PHYS 183 Chapter Notes - Chapter 6: Diffraction Grating, Primary Mirror, Spectral Resolution

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Published on 6 Feb 2013
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6: Telescopes
6.2 Telescopes: Giant Eyes
2 main properties of telescopes:
o light of collecting area: tells us how much total light telescope can collect at one time
characterize telescope’s size as diameter of light-collecting area (since lens is round)
o angular resolution: smallest angle over which we can tell that two dots/stars are
distinct human eye has angular resolution of 1 archminute, so must have that must
angular separation in sky for us to tell they are 2 stars, not 1
dependent on actually separation & distance from us
ultimate limit of telescope properties of light
o since light is an electromagnetic wave, can interfere with one another like ripples
interference limits telescope’s angular resolution
diffraction limit: angular resolution a telescope could achieve if it were limited only by
interference of light waves depends on diameter of primary mirror & wavelength of light
refracting telescope: operates like an eye, using transparent glass lenses to collect & focus light
reflecting telescope: uses precisely curved primary mirror to gather light & reflects it onto a
secondary mirror which reflects the light to a focus at a place where the eye can observe it
nearly all in current research are reflecting telescopes
o light passes through lens of refracting telescope so it must be made from clear, high-
quality glass with precisely shaped surfaces on both sides reflecting surface of mirror
must be precisely shaped, not both sides
o difficult to stabilize refracting telescopes & prevent large lenses from deforming
primary mirror of reflecting at bottom so weight less serious problem
observations through telescopes fall into 3 categories
o imaging photographs or astronomical objects
may place filters in front to allow particular colours/wavelengths
most images of astronomy are combined images of different filters
today many images made from invisible light (anything other than visible light)
no natural colour since colour is a property of visible light
o spectroscopy obtain & study spectra
spectrographs: use diffraction grating to separate various colours of light into
information from image depends on angular resolution & spectral resolution
requires longer exposure time than taking an image
o timing tracks how object changes with time
results often shown as light curves
light curves: graphs that show how an object’s intensity varies with time
6.3 Telescopes and the Atmosphere
telescopes on the ground are less expensive to build, operate & maintain than telescopes in
problems with observing from earth daylight, weather, light pollution, blurry images by
atmospheric motion, light not reaching ground
daylight daytime sky brighter since atmosphere scatters sunlight which drowns dim light of
most astronomical objects
light pollution lights of cities scattered at night making it hard to see light of astronomical
o placing reflective covers on tops of streetlights directs more light toward ground
o low pressure lights can only be seen in a few wavelengths, so can be filtered out
o both of these offer significant energy savings
distortion of light by the atmosphere turbulence constantly changes the atmosphere’s light-
bending properties so light rays are usually bent by different amounts (we see this as twinkling
of stars)
o limits angular resolution
o must choose key sites to observe
o adaptive optics can eliminate a lot of blurring (cancels out atmospheric distortions)
atmosphere prevents most forms of light from reaching ground can’t observe light across the
electromagnetic spectrum from earth
6.4 Telescopes and Technology
radio telescopes
o most common type of telescope every satellite dish is essentially a small radio
dish is a primary mirror, reflecting radio waves toward receiver; receiver acts
like secondary mirror, sending radio waves to decoding device
o rise & set with earth’s rotation
o long wavelengths so large telescopes to achieve reasonable angular resolution
o radio waves & visible light can be observed easily from the ground
o not distorted by the atmosphere
o radio-wave pollution is a serious impediment hope to one day put them deep into
o can be made to work together, so could be spread out over much greater distance in
infrared telescopes
o infrared portion of spectrum close to visible light, so behave similarly
o infrared light from space doesn’t reach the ground though (some at high mountain tops)
a Boeing 747 has a telescope built into it
o longer wavelengths harder only solution to put telescopes in space to get away from
earth’s heat (infrared light)
ultraviolet telescopes
o much of ultraviolet spectrum close enough to visible light to behave similarly
o atmosphere absorbs almost all impossible to observe from ground
o currently 2 telescopes in space
x-ray telescopes
o need to be in space
o x-rays have sufficient energy to penetrate many materials
o grazing incidence mirrors: specifically designed mirrors to deflect x-rays after they graze
the surface
gamma ray telescopes
o gamma rays can penetrate grazing incidence mirrors
o takes massive detector to capture the photons
have learned virtually everything we know about distant objects by observing light 3 other
types of messengers that can tell us information
o neutrino extremely lightweight type of subatomic particle produced by nuclear
reactions like nuclear fusion in sun; neutrino telescopes typically in deep mines or under
o cosmic rays very high energy subatomic particles from space; astronomers using both
satellites & ground-based detectors to catch and study them
o gravitational waves different in nature from light but travel at same speed; hoping to
detect gravitational waves from exotic objects like orbiting neutron stars & black holes
interferometry: technique for improving angular resolution of radio telescopes link two or
more individual telescopes to achieve angular resolution of much larger telescope; takes
advantage of wavelike properties of light that cause interference
o more difficult for shorter wavelength light
o telescopes now being built in pairs so they can be used for interferometry