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 Astronomicaltelescopes

• Convex primary lens & concave eyepiece lens– Upright image Terrestrial

telescopes

Chromatic Aberration In Lenses

Simple lens Achromatic lensOnly one lens Two or more lenses

1 1 2

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

Primefocus

SchmidtCassegrain

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!

http://upload.wikimedia.org/wikipedia/commons/5/5d/GTC_Active_Optics_Acutators.jpg

Thick telescope mirror

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

Adaptive Optics Improve Sharpness

Without With adaptive adaptiveoptics 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 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• 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

The 200-Inch Palomar Telescope

The Observatory on Mauna Kea

Mauna Kea’s Keck I Telescope

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

Multiple Mirror Telescope MakeoverBefore 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 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• 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

Absorptionline

Absorptionline

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 AirRadio 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

Small telescopes

LargeSyntheticaperture

The Very Large Array (Radio)

The Arecibo Radio Telescope• World’s largest radio telescope

– Built in a doline (limestone sinkhole)

Arecibo O

bservatory in a James B

ond Movie

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

Entire Sky at Different Wavelengths

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

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

• 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