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Land Based Telescopes Telescopes: "light buckets" Primary functions: Gather light from a 1. ___________ given region of sky. Focus light. 2. ______ Secondary functions: Resolve detail in image 1. ____________ Magnify angular size 2. _______ of objects. Optical Telescopes wavelengths of light • Designed to collect ____________ visible that are ________ to the human eye. • Data observed by human eyes or recorded on photographs or in computers. The Human Eye: Shortcomings size • Eye has limited _____. – limited light gathering power. frequency response • Eye has limited ___________________. – only detects E-M in visible wavelengths. multiple • Eye distinguishes new image _________ times/second _____________. – cannot be used to accumulate light over long period to intensify faint image. cannot store • Eye _____________ image for future reference. – unlike photographic plate or CCD. Optical Telescope Design • Basic telescope has two parts: Objective 1. _________ gather light Function: to ____________ Lens/mirror of longer focal Materials: __________ length & larger diameter than the eyepiece Eyepiece 2. _____________ magnify image made by Function: to ____________ objective Material: ____ lens with a shorter focal length than the objective Optical Telescopes • ___________ Refractors refraction –Focus light with _________: bend light path in transparent medium –Use _______ lenses –First kind made, used by Galileo • __________ Reflectors reflection –Focus light by __________: bounce light off a solid medium –Use ________ mirror –First designed and created by Sir Isaac Newton • ____________ Catadioptric lenses and _______ mirrors -Uses both ______ First Optical Telescopes: Refractors Image of source is formed on focal plane and magnified by eyepiece. Refracting Telescopes The Yerkes 40” Refracting Telescope Refractors: Disadvantages high tolerance • Quality optics require ______________ perfect – surfaces must be _______ absorb light, especially IR and UV. – glass will _______ – changes in orientation, _____________________ temperature may flex lenses Large size very heavy, hard to support – __________ aberration • Chromatic ___________________ –light passes through glass –refraction a function of wavelength –all wavelengths focus different distances from lens –correctable with compound lenses, expensive Chromatic Aberration • Dispersion of light through optical material causes blue component of light passing through lens to be focused slightly closer to lens than red component. • Known as chromatic aberration. Reflecting Telescopes: Designs Why build reflectors instead of refractors (advantages)? 1.Mirrors don’t have _________ chromatic aberration __________. absorb light 2.Mirrors don’t ____________ (especially infrared and UV). 3.Mirrors can be supported by their edge ____ and back; lenses by ONLY their edge. one surface to be 4.Mirrors have only ____________ machined correctly; lenses have two. Why build reflectors instead of refractors? 5. Telescopes made with mirrors compact in design; can be _________ reflectors cannot. 6. Telescopes using mirrors can have ______________ larger objective ends (because they have bigger mirrors), which means _______________________. more light-gathering power Powers of the Telescope 1. Magnifying Power The ability to enlarge an image. 2. Light Gathering Power The ability to see faint objects. 3. Resolving Power The ability to see fine details. Magnification and Focal Length focal length of objective focal length of eyepiece focal length of objective Magnification = focal length of eyepiece Light-Gathering Power •The objective’s area collects light. •The larger the area, the greater the light-gathering power of telescope. Light-gathering power proportional to (objective diameter)2 ___________________. diameter Resolving Power • Varies directly with the ____________ diameter of objective _________. • Also depends on wavelength of light being observed and – ___________ atmospheric seeing conditions. – __________________ Resolving Power: Diameter and Wavelength Site Selection • Where are the best places for ground-based observatories? • Important factors dark/light pollution – __________ – good _____ weather – dry ___ air air turbulence – _____________ Closer to Sea Level, More air to pass through Higher Altitude, telescopes in the high mountains Earth At Night U.S.A. At Night (circa 1994-95) Detection • Collected light detected in many ways. –image observed and recorded • eye, photographic plate, ____ CCD –measurements • intensity and time variability of source –___________ photometer • spectrum of source –____________ spectrometer CCD Imaging • A charge-coupled device(CCD) – Wafer of silicon divided into a twodimensional array of many tiny elements, known as pixels. – When light strikes a pixel, electric charge builds up on device. – Charge buildup monitored electronically. Radio Telescopes larger than •Much _______ reflecting optical telescopes •Resemble satellite TV dishes •Used to collect radio waves from space •AM, FM, and TV signals interfere, so must be in a radio “protected” area Radio Astronomy: Wavelength Advantages • • • • dependent on time of day/night NOT __________ NOT as __________ dependent on weather Use of interferometry visible light Information other than _____________ – Quasars, pulsars • Generally _____________ not absorbed traveling space – pass through clouds of interstellar dust in our galactic plane Accuracy of dish shape not as hard to • __________ create or maintain – not need to be highly polish, often light weight Arecibo Observatory: Largest Radio and Radar Dish • 1000-ft radio dish • Used to –create maps of Moon, Venus, and Mars –discover pulsars and galaxies –measure the rotation rate of Mercury –discover planetary systems outside of our solar system Very Large Array(VLA) in New Mexico 27 antennas, each 25 m in diameter Effective diameter = 36 km Yields radio-image details comparable to optical resolution Interferometry • Two ___________ or more telescopes used –to observe same object –at same wavelength and –at the same time. • Uses wave interference to yield high resolution. Cheaper than one • ________ (impossibly) large telescope. • Farthest 2 _ telescopes act like the end of one telescope. –Baseline: • distance between 2 farthest ‘scopes. • equals the relative ‘scope size.