Astronomical Tools Optics Telescope Design Optical Telescopes Radio Telescopes Infrared Telescopes X Ray Telescopes Gamma Ray Telescopes
Laws of Refraction and Reflection Law of Refraction n 1 sin θ 1 = n 2 sin θ 2 where n = c/v Law of Reflection θ 1 = θ 2
Lenses and Mirrors A focusing lens can be designed using the law of refraction A focusing mirror can be designed using the law of reflection
Refracting / Reflecting Lenses Focal length Focal length A lens can focus an image on a plane. A source at infinity focuses on the focal plane. A concave mirror can focus an image on a plane. A source at infinity focuses on the focal plane.
The Focal Length Focal length = distance from the center of the lens to the plane onto which parallel light is focused.
Telescope Design Reflecting and Refracting Telescopes Galilean Newtonian
Secondary Optics Galilean Cassegrainian In reflecting telescopes: Secondary mirror, to redirect 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 Refracting Telescopes Chromatic aberration: Different wavelengths are focused at different focal lengths (prism effect). Can be improved, but not eliminated by a second lens out of different material. Difficult and expensive to produce: All surfaces must be perfectly shaped; glass must be flawless; lens can only be supported at the edges.
Reflectors Most research telescopes are reflectors. Types of reflecting telescopes
The Powers of a Telescope: Bigger is better 1. Light-gathering power: Depends on the surface area A of the primary lens/mirror, which is proportional to the diameter squared: D A = π (D/2) 2
The Powers of a Telescope 2. Resolving power: Wave nature of light the telescope aperture produces fringe rings that set a limit to the resolution of the telescope. Astronomers cannot eliminate these diffraction fringes, but the larger the telescope diameter, the diffraction fringes are smaller. Thus the larger the telescope, the better its resolving power. α min = 1.22 (λ/d) For optical wavelengths, this gives α min 11.6 arcsec / D [cm] a min
Resolving Power Effect of improving resolution: (a) 10 ; (b) 1 ; (c) 5 ; (d) 1
Seeing Weather conditions and turbulence in the atmosphere set further limits to the quality of astronomical images. Atmospheric motion blurs the image. Bad seeing Good seeing
The Powers of a Telescope 3. Magnifying Power: ability of the telescope to make the image appear bigger. Magnification is usually changed by changing the focal length of the eyepiece. A larger magnification does not improve the resolving power of the telescope! Higher magnification is useful for extended bodies such as the Sun, the Moon and planets not stars, which are seen as points of light.
The Best Location for a Telescope Far away from civilization to avoid light pollution
The Best Location for a Telescope Paranal Observatory (ESO), Chile On high mountain-tops to avoid atmospheric turbulence (i.e. improve seeing) and other weather effects
Traditional Telescopes Secondary mirror Traditional primary mirror: sturdy, heavy to avoid distortions.
Traditional Telescopes Mount Wilson Observatory Hooker 100 inch reflector Mount Palomar Observatory Hale 200 inch (5.1 m) reflector
Traditional Telescopes Mount Palomar Observatory Schmidt Camera (48 inch) Kitt Peak National Observatory Mayall (4 m) Telescope
Traditional Telescopes Hubble (2.4 m) Space Telescope
Advances in Modern Telescope Design 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 Telescope Design 2. Lighter mirrors with lighter support structures, to be controlled dynamically by computers Floppy mirror Prime Focus Cage Segmented mirror
High-Resolution Astronomy Adaptive optics: track atmospheric changes with a laser, adjust mirrors in real time
Adaptive Optics Computer-controlled mirror supports adjust the mirror surface (many times per second) to compensate for distortions by atmospheric turbulence
Interferometry Recall: Resolving power of a telescope depends on diameter D. Combine the signals from several smaller telescopes to simulate one big mirror Interferometry The amount of radiation collected is smaller, but the improved resolution is important.
Images and Detectors Image acquisition: Photographic plates are being replaced by charge-coupled devices (CCDs), which are electronic devices that can be read out and reset quickly. Smaller CCDs are used in digital cameras.
Radio Astronomy Recall: Radio waves of l 1 cm 1 m also penetrate the Earth s atmosphere and can be observed from the ground.
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.
Radio Maps In radio maps, the intensity of the radiation is color-coded: For example: Red = high intensity going to Black = low intensity Analogy: Seat prices in a baseball stadium: Red = expensive going to Purple = cheap.
Radio Astronomy Largest radio telescope: 300-m dish at Arecibo
Radio Astronomy Disadvantage: Longer wavelength means poor angular resolution hence astronomical interferometry began in radio astronomy. Advantages of radio astronomy: Can observe 24 hours a day. Clouds, rain, and snow don t interfere Observations at a different frequency give different information
Just as for optical telescopes, the resolving power of a radio telescope depends on the diameter of the objective lens or mirror a min = 1.22 l/d. Radio Interferometry For radio telescopes, this is a big problem: Radio waves are much longer than visible light Use interferometry to improve resolution! The Very Large Array (VLA): 27 dish antennae are combined to simulate a large dish of as much as 36 km in diameter.
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.
Infrared Astronomy Most infrared radiation is absorbed in the lower atmosphere. However, from high mountain tops or high-flying aircraft, infrared radiation can be observed at some wavelengths. Infrared astronomy is best done from spacecraft. NASA infrared telescope on Mauna Kea, Hawaii
Infrared Astronomy The Spitzer infrared telescope is in space
Infrared Astronomy Infrared observations of M81 at different wavelengths. The images a, b, and c are colored blue, green and red respectively and combined to give an artificial color image in d. 4 μm 8 μm 24 μm
Ultraviolet Astronomy Ultraviolet radiation with l < 290 nm is completely absorbed in the ozone layer of the atmosphere. Ultraviolet astronomy must be done from spacecraft. Several successful ultraviolet astronomy satellites: IRAS, IUE, EUVE, FUSE Ultraviolet radiation traces hot (tens of thousands of degrees), moderately ionized gas in the universe.
X Ray Astronomy X rays and gamma rays cannot reflect off mirrors as other wavelengths do. X rays can undergo Bragg reflection at very shallow angles and they can be focused in special telescopes.
X Ray Astronomy X-ray image of a supernova remnant
Gamma Ray Astronomy Gamma rays cannot be focused at all; therefore images are coarse. Compton Gamma Ray Observatory (1991-2000) and an image made by it.
Full-Spectrum Coverage Much can be learned from observing the same astronomical object at many wavelengths. Here is the Milky Way.