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PHYS-3380 Astronomy Refracting vs Reflecting Telescopes Reflecting telescopes are primary astronomical tools used for research: 1. Lens of refracting telescope very heavy - must be placed at end of telescope - difficult to stabilize and prevent from deforming 2. Light losses from passing through thick glass of refracting lens 3. Lens must be very high quality and perfectly shaped on both sides 4. Refracting lenses subject to chromatic aberration PHYS-3380 Astronomy Lens and Mirror Aberrations SPHERICAL (lens and mirror) Light passing through different parts of a lens or reflected from different parts of a mirror comes to focus at different distances from the lens. Result: fuzzy image CHROMATIC (lens only) Objective lens acts like a prism. Light of different wavelengths (colors) comes to focus at different distances from the lens. Result: fuzzy image PHYS-3380 Astronomy Chromatic Aberration in Lenses Focal point for blue light Simple lenses suffer from the fact that different colors of light have slightly different focal lengths. This defect is corrected by adding a second lens The problem Focal point for red light Focal point for all light The solution PHYS-3380 Astronomy Spherical Aberration in Lenses Simple lenses suffer from the fact that light rays entering different parts of the lens have slightly difference focal lengths. As with chromatic aberration, this defect is corrected with the addition of a second lens. The problem One focal point for all light rays The solution PHYS-3380 Astronomy Spherical Aberration in Mirrors The Problem Simple concave mirrors suffer from the fact that light rays reflected from different locations on the mirror have slightly different focal lengths. This defect is corrected by making sure the concave surface of the mirror is parabolic The Solution All light rays converge at a single point PHYS-3380 Astronomy Reflecting Telescope The primary mirror focuses light at the prime focus. A camera or another mirror that reflects the light into an eyepiece is placed at the prime focus. PHYS-3380 Astronomy The image from an reflecting telescope is inverted. Focus Inversion Animation The focus is adjusted by changing the secondary mirror position. Mirror Position and Focus Animation PHYS-3380 Astronomy Types of Reflecting Telescopes Each design incorporates a small mirror just in front of the prime focus to reflect the light to a convenient location for viewing. PHYS-3380 Astronomy Cassegrain reflector - most common form of astronomical telescope - allows room for large instruments PHYS-3380 Astronomy Schmidt-Cassegrain focus most common in small telescopes - uses catadioptrics - combines optical advantages of both lenses and mirrors - light enters through a thin aspheric Schmidt correcting lens - focal length increased by the magnification of the correcting lens - lens carefully matched to the primary concave mirror to correct for spherical aberration - too slightly curved to introduce serious chromatic aberration - shorter physical length - lighter and more compact - easy to use PHYS-3380 Astronomy The Keck Telescopes On Mauna Kea in Hawaii. 36 hexagonal mirrors function as single 10meter mirror. - segmented mirrors - more economical - segments can be made separately - weighs less - cools rapidly - less distortion from uneven expansion and contraction - optical shape maintained by computer-driven thrusters PHYS-3380 Astronomy Two Fundamental Properties of a Telescope Resolution smallest angle which can be seen θ = 1.22 λ / D The angular resolution of a reflecting telescope is dependent on the diameter of the mirror (D) and the wavelength of the light being viewed (λ) Light-Collecting Area think of the telescope as a “photon bucket” The amount of light that can be collected is dependent on the mirror area A = π (D/2)2 These properties are much more important than magnification which is produced by placing another lens - the eyepiece - at the mirror focus. Astronomers do not look through telescopes with their eyes - a light gathering detector (for instance a camera) records the image which can later on be magnified to any desired size. PHYS-3380 Astronomy • • • Angular Resolution The ability to separate two objects. The angle between two objects decreases as your distance to them increases. The smallest angle at which you can distinguish two objects is your angular resolution. PHYS-3380 Astronomy Angular Resolution of Car Lights Animation The maximum angular resolution attainable by the human eye is about one arcminute - in other words two stars will appear distinct if they are separated by more than one arcminute - remember that Tycho Brahe produced the best naked eye star charts ever - had resolution of one arcminute. PHYS-3380 Astronomy Angular Resolution 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 amin between two objects that can be separated. amin = 1.22 (λ/D) For optical wavelengths, this gives amin = 11.6 arcsec / D[cm] amin PHYS-3380 Astronomy So: the angular resolution/resolving power of a reflecting telescope is dependent on the diameter of its mirror Mirror Angular Resolution Animation and the wavelength of the light Wavelength Effect on Resolution PHYS-3380 Astronomy Light Gathering Ability: Size Does Matter 1. Light-gathering power: Depends on the surface area A of the primary lens / mirror, proportional to diameter squared: D A = π(D/2)2 PHYS-3380 Astronomy So: light collecting ability of a reflecting telescope is dependent on the area of the mirror Light Collecting Area Animation PHYS-3380 Astronomy Magnifying Power Magnifying Power = 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! PHYS-3380 Astronomy Interferometry Recall: Resolving power of a telescope depends on diameter D: amin = 1.22 λ/D. This holds true even if not the entire surface is filled out. • Combine the signals from several smaller telescopes to simulate one big mirror → Interferometry PHYS-3380 Astronomy Uses of Telescopes 1. Imaging – use a camera to take pictures (images) 2. Photometry measure total amount of light from an object 3. Spectroscopy – use a spectrograph to separate the light into its different wavelengths 4. Timing – measure how the amount of light changes with time (sometimes in a fraction of a second) PHYS-3380 Astronomy Imaging • In astronomy, filters are usually placed in front of a camera to allow only certain colors to be imaged • Single color images are superimposed to form true color images. PHYS-3380 Astronomy Spectroscopy • The spectrograph reflects light of a grating: a finely ruled, smooth surface. • Light interferes with itself and disperses into colors. • This spectrum is recorded by a digital detector called a CCD. PHYS-3380 Astronomy Nonvisible Light • • Special detectors/receivers record light invisible to the human eye - gamma rays, x-rays, ultraviolet, infrared, radio waves. - each type of light can provide information not available from other types. Digital images are reconstructed using false-color coding so that we can see this light. Chandra X-ray image of the Center of the Milky Way Galaxy PHYS-3380 Astronomy Visible The Crab Nebula Radio Waves Infrared X-rays PHYS-3380 Astronomy Atmospheric Effects Earth’s atmosphere causes problems for astronomers on the ground: • Bad weather makes it impossible to observe the night sky. • Man-made light is reflected by the atmosphere, thus making the night sky brighter. – light pollution • The atmosphere absorbs light - dependent on wavelength • Air turbulence in the atmosphere distorts light. – That is why the stars appear to “twinkle”. – Angular resolution is degraded. PHYS-3380 Astronomy Light Pollution PHYS-3380 Astronomy PHYS-3380 Astronomy Radio Astronomy Recall: Radio waves of λ ~ 1 cm – 1 m also penetrate the Earth’s atmosphere and can be observed from the ground. PHYS-3380 Astronomy Radio Telescopes 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. PHYS-3380 Astronomy Radio Telescopes • • The wavelengths of radio waves are long. So the dishes which reflect them must be very large to achieve any reasonable angular resolution. 305-meter radio telescope at Arecibo, Puerto Rico PHYS-3380 Astronomy Radio Interferometry The Very Large Array (VLA): 27 dishes are combined to simulate a large dish of 36 km in diameter. Even larger arrays consist of dishes spread out over the entire U.S. (VLBA = Very Long Baseline Array) or even the whole Earth (VLBI = Very Long Baseline Interferometry)! PHYS 3380 - Astronomy Most sensitive VLBI array in the world - European VLBI Network (EVN). • brings together the largest European radiotelescopes for typically week-long sessions Very Long Baseline Array (VLBA) • uses ten dedicated, 25-meter telescopes spanning 5351 miles across the United States • the largest VLBI array that operates all year round as both an astronomical and geodesy instrument. Global VLBI • Combination of the EVN and VLBA Space Very Long Baseline Interferometry (SVLBI) •dedicated VLBI placed in Earth orbit to provide greatly extended baselines. •HALCA, an 8 meter radio telescope - launched in February 1997 - made observations until October 2003, •small size of the dish - only very strong radio sources could be observed with •Spektr-R (or RadioAstron) - launched in July 2011. When Global VLBI combined with one or more space-based VLBI antennas gives resolution of microarcseconds. PHYS-3380 Astronomy Science of Radio Astronomy 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. PHYS-3380 Astronomy Atmospheric Distortion The turbulence (ever-changing motion) of the atmosphere causes distortion - twinkling of starlight. Bends light in constantly shifting patterns. Like looking down the road on a hot day and seeing distant cars rippling and distorting. Why best viewing is when it is cold and calm. Atmospheric Distortion Animation PHYS-3380 Astronomy • • • Adaptive Optics (AO) It is possible to “de-twinkle” a star. The wavefronts of a star’s light rays are deformed by the atmosphere. By monitoring the distortions of the light from a nearby bright star (or a laser): – a computer can deform the secondary mirror in the opposite way. – the wavefronts, when reflected, are restored to their original state. • • • AO mirror off AO mirror on Angular resolution improves. These two stars are separated by 0.38ʺ Without AO, we see only one star. PHYS-3380 Astronomy PHYS-3380 Astronomy PHYS-3380 Astronomy The Sun PHYS-3380 Astronomy The Sun’s Energy Source The first scientific theories involved chemical reactions or gravitational collapse. - chemical burning ruled out…it can not account for the Sun’s luminosity - conversion of gravitational potential energy into heat as the Sun contracts would only keep the Sun shining for 25 million years - late 19th-century geological research indicated the Earth was older than that Development of nuclear physics led to the correct answer - the Sun generates energy via nuclear fusion reactions - Hydrogen is converted into Helium in the Sun’s core - the mass lost in this conversion is transformed into energy - the amount of energy is given by Einstein’s equation: E = mc2 - given the Sun’s mass, this will provide enough energy for the Sun to shine for 10 billion years PHYS-3380 Astronomy Striking a Balance The Sun began as a cloud of gas undergoing gravitational collapse. - the same heating process, once proposed to power the Sun, did cause the core of the Sun to get hot and dense enough to start nuclear fusion reactions Once begun, the fusion reactions generated energy which provided an outward pressure. This pressure perfectly balances the inward force of gravity. - deep inside the Sun, the pressure is strongest where gravity is strongest - near the surface, the pressure is weakest where gravity is weakest This balance is called gravitational equilibrium. - it causes the Sun’s size to remain stable PHYS-3380 Astronomy One second of output from the Sun (luminosity) would provide power for the human race for the next 500,000 years PHYS-3380 Astronomy Layers of the Sun PHYS-3380 Astronomy Core T = 1.5 x 107 K; depth = 0 – 0.25 R Density - up to 150,000 kg/m³ (154 times the density of water on Earth) Pressure 200 billion times that on the surface of Earth This is where the Sun’s energy is generated.