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Optics Basics All telescopes are based upon two basics properties of optics Mirror: An object which is not transparent to EM waves Lens: An object which is transparent to EM waves. * Reflection: The bouncing of light of surfaces - mirrors * Refraction: The bending of light through transparent materials - lenses Optics Basics - Reflection The angle of incidence ΘI is equal to the angle of reflection Θr Θi Θr Optics Basics - Reflection Optics Basics - Reflection Focal Length Focal Length (f) Image identifies the distance from the mirror where an image of the object will form if the object is very far away. Optics Basics – Refraction An electromagnetic wave traveling through a transparent material (not a vacuum) travels at a speed less than 3 x 108 m/sec The amount of slowing is characterized by the index of refraction of the material, n. c n= Speed in the material The larger the index of refraction, the slower the em wave travels Optics Basics – Refraction Θi Θr Optics Basics – Refraction Θi Θr The bending of the light ray due to the change in speed of the light is called refraction. Optics Basics – Refraction Image Focal Length Focal Length (f) identifies the distance from the lens where an image of the object will form if the object is very far away. Optics Basics – Light Gathering The large mirror or lens that is the “primary” collector of EM energy is called the objective mirror or objective lens. The number of photons collected by the telescope is directly proportional to the area of the objective. Since the area of the objective is proportional to the diameter of the objective squared, Number of photons collected is proportional to d2 Finally, the brightness of the image is proportional to the number of photons collected, Image brightness is proportional to d2 Optics Basics – Refraction Image Eyepiece A short focal length lens, called an eyepiece, is used as a simple magnifier to examine the image. Refractors Refractors Reflectors Newtonian Reflectors Catadioptric Reflectors Also Known as Compound Telescopes Compound telescopes “fold” the path of the light back upon itself. The result is a short barrel telescope that is very convenient for amateur astronomers because of the telescopes transportability. The two most popular designs of catadioptric telescopes are Schmidt- Cassegrain and Maksutov-Cassegrain. They each use differently shaped lenses and mirrors to achieve a similar quality aperture in an easy-to-carry size. For people who like to take their telescope to a remote location and track distant objects across the night sky, a compound telescope is a great fit. Catadioptric Reflectors Schmidt – Cassegrain Telescope Catadioptric Reflectors Maksutov – Cassegrain Telescope Cheap Telescope design – the Dobsonian Telescope Newtonian Reflector See the links in the PHY250 website for building instructions Other Applications Radio Telescope National Radio Astronomical Observatory at Green Bank, Va. Radio Telescope National Radio Astronomical Observatory at Green Bank, Va. Radio Telescope Radio Wave Emission from the Center of the Milky Way Radio Continuum (408 MHz) Intensity of radio continuum emission from high-energy charged particles in the Milky Way,from surveys with ground-based radio telescopes (Jodrell Bank Mark I and Mark IA, Bonn 100-meter, and Parkes 64-meter). At this frequency, most of the emission is from electrons moving through the interstellar magnetic field at nearly the speed of light. Shock waves from supernova explosions accelerate electrons to such high speeds, producing especially intense radiation near these sources. Emission from the supernova remnant Cas A near 110° longitude is so intense that the diffraction pattern of the support legs for the radio receiver on the telescope is visible as a cross shape. Jodrell Bank Lowell Telescope Frequency: 408 MHz Angular resolution: 51 arcminutes Infrared Telescope Infrared Emission from the Center of the Milky Way Infrared Composite mid-and far-infrared intensity observed by the Infrared Astronomical Satellite (IRAS) in 12, 60, and 100 micron wavelength bands. The images are encoded in the blue, green, and red color ranges, respectively. Most of the emission is thermal, from interstellar dust warmed by absorbed starlight, including star-forming regions embedded in interstellar clouds. The display here is a mosaic of IRAS Sky Survey Atlas images. Emission from interplanetary dust in the solar system, the "zodiacal emission," was modeled and subtracted in the production of the Atlas. Frequencies: 3.0 x 103-25 x 103 GHz Angular resolution: 5 arcminutes Infrared Astronomical Satellite Orion – Optical Wavelength Image Infrared Telescope Infrared Telescope Stars bright in different lightThis infrared portrait of the Orion starbirth region was taken by the European Southern Observatory’s new VISTA telescope, the world’s largest wide-field-of view telescope. The image, which measures about 35 light- years from top to bottom, records radiation with about twice the wavelength of light visible to the human eye. Many of the red features just above the center are young stars and the high-speed streams of gas they eject. These stars are completely hidden by dust in visible light but can be seen at dust- penetrating infrared wavelengths. Feb 10, 2010 Optical Telescopes Visible Emission from the Center of the Milky Way Optical Intensity of visible (0.4 - 0.6 micron) light from a photographic survey. Due to the strong obscuring effect of interstellar dust,the light is primarily from stars within a few thousand light-years of the Sun, nearby on the scale of the Milky Way. The widespread bright red regions are produced by glowing, low-density gas. Dark patches are due to absorbing clouds of gas and dust, which are evident in the Molecular hydrogen and Infrared maps as emission regions. Stars differ from one another in color, as well as mass, size and luminosity. Interstellar dust scatters blue light preferentially, reddening the starlight somewhat relative to its true color and producing a diffuse bluish glow. This scattering, as well as absorption of some of the light by dust, also leaves the light diminished in brightness. The panorama was assembled from sixteen wide-angle photographs taken by Dr. Axel Mellinger using a standard 35-mm camera and color negative film. The exposures were made between July 1997 and January 1999 at sites in the United States, South Africa, and Germany. Frequency: 460 x 103 GHz Hubble Space Telescope (HST) Angular resolution: 1 arcminute Ultraviolet Telescopes Ultraviolet You might notice that missing from the list of images is the ultraviolet region of the electromagnetic (EM) spectrum. Ultraviolet radiation begins just past the blue/violet region of the visible (optical) spectrum, and ends when X-rays take over. The boundaries between named regions can get a little blurred, especially if the broad-band regions (example: infrared) are further broken into sub-regions (example: near infrared and far infrared). One reference on EM waves says microwaves extend from about 1 millimeter to about 10 centimeters. In that case, the map of molecular hydrogen would fall into the category of microwaves. The sky has been observed at ultraviolet wavelengths with various detectors [links to EUVE, IUE, etc. will be listed]. An all-sky image has not been presented here because it is relatively featureless. You may view an all- sky survey map in the extreme-ultra-violet at Extreme Ultraviolet Explorer http://archive.stsci.edu/euve/images/jbis_map.gif. The image at that site comes from data obtained with the Extreme Ultraviolet Explorer (EUVE). X-ray Telescopes X-Ray Emission from the Center of the Milky Way X-Ray Composite X-ray intensity observed by the Position-Sensitive Proportional Counter (PSPC) instrument on the Röntgen Satellite (ROSAT). Images in three broad, soft X-ray bands centered at 0.25 , 0.75, and 1.5 keV are encoded in the red, green, and blue color ranges, respectively. In the Milky Way, extended soft X-ray emission is detected from hot, shocked gas. At the lower energies especially, the interstellar medium strongly absorbs X-rays, and cold clouds of interstellar gas are seen as shadows against background X-ray emission. Color variations indicate variations of absorption or of the temperatures of the emitting regions. The black regions indicate gaps in the ROSAT survey. ROSAT Frequency: 60-360 x 106 GHz Angular resolution: 115 arcminutes Gamma Ray Telescopes Gamma Ray Emission from the Center of the Milky Way Gamma Ray Intensity of high-energy gamma-ray emission observed by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory (CGRO). The image includes all photons with energies greater than 300 MeV. At these extreme energies, most of the celestial gamma rays originate in collisions of cosmic rays with hydrogen nuclei in interstellar clouds. The bright, compact sources near Galactic longitudes 185°, 195°, and 265° indicate high-energy phenomena associated with the Crab, Geminga, and Vela pulsars, respectively. CGRO Frequencies: >2.4 x 1013 GHz Angular resolution: ~120 arcminutes Milky Way Object Finder Finder Diagram Major structural features of the Milky Way (red), optical H II regions (blue), radio sources (green), and OB associations (purple) are labeled in the finder chart. The image in the finder chart is derived from the IRAS 100 micron map of intensity with contours from the COBE DIRBE 3.5 micron map overlaid. The axes of the finder diagram are labeled in degrees of Galactic longitude and latitude. Then mosaics in the previous transparencies were taken from the website of the Astrophysics Data Facility (ADF) at the NASA Goddard Space Flight Center (GSFC).
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