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Field lens: A correcting lens placed just before the image plane of a telescope. [citation needed] Telecompressor or focal reducer: Optical element to decrease the telescope's focal length and magnification (usually by a fixed percentage) and widen the field of view, providing opposite effects of a Barlow lens.
Orion ED120 apo refractor on Orion's Sirius EQ-G "GoTo" and GPS equipped German equatorial mount with portable 12 volt power supply. Orion sold a range of telescopes that they characterize as "beginner", "intermediate" or "advanced", including Newtonians, Maksutovs, Schmidt-Cassegrains, Ritchey-Chrétiens and refractors with or without (sold as optical tube assemblies or "OTA") a variety of ...
A telescope on an alt-azimuth GoTo mount. Note the keypad, resting on the platform between the tripod's legs, that is the telescope's hand control. Batteries are stored in the circular compartment just above the tripod. In this picture, the compartment is just above the hand control.
In the largest telescopes, the mass and cost of an equatorial mount is prohibitive and they have been superseded by computer-controlled altazimuth mounts. [5] The simple structure of an altazimuth mount allows significant cost reductions, in spite of the additional cost associated with the more complex tracking and image-orienting mechanisms. [6]
Telescopic sights based on image erector lenses (used to present to the user with an upright image) have two planes of focus where a reticle can be placed: at the focal plane between the objective and the image erector lens system (the First Focal Plane (FFP)), or the focal plane between the image erector lens system and the eyepiece (the ...
William Herschel's 49-inch (1,200 mm) 40-foot telescope on an altazimuth mount. Altazimuth, altitude-azimuth, or alt-az mounts allow telescopes to be moved in altitude (up and down), or azimuth (side to side), as separate motions. This mechanically simple mount was used in early telescope designs and until the second half of the 20th century ...
Multiple mirror telescopes that are on the same mount and can form a single combined image are ranked by their equivalent aperture. Fixed altitude telescopes (e.g. HET) are also ranked by their equivalent aperture. All telescopes with an effective aperture of at least 3.00 metres (118 in) at visible or near-infrared wavelengths are included.
The telescope is more a discovery of optical craftsmen than an invention of a scientist. [1] [2] The lens and the properties of refracting and reflecting light had been known since antiquity, and theory on how they worked was developed by ancient Greek philosophers, preserved and expanded on in the medieval Islamic world, and had reached a significantly advanced state by the time of the ...
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