Parallel Rays From Distant Objects. 6. Optics and Telescopes. Refraction & Reflection

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1 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 Refraction & Reflection Refracting Telescope Designs Convex primary lens & convex eyepiece lens Inverted image Astronomical telescopes Chromatic Aberration In Lenses Convex primary lens & concave eyepiece lens Upright image Terrestrial telescopes Simple lens Achromatic lens Only one lens Two or more lenses

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 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 Reflection by a Concave Mirror (Prime Focus) Corrections for Spherical Aberration Reflecting Telescope Designs Prime focus Schmidt Cassegrain Isaac Newton s Second Telescope 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

3 Active Optics Actuators: Slow! Adaptive Optics Actuators: Fast! Thick telescope mirror Thin deformable mirror Adaptive Optics Improve Sharpness 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 Without adaptive optics With adaptive optics 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 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 The 200-Inch Palomar Telescope

4 The Observatory on Mauna Kea Mauna Kea s Keck I Telescope Mauna Kea s Gemini North scope Multiple Mirror Telescope Makeover Before 1998 After 2000 Secondary mirror Instrument array 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

5 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) 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

6 Two Representations of a Spectrum Thermal Infrared Observations Non-dedicated telescopes Limiting factors Absorption line Dry air minimizes absorption of TIR wavelengths Remote enough to minimize thermal pollution effects Absorption line 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 Kuiper Airborne Observatory & SOFIA Stratospheric Observatory For IR Astronomy Joint NASA & German Aerospace Center project Successor to the Kuiper Airborne Observatory Dedicated on 21 May " 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 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 Interior

7 SOFIA: More Views 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 Are Mostly Air Radio λ s are long enough to reflect from a grating Brief history First EM λ 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 More Telescope Technology Build a Large Synthetic Aperture Basic physical process of telescope arrays Large Synthetic aperture 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 Small telescopes

8 The Very Large Array (Radio) 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 Entire Sky at Different Wavelengths 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 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)

9 Examples of Orbiting Telescopes Ultraviolet Extreme Ultraviolet Explorer Next Generation Space Telescope Renamed James Webb Space Telescope (EUVE) NASA s second Administrator Mission ended in 2000 Hopkins Ultraviolet Telescope Largely responsible for NASA s science programs (HUT) Important facts Far-ultraviolet portion of the EMS Replacement for the Hubble Space Telescope Launch expected in 2017 or 2018 Naked primary mirror ~ 6.5 m (21.3 ft) in diameter Infrared Space Infrared Telescope Facility (SIRTF) Hexagonal segments folded at launch Launch on 25 August 2003 Sun shield the size of a tennis court Operate in the infrared (0.6 to 28 µm) Orbit 1.5 million km from Earth at the L2 Point X-Ray Chandra X-Ray Observatory L2 is a semi-stable point directly opposite the Sun from the Earth Reached its operational orbit on 7 August 1999 Gamma Ray Compton Gamma Ray Observatory Launched 7 April 1991 The Geometry & Location of L2 James Webb Space Telescope Proposed Thirty Meter Telescope 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 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 Astronomical spectroscopes Yield temperature & energy flux Represented as graphs, not pictures Newtonian design is very common Image recording systems Non-optical telescopes Thermal infrared & radio from Earth UV, X-ray & gamma ray from space Interferometer technology Orbiting telescopes Benefits & costs