6. optics and telescopes
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6. Optics and Telescopes. Refractingtelescopes Reflectingtelescopes Image degradation Imaging systems Spectrographs Non-opticaltelescopes Orbitingtelescopes. Parallel Rays From Distant Objects. Refracting Telescopes. A lens is the primary image-forming tool - PowerPoint PPT PresentationTRANSCRIPT
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6. Optics and Telescopes• Refracting telescopes• Reflecting telescopes• Image degradation• Imaging systems• Spectrographs• Non-optical telescopes• Orbiting telescopes
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Parallel Rays From Distant Objects
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Refracting Telescopes• A lens is the primary image-forming tool
– Other lenses and/or mirrors may also be used• Basic physical process
– Refraction• EMR bends due to speed differences in different media
• Basic benefits– Very high contrast of resulting image
• Basic problems– Severe practical limits on the size of the primary
• Lenses cannot be mechanically supported from behind– Chromatic aberration
• Different wavelengths refract by different amounts• Basic solution
– Achromatic lenses
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Refracting Telescope Designs• Convex primary lens & convex eyepiece lens
– Inverted image Astronomicaltelescopes
• Convex primary lens & concave eyepiece lens– Upright image Terrestrial
telescopes
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Chromatic Aberration In Lenses
Simple lens Achromatic lensOnly one lens Two or more lenses
1 1 2
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Reflecting Telescopes• A mirror is the primary image-forming tool
– Other mirrors and/or lenses may also be used• Basic physical process
– Reflection• Re-direction of incoming light rays
– No practical limits on the size of the primary• Mirrors can be mechanically supported from behind
• Basic problems– Relatively low contrast of resulting image– Spherical aberration
• Edge incident rays focus too close to the primary mirror• Basic solutions
– Parabolic, not spherical primary mirror surface
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Reflection by a Concave Mirror
(Prime Focus)
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Reflecting Telescope Designs
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Isaac Newton’s Second Telescope
http://upload.wikimedia.org/wikipedia/commons/c/cc/NewtonsTelescopeReplica.jpg
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Corrections for Spherical Aberration
Primefocus
SchmidtCassegrain
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Reflector Telescope Technology• Active optics
– Purpose Keep the primary in ideal optical shape• Gravity distorts the primary as the telescope moves
– Properties• Numerous actuators on the back of the primary mirror• Computer-adjusted tens of times per second
• Adaptive optics– Purpose Minimize thermal current effects
• “Twinkle, twinkle, little star…”– Properties
• A corrector plate is inserted near the focal plane• Computer-adjusted thousands of times per second• Image quality depends on processing computer speed• Data from a real or synthetic “guide star”
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Active Optics Actuators: Slow!
http://upload.wikimedia.org/wikipedia/commons/5/5d/GTC_Active_Optics_Acutators.jpg
Thick telescope mirror
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Adaptive Optics Actuators: Fast!
http://upload.wikimedia.org/wikipedia/commons/b/bc/Prototype_of_part_of_the_adaptive_support_system_of_the_E-ELT.jpg
Thindeformable
mirror
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Adaptive Optics Improve Sharpness
Without With adaptive adaptiveoptics optics
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Two Properties of All Telescopes• Magnification Apparent closeness
– Lens or mirror without eyepiece• Directly proportional to the focal length of the primary
– Lens or mirror with eyepiece• Primary focal length / Eyepiece focal length
– Double the primary focal length Double the magnification
– Halve the eyepiece focal length Double the magnification
• Light-gathering power Apparent brightness– Unobstructed lens or mirror
• Directly proportional to the surface area of the primary– Obstructed lens or mirror
• Surface area of primary – Surface area of obstruction– Lens or mirror arrays
• Combined surface area of all primaries in the array– Very Large Array (VLA) radio telescope
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Two More Properties of Telescopes• Angular resolution Apparent detail
– Single lens or mirror Smaller is better• Directly proportional to wavelength of observed EMR• Inversely proportional to diameter of the primary
– Multiple lenses or mirrors• Directly proportional to observed EMR wavelength• Inversely proportional to distance between primaries
• Field of view Apparent sky area– Angular diameter of visible telescope sky region– Important variables
• Inversely related to the focal length of the primary– Short primary focal lengths produce wide fields of view
• Directly related to the focal length of the eyepiece– Long eyepiece focal lengths produce wide fields of view
– Rich-field ’scopes: Low magnification & wide field
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The 200-Inch Palomar Telescope
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The Observatory on Mauna Kea
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Mauna Kea’s Keck I Telescope
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Mauna Kea’s Gemini North ’scope
http://zuserver2.star.ucl.ac.uk/~idh/apod/image/9906/gemini_pfa_big.jpgInstrument array
http://www.hia-iha.nrc-cnrc.gc.ca/atrgv/altair2_e.html
Secondarymirror
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Multiple Mirror Telescope MakeoverBefore 1998 After 2000
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Atmospheric Effects• Thermal currents
– Basic physical process• Low-density warm air rises & high-density cool air falls• Rapid heat loss from the atmosphere after sunset• [Early] nighttime atmospheric instability
– Solutions Adaptive optics & optimal locations• Light pollution
– Basic physical process• Light scatters from air molecules• Very few areas are far from large cities
– Solutions Fewer & well-screened city lights• Air pollution
– Basic physical process• Light scatters from air pollution molecules• Very few areas are far from pollution sources & plumes
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Image Recording Systems: Film• Film The historic recording medium
– Black & white Most sensitive type of film• Often taken through blue & red filters• Often heated to increase sensitivity• Always problematic
– Non-linear response to EMR– Sensitivity & development variables– Dimensional instability (film expands & shrinks with humidity)
– Color Least sensitive type of film• Normally used only for very bright celestial objects
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Image Recording Systems: CCD’s• CCD’s
The modern recording medium– Technology of Charge-Coupled-Devices
• Light-sensitive computer chip• Major advantages
– Highly linear response to EMR– No sensitivity or development variables– Extreme dimensional stability
– Black & white• The native mode of astronomical CCD’s
– Color• Multiple exposure through colored filters
– Red, green & blue for natural color– Other filter combinations for other color composites
– False-color• Arbitrary colors applied to non-visible wavelengths
– Various thermal infrared wavelengths
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A Charge-Coupled Device (CCD)
http://www.tech-faq.com/wp-content/uploads/Charge-Coupled-Device.jpg
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Astronomical Spectroscopes• Basic physical process
– Spread starlight into a rainbow• Observe & analyze spectral features
• Basic types of astronomical spectroscopes– Refraction spectroscopes
• Benefit– Well-known properties of lenses & prisms
• Drawback– Differential absorption of EMR by glass
– Reflection spectroscopes• Benefits
– Refraction gratings work on many EMR wavelengths– No differential absorption of EMR by glass
• Drawback– Transmission through the reflective aluminum coating
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A Rare Refraction Spectrograph
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A Common Reflection Spectrograph
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Displaying A Spectrum• Photographic
– Color representation• Color films never accurately represent colors• Computers rarely accurately represent colors
– Analog rather than digital• Ambiguity regarding the actual brightness
• Graphic– Color representation
• Data drawn on Cartesian coordinates– X-axis represents EMR wavelength– Y-axis represents EMR intensity
• Representation is as accurate as the original data– Digital rather than analog
• No ambiguity regarding the actual brightness
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Two Representations of a Spectrum
Absorptionline
Absorptionline
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Thermal Infrared Observations• Non-dedicated telescopes
– Limiting factors• Dry air minimizes absorption of TIR wavelengths• Remote enough to minimize thermal pollution effects
– Existing telescopes at Mauna Kea, Hawai‘i• Keck I & Keck II
– Near Infrared Camera for the Keck I Telescope(NIRC)
– Near Infrared Camera for the Keck II Telescope(NIRC2)
– Near Infrared Spectrometer(NIRSPEC)
– Long Wavelength Infrared Camera(LWIRC)
• Gemini North telescope• Dedicated TIR telescopes
– Existing telescopes at Mauna Kea, Hawai‘i• NASA Infrared Telescope Facility (IRTF)• United Kingdom Infrared 3.8-meter Telescope
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Radio Telescopes• Brief history
– First EM l’s used for astronomy after visible• Karl Jansky (Bell Telephone Laboratories)
– Discovered radio emissions from the galactic center
1932• Grote Reber
– Built the first radio telescope in his Illinois back yard
1936– Discovered radio emissions from many galactic locations
• Modern radio telescopes– Arecibo
Puerto Rico– Very Large Array (VLA)
New Mexico• Classic example of radio telescope interferometry• Better spatial resolution than any optical telescope
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Radio Telescopes Are Mostly AirRadio l’s are long enough to reflect from a grating
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More Telescope Technology• Basic physical process of telescope arrays
– Constructive interference between focused rays– A “synthetic aperture” larger than one telescope
• Existing instruments– Radio telescope arrays [interferometers]
• Relatively common & extremely successful– Very Large Array (VLA)
– Optical telescope arrays [interferometers]• “All-in-one” telescopes with segmented mirrors
– Keck I & Keck II individually, each with 36 hexagonal mirrors– Multi-Mirror Telescope (MMT), now a single large mirror ! ! !
• Independent telescopes– Keck I & Keck II working together
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Build a Large Synthetic Aperture
Small telescopes
LargeSyntheticaperture
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The Very Large Array (Radio)
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The Arecibo Radio Telescope• World’s largest radio telescope
– Built in a doline (limestone sinkhole)
Arecibo O
bservatory in a James B
ond Movie
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Earth’s Atmospheric Transparency
• X-rays Completely opaque• Ultraviolet Completely opaque• Visible Mostly transparent• Infrared Intermittently transparent• Microwaves Part is opaque, part transparent• Radio Part is transparent, part opaque
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Entire Sky at Different Wavelengths
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Orbiting Telescopes• Reasons
– Absorption & scattering by Earth’s atmosphere• Gamma rays Strongly absorbed by air
molecules• X-rays Strongly absorbed by air
molecules• Ultraviolet Strongly scattered by air
molecules• Thermal infrared Absorbed by water
vapor– Atmospheric turbulence
• Rising warm & falling cool air parcels• Corrective measures
– Absorption & scattering Extremely high altitude• Recent NASA balloon missions
– Atmospheric turbulence Adaptive optics• Rapidly increasing computer speed
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Hubble Space Telescope (HST)
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Examples of Orbiting Telescopes• Ultraviolet
– Extreme Ultraviolet Explorer (EUVE)• Mission ended in 2000
– Hopkins Ultraviolet Telescope (HUT)• Far-ultraviolet portion of the EMS
• Infrared– Space Infrared Telescope Facility
(SIRTF)• Launch on 25 August 2003
• X-Ray– Chandra X-Ray Observatory
• Reached its operational orbit on 7 August 1999• Gamma Ray
– Compton Gamma Ray Observatory• Launched 7 April 1991
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Next Generation Space Telescope• Renamed “James Webb Space Telescope”
– NASA’s second Administrator• Largely responsible for NASA’s science programs
– Important facts• Replacement for the Hubble Space Telescope• Launch expected in 2017 or 2018• “Naked” primary mirror ~ 6.5 m (21.3 ft) in diameter
– Hexagonal segments folded at launch• Sun shield the size of a tennis court• Operate in the infrared (0.6 to 28 mm) • Orbit 1.5 million km from Earth at the L2 Point
– L2 is a semi-stable point directly opposite the Sun from the Earth
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The Geometry & Location of L2
http://en.wikipedia.org/wiki/File:L2_rendering.jpg
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James Webb Space Telescope
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Proposed Thirty Meter Telescope
http://en.wikipedia.org/wiki/File:Top_view_of_tmt_complex.jpg
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• Refracting & reflecting telescopes– Refraction systematically bends EMR
• Size limits due to sagging lenses– Reflection systematically rejects EMR
• Theoretically no size limits• Newtonian design is very common
• Active & adaptive optics– Active: Adjust for mirror bending– Adaptive: Adjust for atmosphere
• Angular resolution & field of view– AR: Amount of detail in the image– FoV: Size of visible patch of sky
• Magnification & light gathering power– Mag: Apparent closeness of objects– GP: Brightness of objects
• Atmospheric effects– Thermal currents– Air & light pollution
• Image recording systems– Camera & film– CCD’s
• Astronomical spectroscopes– Yield temperature & energy flux– Represented as graphs, not pictures
• Non-optical telescopes– Thermal infrared & radio from Earth– UV, X-ray & gamma ray from space
• Interferometer technology• Orbiting telescopes
– Benefits & costs
Important Concepts