using the photographic plate to measure the proper motion of stars or the categorization of galaxies according to their form. In photoelectric observation, the photoelectric effect is used to determine the luminosity or color index of celestial bodies, while the spectrometer is used in spectroscopic observation to obtain the spectrum of the celestial bodies and to investigate their physical conditions of fixed stars. Differential photoelectric photometry and all-sky photoelectric photometry are conducted to determine magnitude of a star, and in each case, observations must be made of the respective comparison star and standard star.


  1. What is the function of an optical telescope? Why do observers prefer a larger sized telescope?
  2. If an optical telescope has an aperture of 30cm, a focal distance of 210cm, and the focal distance of its eyepiece is 5cm, what would be its magnifying power? What is the resolving power of this telescope in the yellow wavelength? Compare the resolving power of this telescope to the resolving power of a 300 m radio telescope that observes with a 21 cm wavelength.
  3. What are the respective advantages and drawbacks of reflecting telescope and a refracting telescope?
  4. Explain the concepts of spherical aberration and chromatic aberration.
  5. For each of the following, name the equipment that would be most suited to conduct the observation.
    - Determining the accurate distance to the moon
    - The rotation period of Mercury
    - Photographic observation of a dark galaxy
    - Photographic observation of a broad region
    - Observation of long waves emitted by the sun or Jupiter and low energy radiant rays
    - The decomposition of a binary star
    - A star with low temperature
    - X-ray emission from a quasar
  6. What are some of the advantages and disadvantages of installing a telescope on the moon compared to terrestrial telescopes?
  7. If the diameter of Jupiter is assumed to be 50〃, what is the size of the image obtained by a telescope with a diameter of 5 m and a focal distance of 17 m? If we are to observe Jupiter using a telescope with a diameter of 15 cm and a focal distance of 180 cm, what must be the focal distance of the eyepiece for this telescope?