![]() ![]() More stars are resolved in this drawing by Galileo of the Pleiades than are visible to the unaided eye. This posed the question as to why there were invisible objects in the night sky? On turning his telescope to the band of the Milky Way Galileo saw it resolved into thousands of hitherto unseen stars. This then eased the problem posed by the failure of astronomers to detect stellar parallax that was a consequence of Copernicus' model. Galileo suggested that this was due to their immense distance from Earth. Stars in the Milky WayĮven through a telescope the stars still appeared as points of light. It was not until 1656 that the Dutch scientist, Christiaan Huygens correctly described them as rings. Galileo noted two appendages from the sides of Saturn. Its blemishes and imperfections again undermined the Aristotelian ideal of a perfect cosmos. Galileo, in his Letters on Sunspots supported the sunspot interpretation and used it to show that the Sun was rotating. Debate centered on whether these were satellites of the Sun or actual spots on its surface. SunspotsĪlong with contemporaries such as Thomas Harriot, David Frabicius and Christoph Scheiner, Galileo observed dark regions that appeared to move across the surface of the Sun. Galileo rejected Tycho's model as an unnecessary hybrid and used the discovery to consolidate his support of the Copernican model. This could not be explained in the Ptolemaic model but could be accounted for by either the Sun-centered Copernican model or the Earth-centered Tychonic model that had the other planets orbiting the Sun as it orbited the Earth. Venus was observed to go through a sequence of phases similar to the Moon. ![]() The largest known is 20 m, with a 1 m wide objective lens.Galileo's drawings of the moons of Jupiter of successive nights The Phases of Venus To get a good magnification you need an objective lens with a very long focal length. They do not transmit 100 % of the light some is lost (absorbed/reflected). They refract light of different colours by different amounts. Magnification can also be shown to be related to the focal lengths of theį o is the focal length of the objective lens andį e is the focal length of the eyepiece lens Disadvantages of using lenses in a telescope: The angle to the axis of the ray leaving the telescope is β.Īngular magnification can be worked out by the simple formula:Īngle α is the angle subtended by the object to the unaided eye.Īngle β is the angle subtended by the image to the eye. The observer traces the rays entering his/her eye back to infinity to form an inverted, virtual image of the original object.Ī ray arrives at the objective at a small Rays emerge from the eyepiece and enter the eye of the observer. Light from the intermediary image spreads out until it meets the eyepiece and forms an image at infinity - parallel Is set so that the intermediary image is the focal length away from its principal focus F e. Lens.There it forms an intermediary real inverted image. It will therefore be longer than a less powerful one.įrom a distant object meet at the principal focus F o of the objective ![]() A very powerful telescope will have a big objective lens focal length. Note that the length of the telescope is then set at the sum of the focal lengths of the two lenses. Magnification - the ratio of the angle subtended by the image of the object when using the instrument to view it, to the angle subtended at the eye of the observer when viewing the object without that instrumentĭiagram shows the telescope when it is set up in normal adjustment - that means to view an object at infinity, therefore the focal point of both the eyepiece lens and the objective lens are set to coincide (see diagram below). ![]() Lens can resolve detail nine times better than a 4 cm lens - area depends on the square of the diameter. the curvature of the lens must be perfectly part of a sphere and the density of the glass uniformĭiameter - brightness and detail observed depends on the surface area of the lens. Quality - poor quality lenses produce a poor image The object so that the eyepiece lens can act as a magnifying glass and produce an enlarged image of the objective lens's image!įollowing factors are important in making a good quality instrument: The objective lens makes a small real image of The refracting telescope works by bending
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