Astronomical Telescopes 0 • Primary objective: to gather large amounts of light => large telescopes! • Other forms of radiation (other than visible light) can also be observed, but• The Gemini-North optical very different telescope on Mauna Kea in Hawai’i telescope designs stands over 19 m (60 ft) high, and are needed. the primary mirror at the bottom is 8.1 m (26.5 ft) in diameter.
Radiation: Information 0 from Space The Electromagnetic Spectrum• In astronomy, we cannot perform experiments with our objects (stars, galaxies, …).• The only way to investigate them is by analyzing the light (and other radiation) they emit.
Light as a Wave (I) 0• Light waves are characterized by a wavelength λ and a frequency f.• f and λ are related through f = c/λ c = 300,000 km/s = 3 ×108 m/s
Wavelengths and Colors 0Different colors of visible light correspond to different wavelengths.
0 Light as a Wave (II)Wavelengths of light are measured in units of nanometers (nm) or Ångström (Å): 1 nm = 10-9 m 1 Å = 10-10 m = 0.1 nm Visible light has wavelengths between 4000 Å and 7000 Å (400–700 nm).
Light as Particles 0Light can also appear as particles, calledphotons (explains, e.g., photoelectric effect) • A photon has a specific energy E, proportional to the frequency f: E = h×f h = 6.626 x 10-34 J·s is the Planck constant The energy of a photon does notdepend on the intensity of the light!!!
The Electromagnetic Spectrum 0Wavelength Frequency High Need satellites flying air to observe planes or satellites
0 Refracting / Reflecting Telescopes • Refracting telescope: Lens focuses light onto the focal plane Focal length • Reflecting telescope: Concave mirror focuses light onto the Focal length focal plane• Almost all modern telescopes are reflectingtelescopes.
0 The Focal LengthFocal length = distance from the center of the lens to the plane onto which parallel light is focused.
Secondary Optics 0 In reflecting telescopes: Secondary mirror is needed to re- direct light path towards back or side of incoming light path Eyepiece: to view and enlarge the small image produced in the focal plane of the primary optics
Disadvantages of 0refracting telescopes Chromatic aberration: • Different wavelengths are focused at different focal lengths (prism effect) • Can be corrected, but not eliminated, by second lens out of different material • Second lens is difficult and expensive to produce; all surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges
The Powers of a Telescope: 0 Size Does Matter!1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, D proportional to diameter squared: A = π (D/2)2
The Powers of a Telescope (II) 02. Resolving power: Wave nature of light => the telescope aperture produces fringe rings that set a limit to the resolution of the telescope Resolving power = minimum angular distance αmin between two objects that can be separatedαmin = 1.22 (λ/D) αminFor optical wavelengths, this givesαmin = 11.6 arcsec / D[cm]
Seeing 0 Weatherconditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Bad seeing Good seeing
0 The Powers of a Telescope (III)3. Magnifying Power: The ability of the telescope to make the image appear bigger. • The magnification depends on the ratio of focal lengths of the primary mirror/lens (Fo) and the eyepiece (Fe): M = Fo/Fe • A larger magnification does not improve the resolving power of the telescope!
The Best Location for a Telescope 0 Far away from civilization–to avoid light pollution.
The Best Location for a Telescope (II) 0 On high mountain-tops–to avoid atmospheric turbulence (→ seeing) and other weather effects.
Infrared Telescopes 0• Most infrared radiation is absorbed in the lower atmosphere.• However, from high mountain tops or high-flying air planes, some infrared radiation can still be observed.
Infrared Telescopes 0• Infrared observations can also be performed form high-flying aircraft. • Infrared detectors must be cooled to very low temperatures.
0Traditional Telescopes (I) Secondary mirror Traditional primary mirror: sturdy, heavy to avoid distortions
0TraditionalTelescopes The 4-m Mayall Telescope at Kitt Peak National Observatory (Arizona)
Advances in Modern 0 Telescope Design (I)Modern computer technology has made possible significant advances in telescope design: 1. Simpler, stronger mountings (“alt-azimuth mountings”) to be controlled by computers
Advances in Modern 0 Telescope Design (II)2. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror Segmented mirror
The Keck Telescopes 0 Pictured are the two Keck Telescopes on Mauna Kea, Hawaii.Each telescope has a mirror diameter of 10 meters.
The Future of 0 Optical TelescopesThe Giant Magellan Telescope (2016)The EuropeanExtremely LargeTelescope (E-ELT): 906segments in a42-m mirror!
Adaptive Optics 0Computer-controlled mirror support adjusts the mirrorsurface (many times per second) to compensate for distortions by atmospheric turbulence. Distortions by the atmospheric turbulence are measured using a laser beam.
Interferometry 0 Recall: The resolving power of a telescope depends on diameter D: αmin = 1.22 λ/DThis holds true even if not the entiresurface is filled out. → Combine thesignals from several smaller telescopes to simulate one big mirror → Interferometry
CCD Imaging 0 CCD = Charge-coupled device• More sensitive than photographic plates• Data can be read directly into computer memory, allowing easy electronic manipulations False-color image to visualize brightness contours
Negative Images 0 • The galaxy NGC 891 as it would look to our eyes (i.e., in real colors and brightness) • Negative images (sky = white; stars = black) are used to enhance contrasts
The Spectrograph 0 • Using a prism (or a grating), light can be split up into different wavelengths (colors!) to produce a spectrum. • Spectral lines in a spectrum tell us about the chemical composition and other properties of the observed object.
Radio Astronomy 0Recall: Radio waves of λ ~1 cm to 1 m also penetrate theEarth’s atmosphere and can be observed from the ground.
Radio Telescopes 0• Large dish focuses the energy of radio waves onto a small receiver (antenna). • Amplified signals are stored in computers and converted into images, spectra, etc.
Radio Maps 0• In radio maps, the intensity of the radiation is color-coded: Red = high intensity; Violet = low intensity• Just like optical telescopes, radio telescopes should be built in regions with low average rainfall and cloud cover.
Radio Interferometry 0• Just as for optical telescopes, the resolving power of a radio telescope is αmin = 1.22 λ/D.• For radio telescopes, this is a big problem: radio waves are much longer than visible light.→ Use • The Very Large Array (VLA): 27interferometry to dishes are combined to simulate aimprove resolution! large dish of 36 km in diameter.
Science of Radio Astronomy 0 Radio astronomy reveals several features, not visible at other wavelengths: • Neutral hydrogen clouds (which don’t emit any visible light), containing ~90% of all the atoms in the Universe • Molecules (often located in dense clouds, where visible light is completely absorbed) • Radio waves penetrate gas and dust clouds, so we can observe regions from which visible light is heavily absorbed
Observatories in Space (I) 0The Hubble Space • Launched in 1990; Telescope maintained and upgraded by several space shuttle service missions throughout the 1990s and early 2000’s • Avoids turbulence in the Earth’s atmosphere • Extends imaging and spectroscopy to (invisible) infrared and ultraviolet
The Future of Space-Based 0Optical/Infrared Astronomy: The James Webb Space Telescope
High-EnergyAstronomy: 0X-rays and gamma-rays from space can also be observed from astronomy satellites: Combined visual + X-ray image (taken by Chandra) of a supernova remnant