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Basic definition, page 5 Astronomy - Telescopes, 1 of 3 Telescopes, page 2

Since the day of Galilei bigger and more sophisticated telescopes have been built. Due to unimaginable size of the universe, telescope is still the only instrument astronomers can rely on to study distance planets, stars, galaxies and nebulas. Space probes have been used to investigate planets and comets but these are only confined to our solar system. In fact telescopes are still important to study these neighboring objects.

Telescopes are just like eyes: They collect light (either given off or reflection) from objects. However, the diameter of telescope lenses, or objective lenses, are much larger than those of our eyes, the formers can gather more light and hence able to detect very faint objects. The light that is gathered can then be magnified (by a small magnifying lens called eyepiece) so that an object can be seen with greater details.

The basic design of a telescope usually falls into one of the two main kind: refractive or reflective. A refractor telescope consists of an object glass placed at front end of a telescope tube and collects light from the object to be observed. The light is brought into focus and then the image is magnified by an eyepiece at the other end of the tube. On the other hand, the objective lens on a reflector telescope is actually a concave-shape mirror. It is coated with aluminium or silver to make it highly reflective. It is usually placed at the base of the telescope tube. Instead of passing through the lens, light travels into the tube and reflects by the concave mirror and brings it into focus. Once again, the image is magnified by an eyepiece.

refractor telescope object reflector telescope

Refractor telescope


Reflector telescope

WARNING! Never look at the Sun directly unaided with your telescopes. This may cause permanent eye damage.

For a given diameter, an object glass of a refractor is usually much heavier than the mirror counterpart from a reflector. In fact, the designs of most large telescopes for research studies are based on the reflector variations. It is more cost effective and larger mirrors can be made and they are suited to study very dim (deep-sky) objects.

Just like a rainbow, light from stars also consist of several component wavelengths (colors). These components are essential to determine a celestial's characteristics such as its temperature, chemical composition and even motion. Resolving these components is a technique called spectroscopy, which can be achieved by placing a prism in the telescope.

However, what we see is only the light which consists of a small part of the whole spectrum of light. In astronomy, seeing is not always believing. Some celestial objects look 'boring' in visible light but reveal in great details when using telescopes designed to detect the invisible sections of light such as X-rays, ultraviolet and infrared. Images below show the planet Venus in X-ray (green) and normal optical (blue) emission. Notice that the X-ray emission extends slightly further than the optical.

Venus in X-ray and optical
Credit: Xray: NASA/CXC/MPE/K. Dennerl et al., Optical: K. Dennerl

Telescopes that detect radio signals from space are called radio telescopes. These telescopes are usualy much larger than those of conventional telescopes mentioned above. A large dish are used to detect the long-wave, extremely weak radio signals coming from stars thousands of light years away. It is used to study objects such as pulsars (radio-emitting fast-spinning stars) and active galaxies.

Basic definition, page 5 Telescopes, page 2


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