3RefractionRefraction is the bending of light when it passes from one substance into anotherYour eye uses refraction to focus light
4Refraction makes a pencil appear to be bent when placed in water. When light passes through a glass slab it first refracts towards the normal then away from the normal.
5Example: Refraction at Sunset Sun appears distorted at sunset because of how light bends in Earth’s atmosphere
6Refraction telescope images The main lens in a refracting telescope is called the primary lens (or the objective lens). This lens is a part of the telescope and is fixed.
7Problems with refractors. Light of differing frequency travel at different speeds in glass. This results in a spectrum from a prism and leads to chromatic aberration in lenses.Large achromatic lenses are expensive.Lenses must be supported at the edges. Large lenses sag in the middle under their own weight.Long focal length= long tubes. Costly mounts and domes.
10Yerkes Observatory Williams Bay Wis. Worlds largest at 40 inches in diameter63ft long
11Reflectors Most large telescopes are reflectors. Isaac Newton presented a reflecting telescope to the Royal Society in 1671.Mirrors avoid chromatic aberration.Objective mirror instead of an objective lens.Early reflectors were made of polished metal alloys. Tarnished rapidly dimming images.
12Reflectors 21850 method for depositing silver on glass. Most telescopes became reflectorsStill tarnished, normal glass changes shape with temperature changes1940’s technique for casting Pyrex glass and aluminum coatings (more durable)Now Pyrex or Fused QuartzPalomar 200 inch mirror took 11 years to build and required 5 tons of glass.
13Reflecting telescope images Light in a reflecting telescope does not have to pass through glass at all.The main mirror in a reflecting telescope is called the primary mirror (objective mirror).
28Light Gathering Power A telescope is a light bucket. Like a bucket catching rain the diameter is the determining factor in hiw much light gets caught.Area= π r2The large the radius, diameter, the greater the area.Bigger is better. The more light you collect the farther out you see.
29Resolving Power … ability of a telescope to see fine detail. Defined as the angular distance between two objects that are barely visible as separate.Due to the wave nature of light magnified images have diffraction fringes.
30Diffraction LimitsThe edge of a telescope acts as slits causing diffraction patterns. These are only seen at the highest magnification for a telescope.
31OverlapClosely spaced objects begin to overlap, becoming indistinguishable.
32Increasing resolving power. The smaller the number the better the resolution.
33Resolution formula Angular resolution = 0.25 * For visible wavelengths in the middle of the visible spectrum“ α” ≈Note aperture is in the denominator. Large D smaller resolution.Bigger telescope is better.
34Magnifying PowerUsually the big sell. Least important. It can be changed.Magnification is calculated by the focal length of the objective divided by the focal length of the eyepiece.To Increase magnification just change to a shorter focal length eyepiece.M =
35Maximum Magnification As magnification of an instrument is increased the image will be dimmer.As a rule of thumb the maximum magnification of a telescope can be found byMmax = 20(X/cm) *D(cm)
36F5_4 Focal Length of a Lens The focal length of a lens or mirror is the distance from the center of the lens to the image formed from an object placed at a great distance.
37Observing Problems: Light Pollution Many of the most interesting objects are dim. Bright skies wash out the images. The darker the sky the better.
41SeeingTo reduce the effects of the atmosphere observatories are placed at high elevations and in regions where the air is dry.
42Paranal Observatory ESO The best seeing is from remote, high and dry locations.
43TelescopesNew Generation telescopes use advances in technology to correct images for bad seeing conditions.
44New Generation Telescopes Large mirrors had to be made thick to avoid sagging.A mirror can be supported from the back. Traditional telescopes were big, heavy, and expensive. Control devices had to be massive too.High speed computers have helped to reduce costs and improve performance.Computer control makes alt-azimuth mounts usable.Computer control makes it possible to control the shape of thin mirrors rapidly. This reduces the costs of making the mirror, smaller mounts and allows for…
45Thin floppy mirrorsMirrors are backed by movable pistons able to change the shape of the mirror quickly under computer control.One of the Keck hexagonal mirror segments.Thin mirrors are lighter require less support and they change temperature faster (less trouble with convection currents at surface)
47Nordic 2.6 M Canary Islands 1989 First large instrument whose dome and primary mirror shape are continually adjusted for the sharpest possible image.
48Gemini 8.1 m mirrorsRapid control of mirror shapes makes it possible to correct some distortions caused by poor seeing.Real time mirror control achieved by 120 actuators under the mirror and 60 around the edge.Right: The Gemini mirrors have adaptive optics
49Adaptive opticsAO allows this 1.5m telescope to reach 0.1” resolution
70Optical Interferometry Combining signals requires control of signal path lengths to a fraction of the wave length of the e-m radiation being combined.Only radio waves could be used until recently.The resulting image has the resolution of the distance between mirrors.
71Very Large Telescope Array Paranal Observatory at Atacama ChileFour 8.2 m reflecting telescopes .Used together have the effective area of a 16 m .
92Very Long Baseline Array VLBA Combines electronically signals from Hawaii and the Virgin Islands. This gives the resolution of an Earth sized telescope
93IR Infrared Telescopes Top:Longer wave length light doesn’t see the smog particles.Dust doesn’t block IR as easily in space either.
94NASA 3 m Infrared Telescope Water absorbs IRIR Telescopes must be at high elevations, in the mountains, on balloons or in space. Still only the near infrared is visible under even the thin atmosphere.
97Hubble launched April 1990. Visible light and UV ( Shuttle Discovery) Compton Gamma-Ray Observatory.(Shuttle Atlantis). April 1991 to June 2000 (lost gyroscope)Fermi Gamma-Ray Observatory (formerly GLAST) 2008 replaces ComptonSwift Has gamma ray detectors, as well as x-ray and visible light telescopes.Chandra X-ray Observatory July 1999 (Shuttle Columbia)Space Infrared Telescope Facility (SIRTF), now called the Spitzer Space Telescope
98SIRTF Space Infrared Telescope Facility [Spitzer]
99SIRTF, Spitzer Space Telescope Was launched by Delta rocket first expected in mid April Saved a lot of money, but required a redesign. Was to have been launched by a shuttle.
100IRASInfrared Astronomy Satellite surveyed cooler gas and dust.