Telescopes and Technology: A history and close up look at the amazing telescope.
Telescopes and Technology Common questions about telescopes: What is a telescope? What is the history and origin of the telescope? How do telescopes work? What are the most common telescope types in amateur astronomy? What are the pros and cons of the common telescope types?
Covered in this section: Define what a telescope is. 1. 2. Brief history and earliest known telescopes. Where the word “telescope” is derived from. 3. Profile of the inventors of the common types we see today. 4.
What is a telescope? Wikipedia definition: A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation (such as visible light). Online Dictionary definition: An arrangement of lenses or mirrors or both that gathers light, permitting direct observation or photographic recording of distant objects. Any of various devices, such as a radio telescope, used to detect and observe distant objects by their emission, absorption, or reflection of electromagnetic radiation.
History of the telescope The first known practical telescopes were invented in the Netherlands at the beginning of the 17 th century, using glass lenses. These are known as refracting telescopes. The word "telescope" (from the Greek τ ῆ λε, tele "far" and σκοπε ῖ ν, skopein "to look or see"; τηλεσκόπος, teleskopos "far-seeing") was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galilei's instruments presented at a banquet at the Accademia dei Lincei . In the Starry Messenger , Galileo had used the term " perspicillum ".
Earliest working telescopes The earliest records indicate they were refracting telescopes, appearing in the Netherlands in 1608. The three men credited with the invention of the first telescope were: Hans Lippershey, Zacharias Janssen (spectacle makers in Middleburg) and Jacob Metius of Alkmaar. Disputes have arisen as to the validity of the claims from these men. Much of the history of the time period was lost due to the Nazi’s bombing of Middleburg in 1940. Galileo Galilei’s refracting instrument was the first to be called a telescope.
Hans Lippershey Born 1570 in Wesel, Duchy of Cleves, Germany Died September 1619, Middleburg, Netherlands Was a master lens grinder and spectacle maker. Lippershey applied, to the States General of the Netherlands on 2 October 1608, for a patent for his instrument "for seeing things far away as if they were nearby", beating another Dutch instrument-maker's patent, Jacob Metius, by a few weeks. Although he failed to obtain the patent, he is given credit for the design of the refracting telescope.
James Gregory Lived 1638-1675 Scottish mathematician and astronomer, he advanced the understanding of trigonometry and calculus. In the Optica Promota , 1663, he described his design for a reflecting telescope, known as the “Gregorian Telescope”. Ten years later, Oxford physicist Robert Hooke and Sir Robert Moray built the first Gregorian Telescope. Gregorian optics are rarely used today, being seen mainly in radio telescopes. Source: http://en.wikipedia.org/wiki/James_Gregory(mathmetician)
Gregorian Telescope Gregorian Telescope circa 1735 Source: http://en.wikipedia.org/wiki/Gregorian_telescope
Sir Isaac Newton Lived 1640-1726 In late 1668 he built his first practical reflecting telescope, a design which bears his name: Newtonian reflector • Shown to the left is a replica of Newton’s second reflecting telescope that he presented to the Royal Society, circa 1672. • He was not the first to invent the reflecting telescope, but he made the first practical one.
Profile: Laurent Cassegrain Lived 1629-1693 Laurent Cassegrain was a Catholic priest who is associated with the invention of the Cassegrain reflecting telescope that bears his name. The identity of this "Cassegrain" has had many theories. His only known publication was the letter on the megaphone/reflecting telescope in the April 25, 1672 Recueil des mémoires et conférences concernant les arts et les sciences . Source: http://en.wikipedia.org/wiki/Laurent_Cassegrain
Profile: John Lowry Dobson Born: Sept 14,1915 Beijing, China Died: Jan 15, 2014 (age 98) Burbank, California John Dobson was an amateur astronomer and monk who is best known for the Dobsonian reflector, a low cost, portable Newtonian reflector telescope. His design is considered revolutionary since it allowed amateur astronomers to build large telescopes. His telescopes are often called “light buckets”. He was also reluctant to take credit for the design, saying he built it because it was all he needed, and that he didn’t know how to build anything with an equatorial mount.
Dmitry Dmitrievich Maksutov Lived 1896-1964 Russian optical engineer and amateur astronomer. In 1941 he invented the Maksutov telescope, a variant of the Cassegrain telescope. He published the design in 1944 in a paper entitled " Новые катадиоптрические менисковые системы" [ New catadioptric meniscus systems].
Bernhard Schmidt Lived 1879-1935 German optician In 1930 he invented the Schmidt telescope which corrected for the optical errors of spherical aberration, coma, and astigmatism, making possible for the first time the construction of very large, wide-angled reflective cameras of short exposure time for astronomical research.
In this section we will cover how telescopes work, and the different types common in amateur astronomy. We will also cover the different mounting mechanisms and types.
Telescope Types Here are the common types you typically find in amateur astronomy and that are available on the mass market. These include: Refracting tube Newtonian reflector Catadioptric Schmidt-Cassegrain Catadioptric Maksutov-Cassegrain Dobsonian reflector (derivative of the Newtonian)
Refracting Telescopes Refractors consist of the following: A long tube made of metal, plastic or wood. A glass combination lens at the front end ( objective lens) A second glass combination lens ( eyepiece) The tube holds the lenses in place, at the correct distance from each other. It also serves to keep out dust, moisture and light that would interfere with producing a good image.
Refracting Telescopes The objective lens gathers the light, and bends or refracts it to a focus near the back of the tube. The eyepiece brings the image to your eye and magnifies it. Achromatic refractors use lenses that are not extensively corrected to prevent chromatic aberration , which is a rainbow halo that sometimes appears around images in a refractor. Apochromatic refractors use either multiple-lens designs or lenses made of other types of glass (such as fluorite) to prevent chromatic aberration. Apochromatic refractors are much more expensive than achromatic refractors.
Refracting Telescope Diagram
Refracting Telescopes Advantages: Easy to use and consistent due to simplicity of design. Good for distant terrestrial viewing. Excellent for lunar, planetary and binary stargazing especially with larger apertures. Sealed tube protects optics and reduces image degrading air currents. Rugged, need little or no maintenance.
Refracting Telescopes Disadvantages: Generally have small apertures, typically 3 to 5 inches. Less suited for viewing small and faint deep sky objects such as distant galaxies and nebulae. Heavier, longer and bulkier than equivalent aperture reflectors and catadioptrics. Limited practical usefulness. Good-quality refractors cost more per inch of aperture than any other kind of telescope.
World’s Largest Refractor The largest refractor telescope ever built is the in the Yerkes Observatory at the University of Chicago in Williams Bay, Wisconsin. The 40” refractor located there was built by master optician Alvan Clark. It is the largest refractor telescope used for scientific research.
Yerkes Observatory Refractor
Newtonian Reflector Newtonian Reflectors consist of the following: Parabolic mirror ( primary mirror )at the end of the tube focuses the light back at the front of the tube, where the eyepiece sits. This is after it has been deflected by a small small secondary mirror in the light path. The secondary mirror is small enough that it does not block the image. No chromatic aberration is inherent in this design.
Inside the Newtonian reflector 1. Tube assembly 2. Primary mirror 3. Secondary diagonal mirror support, also called the “spider mirror”.
Newtonian reflector diagram
Newtonian Reflector Advantages: Free of chromatic aberration found in refractor type telescopes. They are usually less expensive for any given objective diameter ( or aperture ). Only one surface needs to be ground and polished into a complex shape, resulting in simpler manufacturing. A short focal ratio can be more easily obtained, leading to a wider field of view. The eyepiece is located at the top of end of the telescope. Combined with short f-ratios this can allow for a much more compact mounting system, reducing cost and adding to portability.
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