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6. Optics and Telescopes. Refracting telescopes Reflecting telescopes Image degradation Imaging systems Spectrographs Non-optical telescopes Orbiting telescopes. Parallel Rays From Distant Objects. Refracting Telescopes. A lens is the primary image-forming tool

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6 optics and telescopes
6. Optics and Telescopes
  • Refracting telescopes
  • Reflecting telescopes
  • Image degradation
  • Imaging systems
  • Spectrographs
  • Non-optical telescopes
  • Orbiting telescopes
refracting telescopes
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
refracting telescope designs
Refracting Telescope Designs
  • Convex primary lens & convex eyepiece lens
    • Inverted image Astronomical telescopes
  • Convex primary lens & concave eyepiece lens
    • Upright image Terrestrial telescopes
chromatic aberration in lenses
Chromatic Aberration In Lenses

Simple lens Achromatic lens

Only one lens Two or more lenses




reflecting telescopes
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 EMR due to organized rejection
  • Basic benefits
    • 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
isaac newton s second telescope
Isaac Newton’s Second Telescope


reflector telescope technology
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”
active optics actuators slow
Active Optics Actuators: Slow!

Thick telescopemirror


adaptive optics actuators fast
Adaptive Optics Actuators: Fast!





adaptive optics improve sharpness
Adaptive Optics Improve Sharpness


adaptive adaptive

optics optics

two properties of all telescopes
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
two more properties of telescopes
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
mauna kea s gemini north scope



Instrument array



Mauna Kea’s Gemini North ’scope
atmospheric effects
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
image recording systems film
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
image recording systems ccd s
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
a charge coupled device ccd
A Charge-Coupled Device (CCD)


astronomical spectroscopes
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
displaying a spectrum
Displaying A Spectrum
  • Photographic
    • Color representation
      • Color films never accurately represent colors
      • Computers rarely accuratelyrepresentcolors
    • 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
thermal infrared observations
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
nasa s sofia
  • Stratospheric Observatory For IR Astronomy
    • Joint NASA & German Aerospace Center project
    • Successor to the Kuiper Airborne Observatory
      • Dedicated on 21 May 1975
      • 36" diameter mirror
    • Based on a highly modified Boeing 747SP aircraft
      • 100" diameter mirror
      • Telescope door installed behind the left wing
      • Aircraft shortened to maintain balance
      • PanAM name Clipper Lindbergh restored on 21 May 2007
  • Serious funding issues in early 2014
    • Supplemental funding approved by U.S. Congress
sofia s telescope
SOFIA’s Telescope
  • Optimized for infrared (radiant heat) astronomy
    • Slightly longer wavelengths than visible red light
    • Also able to observe using visible wavelengths
  • Bent Cassegrain (Coudé) optics
    • Long focal length but short tube length
    • 45° tertiary mirror directs image sideways
sofia s science
SOFIA’s Science
  • Four basic science objectives
    • Composition of planetary atmospheres
    • Structure, evolution & composition of comets
    • Physics & chemistry of the interstellar medium
    • Formation of stars & other stellar objects
  • Some major successes
    • Images showing starburst galaxy M82's core
    • Heat from Jupiter's formation
    • Milky Way galaxy's core
radio telescopes
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
radio telescopes are mostly air
Radio Telescopes Are Mostly Air

Radio l’s are long enough to reflect from a grating

more telescope technology
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
build a large synthetic aperture
Build a Large Synthetic Aperture




Small telescopes

the arecibo radio telescope
The Arecibo Radio Telescope
  • World’s largest radio telescope
    • Built in a doline (limestone sinkhole)

Arecibo Observatory in a James Bond Movie

earth s atmospheric transparency
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
orbiting telescopes
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
examples of orbiting telescopes
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
next generation space telescope
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
proposed thirty meter telescope
Proposed Thirty Meter Telescope


important concepts
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


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