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Material Type: Project; Class: DESCRIPTIVE ASTRONOMY; Subject: Physics; University: Oregon State University; Term: Unknown 1989;
Typology: Study Guides, Projects, Research
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-^ Telescopes have been usedfor hundreds of years tocollect light from the sky andfocus it into an eyepiece. Anastronomer would then lookthrough the eyepiece atplanets, nebulae, etc. •^ The human eye is not verysensitive to dim light, andwas replaced in astronomyby the film camera. •^ Film is sensitive to onlyaround 10% of the impinginglight, and is often todayreplaced by a…
-^ The CCD, similar tothose found incommercial digitalcameras and phones, cancollect around 75% ofthe visible light that isfocused on it. •^ It has revolutionizedastronomy – images canbe recorded anddownloaded to acomputer anywhere inthe world for analysis. -^ The science of developing newmethods for sensing, focusing andimaging light in astronomy is called^ instrumentation.
www.physics.oregonstate.edu/~hetheriw/astro/index.html
-^ Modern telescopes aredesigned to collect as muchlight as possible. •^ Collected visible light is ofnanometer wavelength; thetelescopes must be extremelyprecise to keep the imagesclear and sharp.
-^ Aperture
Area Collecting
D × π=
-^ A lens is a speciallyshaped piece of glass thatbends light rays passingthrough it so that theyfocus a particulardistance away (the focallength) at a particularlocation (the focal plane). •^ A^ sensor -
such as the human retina, camerafilm, or a CCD - placedin the focal plane canimage the light.
-^ The amount that light is diffracted depends on its wavelength. •^ A prism or raindrop spreads white light out into colors. •^ This dispersion of light is a problem in refracting telescopes (andcameras too), as the focal plane will be at a slightly different locationfor each wavelength (color) of light. •^ This leads to
chromatic aberration
, a blurring effect.
-^ Mirrors can be supported frombehind, and so can be muchlarger than refractors. •^ Larger sizes mean that morelight can be collected andfocused, allowing astronomers toimage dimmer or more distantobjects. •^ Most modern telescopes arereflectors.
-^ X-rays only reflect at glancing angles, otherwise they areabsorbed or pass through the mirror. •^ X-Ray mirrors are designed to gently reflect theincoming photons, focusing them at the end of a longtube-shaped array of mirrors
-^ Reflectors can be made even largerif multiple mirrors are used as theprimary mirror. •^ The Keck Telescope uses 36 largemirrors to create a single hugeprimary reflecting surface. •^ The positions of the mirrors areprecisely measured by lasers, andcan be individually adjusted tokeep them perfectly aligned.
-^ Diffracted light waves spread andinterfere with each other. •^ This results in a
diffraction pattern
, a
blurring of the image as it passesthrough the telescope. • Larger apertures have less diffraction,and therefore have higher resolutionthan smaller apertures. • For observing light of wavelength
λ,nm
the smallest separation angle
αaarcsec^
telescope can resolve is related to thetelescope aperture D
by:cm λ nm D cm
α^
× (^02). (^0) = arcsec
Interferometers
-^ To counter diffraction effects (andobtain higher resolution),astronomers use
interferometers
-^ Signals from these arrays ofwidely-separated telescopes areadded together to create imageswith very high resolution. •^ In fact, the resolution isequivalent to that of a singletelescope with an aperture aslarge as the separation betweentelescopes in the array
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