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Transcript
TELESCOPIC ASTRONOMY What is a telescope? • What are telescopes used for? • View distant objects • Collect light First Telescope • 1608- Hans Lippershey • Hans Lippershey was a Dutch lens maker. • 1609- Galileo. Built his own telescope, used it for scientific study. – – – – Mountains and valleys on Moon Moons of Jupiter Phases of Venus Saturn’s rings Galileo’s Telescope Optical Telescopes Optical telescopes use a lens and light. One type: Refracting telescope uses two large lenses to gather and focus light. Primary lens: the main lens in a refracting telescope. It is also called an objective lens. Eyepiece: A small lens to magnify the image produced by the objective (primary) lens Focal length – the distance from the lens or mirror to the image formed of a distant light source Pros • • • • • Simple design Minimal maintenance Good for planets and moons Good for photography High contrast Cons • Costly • Bulky and large (large focal length) • Chromatic aberration Refraction Limitation When light is refracted through glass, shorter wavelengths bend more than longer wavelengths, and blue light comes to a focus closer to the lens than does red light. Refraction Limitation If we focus on the blue image, the red image is out of focus and we see a red blur around the image. This color separation is called chromatic aberration. An achromatic lens is made of two components made of different kinds of glass and brings the two different wavelengths to the same focus. Other wavelengths are still out of focus. Do we still use these types of telescopes?? Yerkes Refracting Telescope • Largest refracting telescope in the world is at Yerkes Observatory in Wisconsin • Lens is 1m in diameter • ½ tonne • The glass sags under its own weight Reflecting Telescope • 1666- Newton found that a prism breaks up white light into a rainbow of colours • Telescope lenses do the same – Creates haloes of coloured light around objects being viewed Newton’s Telescope Primary mirror: the main mirror in a reflecting telescope. It is larger than the secondary mirror. It is also called an objective mirror. • NOTE: The primary mirror is in a different spot than the primary lens of a refracting telescope. Secondary mirror: the smaller mirror in a reflecting telescope. It directs the light through a small hole in the primary mirror to the eyepiece. Benefits of Reflecting Telescopes Less expensive. Only the front surface of the mirror must be ground. The mirror can be supported to reduce sagging. They do not suffer from chromatic aberration because the light is reflected toward the focus before it can enter the glass. Cons of Reflecting Telescopes • More maintenance William Hershel (1757-1822) • By late 1770s, he was making the best metallic mirrors and telescopes in the world. • 1781- Discovered Uranus • His telescope had a 125cm mirror and was 40ft in length Hershel’s Telescope Large Telescopes • Up until mid-1800s telescopes were generally small and not very powerful • 1838- Earl of Rosse, Ireland, taught himself art of mirror-making and built a 91cm telescope • 1842- attempted to build a 181cm telescope but it broke when moved – built another one that couldn’t be moved Rosse’s Telescope Rosse is important because he built the largest telescope the world had ever seen and he saw further into space than anyone had before him. Cassegrain Telescope • Specific type of reflecting telescope • a wide-angle reflecting telescope • the eyepiece or camera is mounted at the back end of the tube • developed in 1672 Schmidt-Cassegrain Telescope • Specific type of reflecting telescope • a wide-angle reflecting telescope (Cassegrain telescope) with a correcting lens that minimizes spherical aberration • The correcting plate (a lens) was added in 1930 by the Estonian astronomer and lensmaker Bernard Schmidt (1879-1935). New Generation Telescopes • Refraction and Reflection telescopes are OPTICAL telescopes (uses light) • New generation telescopes are RADIO telescopes (uses waves) • Radio telescopes use dishes to pick up radio waves. Sensors on the dishes collect the waves and turn them into a picture. Radio Telescopes New Generation Telescopes • 1993 – Keck telescope 1000cm mirror, made of smaller segments • Photographic plates were more sensitive and permitted a permanent record of observations – Photographic plates have since been replaced by electronic imaging devices Keck Telescope Handicaps to Radio Telescopes 1) Poor resolution: To improve resolution, two or more radio telescopes can be combined to improve the resolving power Handicaps to Radio Telescopes 2. Low intensity In order to get strong signals focused on the antenna, the radio astronomer must build large collecting dishes. The largest dish is the 300 m dish at Arecibo, Puerto Rico. Handicaps to Radio Telescopes • Interference: This occurs because of poorly designed transmitters in Earth satellites to automobiles with faulty ignition systems. Powers of a Telescope 1. Lightgathering powerThis is the ability of a telescope to collect light. The larger the telescope’s lens, the more light it can gather. Powers of a Telescope 2. Resolving Power – the ability of a telescope to reveal fine detail. • The larger the telescope, the better its resolving power. However, optical quality of the lens and atmospheric conditions limit the detail we can see. Powers of a Telescope Magnifying power – the ability to make the image bigger. Magnification of a telescope can be changed by changing the eyepiece. Astronomers identify telescopes by diameter because that determines both light-gathering power and resolving power. Telescopes and Observatories • An observatory is a place for viewing the sky through telescopes. • Observatories can be small with just a single telescope outside or they can be large buildings housing a number of telescopes, each in their own room. Hale Observatory, California • The traditional image of an observatory is probably that of a large building with domed rooms housing telescopes. But an observatory can be just a single room with a telescope, or nothing more than an open area with telescopes. Observatories are built on top of mountains because: 1) air is thin and more transparent 2) the sky is darker 3) stars are brighter 4) wind blows smoothly over some mountaintops 5) there is less pollution Telescopes and the Scientific Method The Scientific Method The Scientific Method’s 4 Steps 1) Observation and description of a phenomenon. The observations are made visually or with the aid of scientific equipment (like a telescope). 2) Formulation of a hypothesis to explain the phenomenon (usually in the form of math or a causal mechanism or a mathematical relation. • Test the hypothesis by analyzing the results of observations or by predicting and observing the existence of new phenomena that follow from the hypothesis. If experiments do not confirm the hypothesis, the hypothesis must be rejected or modified (Go back to Step 2). • Establish a theory based on repeated verification of the results.