A Brief History of Astronomical A Brief History of Astronomical - PDF document

A Brief History of Astronomical A Brief History of Astronomical Imaging Systems Imaging Systems 1

Oldest “ “Imaging Imaging” ” Instruments Instruments Oldest • circa 1000 CE – 1600 CE • Used to measure angles and positions • Included No Optics – Astrolabe – Octant, Sextant – Tycho Brahe’s Mural Quadrant (1576) • Star Catalog accurate to 1' (1 arcminute = 1/60 ° ≈ limit of resolution of unaided human eye) – Astronomical Observatories were built by church as part of European Cathedrals • (possible subject for course term paper) 2

Early “ “Imaging Imaging” ” System System Early the Mural Quadrant the Mural Quadrant • Most accurate positions of stars and planets then available • Used by Johannes Kepler to derive the three laws of planetary motion – Laws 1,2 published in 1609 – Third Law in 1619 H.C. King, History of the Telescope 3

Kepler’ ’s s Three Laws of Three Laws of Kepler Planetary Motion Planetary Motion 1. The orbits of planets are ellipses with the Sun at one focus. 2. The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse, thus the planet travels faster when it is closer to the Sun. 3. The ratio of the squares of the periods (“years”) for two planets is equal to the ratio of the cubes of their semimajor axes. 4

Optical Instruments, (1609+) Optical Instruments, (1609+) • Refracting Telescope – uses lenses to redirect light – Invented in 1608 • Hans Lippershey (1570? – 1619) – Early Use in Astronomy • 1609, by Galileo Galilei (1564 – 1642) • Johannes Hevelius (1611 – 1687) • Reflecting Telescope – Invented ca. 1671 • Isaac Newton (1642-1727) • Spectroscope – Invented ca. 1669, also by Newton 5

Galileo’ ’s Telescopes s Telescopes Galileo • Combination of Two Lenses – Objective • Two surfaces: flat and convex – Eye Lens (Ocular) • Two surfaces: flat and concave • Magnified by 20 × Cracked Objective Lens 6

Galilean Telescope Galilean Telescope f objective • Ray incident “above” the optical axis emerges “above” the axis • image is “upright” • Small Field of View 7

Galilean Telescope Galilean Telescope θ θ′ Ray entering at angle θ emerges at angle θ′ > θ Larger ray angle ⇒ angular magnification 8

Keplerian Telescope Telescope Keplerian f eyelens f objective Ray incident “above” the optical axis emerges “below” the axis image is “inverted” 9

Keplerian Telescope Telescope Keplerian θ′ θ Ray entering at angle θ emerges at angle θ′ where | θ′ | > θ Larger ray angle ⇒ angular magnification 10

“Refractive Index Refractive Index” ” “ • Denoted by n • Measure of ratio of velocity of light in matter to that in vacuum c n = v c = velocity in vacuum ≈ 3 × 10 8 meters/second v = velocity in medium measured in same units n ≥ 1.0 11

Sample Refractive Indices Sample Refractive Indices • Vacuum: n = 1.0 n ≈ 1.00003 ≈ 1.0 • Air: n ≈ 1.33 • Water 1.71 ≤ n ≤ 1.46 • Glass: 12

Lenses Redirect (“ “Bend Bend” ”) Light ) Light Lenses Redirect ( by Refraction due to Different n n by Refraction due to Different Incident Ray θ 1 - θ 1 Reflected Ray n 1 n 2 Refracted Ray θ 2 13

Refracted Angles Determined Refracted Angles Determined by “ “Snell Snell’ ’s Law s Law” ” by θ = θ n sin n sin 1 1 2 2 L O M n P − ⇒ θ = θ 1 1 sin sin N Q 2 1 n 2 14

Problem with Refracting Problem with Refracting Telescopes: “ “Optical Dispersion Optical Dispersion” ” Telescopes: • Refractive index n of glass is not constant • n of glass tends to DECREASE with increasing wavelength λ • ⇒ Refracted angles change with wavelength λ • Focal length f of lens tends to INCREASE with increasing wavelength λ – Different colors “focus” at different distances – “Chromatic Aberration” 15

Optical Dispersion Optical Dispersion n Ultraviolet Infrared λ 16

Chromatic Aberration Chromatic Aberration 17

Minimize Chromatic Aberration Minimize Chromatic Aberration • Chromatic aberration is less noticeable for lenses with long focal lengths f 18

Hevelius’ ’ Refractor, ca. 1650 Refractor, ca. 1650 Hevelius Objective Lens Eye Lens H.C. King, History of the Telescope 19

Later Methods to Diminish Later Methods to Diminish Chromatic Aberration Chromatic Aberration • Create lens systems from multiple lenses made from different glasses – “doublets” or “triplets” – Designed so that chromatic aberrations “cancel” for some wavelengths • Difficult to design and fabricate • Beyond capability of early optical technicians 20

“Achromatic Achromatic” ” Lenses Lenses “ • “Achromatic” means “no color” • Two (or more) lenses with different glasses – “Crown”, with smaller n – “Flint”, with larger n 21

Easy Way to Eliminate Easy Way to Eliminate Chromatic Aberration Chromatic Aberration • Don’t use lenses!! f All colors “focus” at same distance f 22

Newton’ ’s Reflector s Reflector Newton • ca. 1671 • 1"-diameter mirror • no chromatic aberration – mirrors reflect all wavelengths at the same angle! H.C. King, History of the Telescope 23

Large Historical Reflecting Large Historical Reflecting Telescope Telescope Lord Rosse’s 1.8 m (6'-diameter) telescope metal mirror, 1845 H.C. King, History of the Telescope 24

History of Imaging Sensors History of Imaging Sensors • Eye – Limited sensitivity – Limited range of wavelengths – Images can be “stored” only “by hand” (drawings) • Image Recording Systems – Chemical-based Photography • wet plates, 1850 + • dry plates, 1880+ • Kodak plates, 1900+ – Physics-based Photography, 1970 + • Electronic Sensors, CCDs 25

History of Imaging Sensors History of Imaging Sensors • Groundbased Infrared Imaging – 1856: using thermocouples and telescopes (“one-pixel sensors”) – 1900+: IR measurements of planets – 1960s: IR survey of sky (Mt. Wilson, lead sulfide − PbS − detector) • Spacebased Infrared Imaging – 1983: IRAS (Infrared Astronomical Satellite) • cooled Silicon and Germanium detectors – 1989: COBE (Cosmic Background Explorer) 26

History of Imaging Sensors History of Imaging Sensors • Airborne Infrared Observatories – Galileo I (Convair 990), 1965 – 4/12/1973 (crashed) – Frank Low, 12"–diameter telescope on NASA Learjet, 1968 – Kuiper Airborne Observatory (KAO) (36"- diameter telescope) 27

Galileo I (Convair Convair 990) 990) Galileo I ( • Started in 1965 • Several “ports” available for cameras • Crashed 4/12/1973 – midair collision on landing at NAS Moffet 28

NASA Learjet, 1968 NASA Learjet, 1968 • 12"–diameter telescope, by Frank Low 29

Kuiper Airborne Observatory Airborne Observatory Kuiper • Modified C-141 Starlifter • 2/1974 – 10/1995 • ceiling of 41,000' is above 99% of water vapor, which absorbs most infrared radiation 30

Stratospheric Observatory for Stratospheric Observatory for − SOFIA Infrared Astronomy − SOFIA Infrared Astronomy • Boeing 747SP • 2.7-m Mirror (106") 31

Spaceborne Observatories Observatories Spaceborne • “Orbiting Astronomical Observatory” (OAO), 1960s • “Infrared Astronomical Satellite” (IRAS), 1980s • Hubble Space Telescope (HST), 1990 • Chandra (Advanced X-ray Astrophysics Facility= AXAF), 7/1999 32

History of Imaging Systems for History of Imaging Systems for Radio Astronomy Radio Astronomy • Wavelengths λ are much longer than visible light – millimeters (and longer) vs. hundreds of nanometers • History – 1932: Karl Jansky (Bell Telephone Labs) investigated use of “short waves” for transatlantic telephone communication – 1950s: Plans for 600-foot “Dish” in Sugar Grove, WV (for receiving Russian telemetry reflected from Moon) – 1960s 305m Dish at Arecibo, Puerto Rico – 1963: Penzias and Wilson (Bell Telephone Labs), “Cosmic Microwave Background” – 1980: “Very Large Array” (= VLA) in New Mexico 33

Jansky Radio Telescope Radio Telescope Jansky 1932 Image courtesy of NRAO/AUI 34

Large Radio Telescopes Large Radio Telescopes 100m at Green Bank, WV 305m at Arecibo, Puerto Rico Image courtesy of NRAO/AUI http://www.naic.edu/about/ao/telefact.htm 35

Very Large Array = VLA Very Large Array = VLA • 27 telescopes • 25m diameter • transportable on rails • separations up to 36 km (22 miles) Image courtesy of NRAO/AUI 36