Transcript Chapter 6 PowerPoint

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
– 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
Reflection By a Planar (Flat) Mirror
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
1
Simple lens
Only one lens
12
Achromatic lens
Two or more lenses
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
Reflection by a Concave Mirror
(Prime Focus)
Reflecting Telescope Designs
Isaac Newton’s Second Telescope
http://upload.wikimedia.org/wikipedia/commons/c/cc/NewtonsTelescopeReplica.jpg
Corrections for Spherical Aberration
Prime
focus
Schmidt
Cassegrain
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!
Thick telescope mirror
http://upload.wikimedia.org/wikipedia/commons/5/5d/GTC_Active_Optics_Acutators.jpg
Adaptive Optics Actuators: Fast!
Thin
deformable
mirror
http://upload.wikimedia.org/wikipedia/commons/b/bc/Prototype_of_part_of_the_adaptive_support_system_of_the_E-ELT.jpg
Adaptive Optics Improve Sharpness
Without
adaptive
optics
With
adaptive
optics
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
– Halve the eyepiece focal length
• Light-gathering power
Double the magnification
Double the magnification
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
• Angular resolution
– Single
lens
or mirror
Apparent detail
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
The 200-Inch Palomar Telescope
The Observatory on Mauna Kea
Mauna Kea’s Keck I Telescope
Mauna Kea’s Gemini North ’scope
Secondary
mirror
http://zuserver2.star.ucl.ac.uk/~idh/apod/image/9906/gemini_pfa_big.jpg
Instrument array
http://www.hia-iha.nrc-cnrc.gc.ca/atrgv/altair2_e.html
Multiple Mirror Telescope Makeover
Before 1998
After 2000
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
• Light
Adaptive optics & optimal locations
pollution
– Basic physical process
• Light scatters from air molecules
• Very few areas are far from large cities
– Solutions
• Air
Fewer & well-screened city lights
pollution
– Basic physical process
• Light scatters fromair pollution molecules
• Very few areas are far from pollution sources & plumes
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
• 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)
http://www.tech-faq.com/wp-content/uploads/Charge-Coupled-Device.jpg
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
A Rare Refraction Spectrograph
A Common Reflection Spectrograph
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
Two Representations of a Spectrum
Absorption
line
Absorption
line
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
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 l’s are long enough to reflect from a grating
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
Large
Synthetic
aperture
Small
telescopes
The Very Large Array (Radio)
• World’s largest radio telescope
– Built in a doline (limestone sinkhole)
Arecibo Observatory in a James Bond Movie
The Arecibo Radio Telescope
Earth’s Atmospheric Transparency
•
•
•
•
•
•
X-rays
Ultraviolet
Visible
Infrared
Microwaves
Radio
Completely opaque
Completely opaque
Mostly transparent
Intermittently transparent
Part is opaque, part transparent
Part is transparent, part opaque
Entire Sky at Different Wavelengths
Orbiting Telescopes
• Reasons
– Absorption & scattering by Earth’s atmosphere
•
•
•
•
Gamma rays
X-rays
Ultraviolet
Thermal infrared
Strongly absorbed by
Strongly absorbed by
Strongly scattered by
Absorbed by
air molecules
air molecules
air molecules
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
Hubble Space Telescope (HST)
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
• 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
The Geometry & Location of L2
http://en.wikipedia.org/wiki/File:L2_rendering.jpg
James Webb Space Telescope
Proposed Thirty Meter Telescope
http://en.wikipedia.org/wiki/File:Top_view_of_tmt_complex.jpg
Important Concepts
•
Refracting & reflecting telescopes
•
– Refraction systematically bends EMR
– Camera & film
– CCD’s
• Size limits due to sagging lenses
– Reflection systematically rejects EMR
• Theoretically no size limits
• Newtonian design is very common
•
Active & adaptive optics
– Active:
– Adaptive:
•
Adjust for mirror bending
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
•
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