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Radio Telescope

Posted on August 5th, 2007 by Admin.
Categories: Telescopes.

Radio Telescope

From Wikipedia, the free encyclopedia
 
The 64 meter radio telescope at Parkes ObservatoryA radio telescope is a form of directional radio antenna used in radio astronomy and in tracking and collecting data from satellites and space probes. In their astronomical role they differ from optical telescopes in that they operate in the radio frequency portion of the electromagnetic spectrum where they can detect and collect data on radio sources. Radio telescopes are typically large parabolic (”dish”) antenna used singularly or in an array. Radio observatories are located far from major centers of population in order to avoid electromagnetic interference (EMI) from radio, TV, radar, and other EMI emitting devices. This is similar to the locating of optical telescopes to avoid light pollution, with the difference being that radio observatories will be placed in valleys to further shield them from EMI as opposed to clear air mountain tops for optical observatories.

 Early radio telescopes
 
Reber’s first “dish” radio telescope - Wheaton, IL 1937The first radio antenna used to identify an astronomical radio source was one built by Karl Guthe Jansky, an engineer with Bell Telephone Laboratories, in the early 1930s. Jansky was assigned the job of investigating sources of static that might interfere with radio telephone service. Jansky’s antenna was designed to receive short wave radio signals at a frequency of 20.5 MHz (wavelength about 14.6 meters). It was mounted on a turntable that allowed it to rotate in any direction, earning it the name “Jansky’s merry-go-round”. It had a diameter of approximately 100 ft. and stood 20 ft. tall. By rotating the antenna on a set of four Ford Model-T tires, the direction of the received interfering radio source (static) could be pinpointed. A small shed to the side of the antenna housed an analog pen-and-paper recording system. After recording signals from all directions for several months, Jansky eventually categorized them into three types of static: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of unknown origin. Jansky finally determined that the “faint hiss” repeated on a cycle of 23 hours and 56 minutes. This four-minute lag is a typical an astronomical sidereal day, the time it takes any “fixed” object located on the celestial sphere to pass overhead twice. By comparing his observations with optical astronomical maps, Jansky concluded that the radiation was coming from the Milky Way and was strongest in the direction of the center of the galaxy, in the constellation of Sagittarius.

Grote Reber was one of the pioneers of what became known as radio astronomy when he built the first parabolic “dish” radio telescope (9m in diameter) in 1937. He was instrumental in repeating Karl Guthe Jansky’s pioneering but somewhat simple work, and went on to conduct the first sky survey in the radio frequencies. After World War II, substantial improvements in radio astronomy technology were made by astronomers in Europe, Australia and the United States, and the field of radio astronomy began to blossom.
 Radio telescope types
 
A cylindrical paraboloid antenna.The range of frequencies in the electromagnetic spectrum that makes up the radio spectrum is very large. This means the variety and types of antennas that are used as radio telescopes vary in design, size, and configuration. At wavelengths of 30 meters to 3 meters (10 MHz - 100 MHz), they are generally directional antenna arrays similar to “TV antennas” or large stationary reflectors with moveable focal points. Since the wave length being observed with these types of antennas are so long, the “reflector” surfaces can be constructed from course wire mesh. At shorter wavelengths “dish” style radio telescopes predominate. The angular resolution of a dish style antenna is a function of the diameter of the dish in proportion to the wavelength of the electromagnetic radiation being observed. This dictates the size of the dish a radio telescope needs to have a useful resolution. Radio telescopes operating at wavelengths of 3 meters to 30 cm (100 MHz to 1 GHz) are usually well over 100 meters in diameter. Telescopes working at wavelengths above 30 cm (1 GHz) range in size from 3 to 90 meters in diameter.
 Big dishes
 
The 76.0m Lovell radio telescope at Jodrell Bank Observatory which, at the time of its construction, was the largest stearable dish radio telescope in the world.In the late 1950s and early 1960s saw the development of large single-dish radio telescopes. The largest individual radio telescope is the RATAN-600 (Russia) with 576 meter diameter of circular antenna (RATAN-600 description). Other two individual radio telescopes at Pushchino Radio Astronomy Observatory, Russia, designed specially for the low frequency observations, are between the largest in their class. LPA (LPA description (in Russian)) is 187 x 384 m size phased array meridional radio telescope, and DKR-1000 is 1000 x 1000 m cross radio telescope (DKR-1000 description (in Russian) ). The largest radio telescope in Europe is the 100 meter diameter antenna in Effelsberg, Germany, which also was the largest fully steerable telecope for 30 years until the Green Bank Telescope was opened in 2000. The largest radio telescope in the United States until 1998 was Ohio State University’s The Big Ear.

Other well known disk radio telescopes include the Arecibo radio telescope located in Arecibo, Puerto Rico, which is steerable within about 20° of the zenith and is the largest single-aperture telescope (cf. multiple aperture telescope) ever to be constructed, and the fully steerable Lovell telescope at Jodrell Bank in the United Kingdom. A typical size of the single antenna of a radio telescope is 25 metre, dozens of radio telescopes with comparable sizes are operated in radio observatories all over the world.
 Radio interferometry
 
The Very Large Array, an interferometric array formed from many smaller telescopes, like many larger radio telescopes.One of the most notable developments came in 1946 with the introduction of the technique called astronomical interferometry. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g. the One-Mile Telescope), arrays of one-dimensional antennas (e.g. the Molonglo Observatory Synthesis Telescope) or two-dimensional arrays of omni-directional dipoles (e.g. Tony Hewish’s Pulsar Array). All of the telescopes in the aray are widely separated and are connected together using coaxial cable, waveguide, optical fiber, or other type of transmission line. This not only increases the total signal collected, it can also be used in a process called Aperture synthesis to vastly increase resolution. This technique works by superposing (interfering) the signal waves from the different telescopes on the principle that waves that coincide with the same phase will add to each other while two waves that have opposite phases will cancel each other out. This creates a combed telescope that is the size of the antennas furthest apart in the array. In order to produce a high quality image, a large number of different separations between different telescopes are required (the projected separation between any two telescopes as seen from the radio source is called a baseline) - as many different baselines as possible are required in order to get a good quality image (For example the Very Large Array (VLA) in Socorro, New Mexico has 27 telescopes giving 351 independent baselines at once to achieve resolution of 0.2 arc seconds at 3 cm wavelengths[1]). Martin Ryle’s group in Cambridge obtained a Nobel Prize for interferometry and aperture synthesis[2]. The Lloyd’s mirror interferometer was also developed independently in 1946 by Joseph Pawsey’s group at the University of Sydney[3]. In the early 1950s the Cambridge Interferometer mapped the radio sky to produce the famous 2C and 3C surveys of radio sources. The largest existing radio telescope array is the Giant Metrewave Radio Telescope, located in Pune, India. A larger array, LOFAR (the ‘LOw Frequency ARray’) is currently being constructed in western Europe, consisting of 25 000 small antennas over an area several hundreds of kilometres in diameter.

 References
astronomytoday.com - “Radio Astronomy” by Sancar J Fredsti
Rohlfs, K., & Wilson, T. L. (2004). Tools of radio astronomy. Astronomy and astrophysics library. Berlin: Springer.
Asimov, I. (1979). Isaac Asimov’s Book of facts; Sky Watchers. New York: Grosset & Dunlap. Page 390 - 399. ISBN 0803893477
Retrieved from “http://en.wikipedia.org/wiki/Radio_telescope”

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Meade ETX Telescope

Posted on August 5th, 2007 by Admin.
Categories: Telescopes.

Meade ETX Telescope

From Wikipedia, the free encyclopedia
 
A 105mm diameter Meade ETXThe ETX is a line of Maksutov-Cassegrain telescopes made by Meade Instruments Corporation. They have been produced in diameters of 90mm, 105mm, and 125mm. They are moderately priced compared to other commercialy produced telescopes with their level of performance. Other than Maksutov, another line of ETX based on achromatic refractor telescopes are available in sizes of 60mm and 70mm, but these two models were later replaced by a 80mm model.

“EC” models have provision for automatic control. Older model, called ETX RA, have a clockwork mechanism to track movement of the earth with an equatorial wedge, but it cannot be retrofitted for computer control,.

New ETX telescopes (”AT” models) come with the #497 Autostar controller. This provides “goto” capabilities at an affordable price. (Compare with LX and RC lines of Meade telescopes) Smaller refractor models come with the #494 Autostar controller.

They can be used on a tabletop, or on a field tripod.

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Pfund Telescope

Posted on August 5th, 2007 by Admin.
Categories: Telescopes.

Pfund Telescope

From Wikipedia, the free encyclopedia

The Pfund telescope, originated by A. H. Pfund, provides another method for achieving a fixed telescope focal point in space regardless of where the telescope line of sight is pointed. This configuration utilizes a two-axis feed flat mirror to reflect starlight into a fixed paraboloid of revolution (paraboloidal) mirror, usually with a horizontal optical axis. The paraboloid focuses through a central hole in the feed flat to a convenient distance behind the flat. No spider vanes or Newtonian secondary fold mirrors are required in this configuration. This eliminates vane diffraction and blockage, as well as secondary mirror scattering and absorption, thus improving image brightness and contrast.

The feed flat is mounted on a two-axis azimuth/elevation mount. The azimuth and elevation drive servos must be continuously controlled as objects move across the sky, using vector addition to calculate the mirror motion in real time. One vector (V1) is stationary and points from the center of the feed flat to the center of the fixed paraboloid mirror. The other vector (V2) points from the center of the feed flat to the object to be tracked, which of course moves across the sky in time. The surface normal of the feed flat mirror is the 3D bisector of vectors V1 and V2, normalized to unity length. If (Nk,Nl,Nm) are the instantaneous unit vector components of the mirror surface normal, then the mirror elevation angle is arcsin(Nl), and the mirror azimuth angle is arcsin[Nk/cos(Elevation)]. The field of a Pfund telescope rotates at a nonuniform rate during tracking, precluding it from long-exposure astrophotography unless a derotation control matrix and optics are used to compensate field rotation.

The hole in the front face of a Pfund tracking flat must be large enough to pass the desired field of view with minimum vignetting (blocking of part of the light from the paraboloid). The hole through the flat must be conically shaped, to at least a 45º cone or more, to prevent vignetting of the image by the back of the steering flat at high mirror angles.

The front reflective face of the Pfund flat must be polished extremely flat, smooth and zone-free. The flat should ideally be flat to within about 25 nanometers peak-to-valley error. Departures from flatness, from figure error or deflection, or both, rapidly introduce unacceptable astigmatism in the image. The front face should lie precisely in the plane of the elevation rotation axis to minimize the required flat mirror aperture. This creates the need for counterweights extending forward from the mirror cell to balance the load on the elevation servo drive.

The diameter of the Pfund flat is generally larger than the focusing paraboloid, and is a design trade between fully illuminated field of view coverage and flat cost and weight. If the Pfund is intended to provide fully illuminated field coverage at a 90º flat angle, then the minimum flat diameter must be at least  times the paraboloid diameter. The McDonald Observatory Supernova Search Telescope (SNST) used the Pfund configuration, and its feed flat diameter was 24″, while the focusing mirror was an 18″ f/4.5 paraboloid. The aperture stop is the rim of the focusing paraboloid, thus the feed flat has to be slightly larger than the on-axis diameter required to maximize illumination over the desired field.

Examples of Pfund telescopes are the Infrared Spatial Interferometer Array at the University of California at Berkeley. In addition to the array’s website, the instrument is described in Dr. Charles Townes’ book, How The Laser Happened[1]. The same telescope is briefly described and diagrammed on pages 136-137 in Peter Manly’s book Unusual Telescopes[2].

Apparently unaware of Pfund’s telescope design, John O. Fundingsland independently developed the same optical configuration, and published his 4″ aperture prototype instrument in an August, 1992 Sky and Telescope article entitled “Easy Viewing with a Fixed Telescope”[3].

The George B. Wren Supernova Search Telescope (SNST) at McDonald Observatory and the new Wren-Marcario Wheelchair Access Telescope (WAT) at the McDonald Observatory Visitor Center (to be operational early 2007) are both based on the Pfund configuration. WAT is unique in that it will employ two 18″ f/8 mirrors arranged on a north-south line and facing each other, with the steering flat halfway between. The north 18″ mirror covers the northern half-hemisphere of the sky, and the south 18″ mirror covers the south sky, thus providing full sky coverage, which is not possible with a single-mirror Pfund. The 24″ steering flat and viewing port assembly rotate in azimuth to either mirror. Each half-hemisphere has its own fixed image location. WAT is fully compliant with all Americans with Disabilities Act (ADA) requirements and will provide both fully and differently abled visitors to McDonald Observatory with superb and comfortable viewing. A detailed Wikipedia article on the WAT is in preparation.[this source’s reliability may need verification]

References
^ Townes, Charles H., How the Laser Happened, Oxford University Press, 1999, pp. 184-185, ISBN 0-19-515376-6 (pbk).
^ Manly, Peter L., Unusual Telescopes, Cambridge University Press, 1999, pp. 136-137, ISBN 0-521-48393-X (paperback).
^ Fundingsland, John O., “Easy Viewing with a Fixed Telescope”, Sky and Telescope, August 1992, pp. 212-215.

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Types of Combined Lens-Mirror Telescopes

Posted on August 4th, 2007 by Admin.
Categories: Telescopes.

Combined Lens-Mirror Systems (Catadioptrics)

• Maksutov telescope
• Schmidt camera
• Schmidt-Newton telescope
• Schmidt-Cassegrain telescope
• Houghton
• Modified Dall-Kirkham
• Klevtsov
• Argunov

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Types of Reflector Telescopes

Posted on August 4th, 2007 by Admin.
Categories: Telescopes.

Reflectors (Catoptrics)

• Newtonian
• Gregorian 
• Classical Cassegrain
• Dall-Kirkham
• Large liquid mirror telescope
• Pfund telescope
• Ritchey-Chrétien
• Schiefspiegler
• Yolo
• Herrig
• Stevick-Paul

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Types of Refractor Telescopes

Posted on August 4th, 2007 by Admin.
Categories: Telescopes.

Refractors (Dioptrics)

• Non-achromatic
• Achromatic
• Apochromatic
• Superachromatic

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Top Telescope Keywords

Posted on June 4th, 2007 by Admin.
Categories: Telescopes.

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Telescope Types

Posted on December 19th, 2006 by Admin.
Categories: Telescopes.

Optical telescopes come in three types and these are further divided into more descriptive subgroups. Each type has it’s specific advantages and disadvantages and are used in different applications within the field of astronomy.

Refractors (Dioptrics)

• Non-achromatic
• Achromatic
• Apochromatic
• Superachromatic

Reflectors (Catoptrics)

• Newtonian
• Gregorian 
• Classical Cassegrain
• Dall-Kirkham
• Large liquid mirror telescope
• Pfund telescope
• Ritchey-Chrétien
• Schiefspiegler
• Yolo
• Herrig
• Stevick-Paul

Combined Lens-Mirror Systems (Catadioptrics)

• Maksutov telescope
• Schmidt camera
• Schmidt-Newton telescope
• Schmidt-Cassegrain telescope
• Houghton
• Modified Dall-Kirkham
• Klevtsov
• Argunov

The telescope mounts vary as much as the types of telescopes. Here is a listing of the more common telescope mounts.

Altazimuth (ALT-AZ) Mounts

• Altazimuth mount

Equatorial mounts

• Equatorial mount
• German equatorial mount
• Fork mount
• Springfield mount
• Poncet table

Other mounts

• Transit mount
• Zenith mount

Telescopes with a unique design

• Dobsonian telescope
• The Turret telescope

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