The tube also helps to keeps out dust, moisture and light that would interfere with forming a good image. The tube holds the lenses in place at the correct distance from one another. Diagram of a refractor showing the light path inside. The eyepiece is just the case for the eyepiece lens. Externally, the parts of a refracting telescope include the eyepiece, finderscope, optical tube, aperture, focuser and mount. His telescope could magnify objects 3 times. After hearing of the invention of the telescope in, he built one of his own, called a Gallilean Telescope, in using a convex objective lens and a concave eyepiece lens. A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as Optical diagram of Galilean telescope y – Distant object y′ – Real image from objective y″ – Magnified virtual image from eyepiece D.Galileo is credited with being the first person to use a telescope to make observations of the night sky. Amateur astronomers use two main types of telescopes: reflecting and refracting.Ī reflecting telescope uses mirrors to focus light from a distant object, while a. Diagram illustrating the basic optical design of Galileo's reracting telescope. Two converging lenses, an objective and an eyepiece, make a refacting or Keplerian telescope. How Telescopes Work Refracting Telescopes In the diagram below, light is leaving air and entering glass, so it bends towards the normal on the.Figure 12: A rather extreme example of chromatic aberration. The picture of the moon below is an extreme example of chromatic aberration, and usually the effect isn’t so drastic. This is a fundamental property of refraction, and as such only affects refracting telescopes Figure 11: Chromatic aberration in a lens Blue light is brought to a focus closer to the lens and red light will have a longer focal length. This is caused by different wavelengths of light being diffracted by a different amount as they pass through a lens. The other type of aberration that telescopes suffer from is chromatic aberration. However parabolic mirrors and lenses are much more expensive to produce, so cheaper telescopes suffer more from this aberration more than expensive ones with higher specification optics. Spherical aberration can be corrected by using a parabolic mirror or lens, as this will bring rays to a single focus no matter their distance from the principle axis, as in the diagram below: Figure 10:Spherical aberration can be corrected by using a parabolic mirror rather than a spherical one. ![]() The objective lens collects the light from stars and brings it to a focus at its focal length, $f_$, 40 times smaller. They consist of an objective lens and an eyepiece lens. These are the easiest to understand, and were the first to be developed. These telescopes use two lenses to collect light and allow astronomical objects to be viewed. Reflecting telescopes - Constructed from a concave mirror.Įach has their own set of limitations and advantages.Refracting telescopes - Constructed from two or more convex lenses.Figure 1: An image of the star Antares taken by the ESO's Very Large Telescope Interferometer.ĭespite the fact that through most telescopes stars still appear as just points of light, we have still managed to learn a huge amount about the universe just be carefully studying the light, and by building better and better telescopes.Īstronomical telescopes come in two different types, The image below is the first ever image taken of a star other than the Sun. ![]() In fact, it wasn’t until very recently that images have been able to be made of any stars, and even then the images require multiple telescopes and a lot of computer processing. So although stars are not an infinite distance away, even the closest star is so far from Earth that it appears to be a point source of light.Īstronomical telescopes do not magnify stars, they still appear to be barely more than points of light through even the most powerful telescopes. By definition a light source that produces parallel rays of light is said to be at infinity. Stars are point sources of light, which are so far away that the light that reaches us is almost exactly parallel. ![]() So far we have only considered light passing through one lens, being scattered by an object close to the lens.
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