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DEFINITION OF TERMS (GLOSSARY): O - Q

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O

OAG - Off-axis Guider - a device used to allow manual guiding of photos being taken through a telescope.

Objective The principle optical element of a telescope that collects light and brings it to a focus.

Occultation An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon.

Off-axis: used most frequently among amateur astronomers to denote something that is not at the center of the field of view. In optics, the "axis" is the axis of (symmetrical) rotation of an optical surface, and therefore "off-axis" refers to something not along this line.

Off-Axis Guider - An astrophotographic accessory featuring an off-axis prism that intercepts unused light and uses this light to transmit images of guide stars.

Off-Axis Masks - Masks that stop down the aperture of a reflecting or catadioptric telescope. Their apertures are located in an off-axis position to avoid obstruction by the telescope's secondary mirror.

Offsetting - the procedure of finding objects in the sky by noting the movements necessary in an equatorial mount to find the object (e.g.2 degrees north and sweep to the west)

Oort Cloud A theoretical shell of icy bodies (comets) that is believed to exist at the outermost regions of our solar system. The Oort cloud was named after the Dutch astronomer who first proposed it.

Open Cluster A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.

Opposition The position of a planet when it is exactly opposite the Sun as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.

Optically-ground - a description referring to any optical surface ground or polished to a high standard of accuracy. In a filter, it refers to having the two sides of the optical window polished to a flatness not found in normally-created glass.

Orbit The path of a celestial body as it moves through space, in relation to a much larger body around which it moves.

Orthoscopic Free of distortion, a type of eyepiece designed by Ernst Abbe in 1880. This eyepiece employs a triplet field lens and a singlet eye lens. It excels in planetary observing: a lens that is free of spherical aberration, and magnifies an image uniformly throughout the field (i.e., the lens satisfies the tangent condition in which the ratio of the tangent of a' to the tangent of a is a constant for every ray on the lens). For a description of the "orthoscopic" eyepiece, see Abbe orthoscopic. Usually limited to a very small apparent field of view.

OTA - Optical Tube Assembly - a telescope minus the mount.

Oxygen III (O-III) Filter - A type of nebular filter that isolates just the two doubly ionized oxygen lines at 496 and 501nm wavelengths.

P

Paraboloid The shape given to the primary mirror of a Newtonian or Classical Cassegrain telescope.

Paracorr: a coma-correcting lens by TeleVue. It is not the same as a Ross coma corrector. It is composed of two doublets, and is designed for good axial and lateral color correction and coma correction, with reasonable control of spherical aberration. See also coma.

Park's Gold Series Plossl: A Japanese-made version of the five-element Pl?ssl, with a 52 degree apparent field. Excellent.

Parfocal: two eyepieces, the focal planes of which are the same distance from the barrel lip on which the eyepiece rests when in a focuser, are said to be parfocal. Many people note that, for example, a 12mm and an 8mm eyepiece in a supposedly "parfocal" line focus at slightly different positions for them. The reason is that the eye's lens exhibits spherical and other aberrations. As a result, pupil size affects the position of best focus for the eye. When reducing the pupil size by going to the shorter eyepiece, less aberration is encountered, and the focus point shifts accordingly. In addition, there is greater focus accommodation with longer focal length eyepieces, further obscuring an eyepiece set's true parfocality.

Parsec The distance an object lies from the Earth to exhibit a parallax of one arc-second from one end of the Earth's orbit to the other. A parsec is equal to 3.26 light years.

PE - Periodic Error (for Equatorial Drives). Tracking errors caused by the imperfections inherent in any worm gear system. These errors repeat every rotation of the worm, hence Periodic. See also PEC.

PEC - Periodic Error Control or Correction. Records corrections made during RA tracking, either by hand or autoguider; when activated, PEC replays the corrections, reducing periodic error (PE). Mount either does or does not retain the adjustments after powering down.

Perigee The point in the orbit of the Moon or other satellite at which it is closest to the Earth.

Perihelion The point in the orbit of a planet or other body where it is closest to the Sun.

Phase The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.

Photopic - daytime vision. The cones in the eye are turned on and we have color vision.

Photosphere The bright visible surface of the Sun.

Pickering Scale of Seeing - a ten point scale of degrees of flickering of the star image in a telescope due to turbulence in the atmosphere.

Piggyback Photography - Wide Field astrophotography where the camera is piggybacked on top of a telescope, and the photo is taken through the camera's normal lens.

Plages - a bright and intensely hot spot in the Sun's chromosphere, usually associated with a sunspot.

Planet A large body in orbit around a star. Planets can be composed mainly of rock, ices, or of dense gases. Hydrostatic equilibrium has been overcome by gravity and the object has been pulled into a nearly spherical shape. Additionally, the body must have swept up most of the debris in its orbital zone so that all that is left is a minor amount of mass not exceeding 1% of the mass of the planet.

Planetary Filters - Color Filters used to enhance light and dark features of the moon and planets.

Planetary Nebula A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colors and shapes.

Planisphere - a circular device that allows one to tell at a glance which constellations are above the horizon at a specific day and time.

Plossl A popular, high-performance eyepiece employing two cemented doublet lenses. The original Plossl eyepiece was symmetrical in design. The most traditional design seems to be one in which all air-glass surfaces are convex and the crown elements face one another, which minimizes aberrations at the exit pupil and minimizes distortion. The design is easy to make, as the spacing between the groups is not critical. As the spacing of the groups increases, focal plane curvature decreases and astigmatism increases. It is therefore possible to make an orthoscopic Plossl, but only at the expense of pronounced off-axis aberrations. There are also designs in which the external flint surfaces are plano. The asymmetrical Plossl designs, assumed to be variants on the traditional symmetrical design, may be better corrected than the symmetrical, depending on the design and execution, but there is no inherent advantage to an asymmetrical Plossl. This TeleVue design reduces astigmatism at the edge of the field and provides some correction for coma, at the expense of slight distortion in the field and exit pupil aberrations. Plossls, including variants, offer eye relief about 0.7x their focal length.

Plossl, George Simon: born September 19, 1794 near Vienna. In May 1812 he began work at the Voigtlaender optical firm. Supported by J.F. Jacquin and J.J. Littrow he studied mathematics and optics, founding his own firm in 1823 in his parent's house in Vienna. He encountered severe capital problems starting his firm but moved to much larger shops in 1831 and again in 1835. He made telescopes of the dialytic type, selling a 10.5" in 1850 and several other instruments of 6" to 8" aperture. He died as the result of an injury incurred dropping a sheet of glass, which severed an artery in his hand, causing blood loss and gangrene. He died January 30 1868 in the same house where he was born. His business was carried on as Fa. Pl?ssl & Co. by M. Wagner until the last shop was closed in 1905. In 1875 there was an unrelated shop named Pl?ssl in Vienna. George Pl?ssl is best known today as the designer of the Pl?ssl eyepiece.

Polar Alignment - The alignment of an equatorial mounting's polar axis parallel with the axis of rotation of the Earth.

Polar Axis - The axis of an equatorial mounting intended for alignment with the North Celestial Pole and thus with the rotational axis of the Earth, enabling a telescope to track celestial objects without rotating the field of view.

Polaris - In the northern hemisphere, the bright star that is nearest to the north celestial pole and whose altitude above the horizon describes ones latitude.

Polarizing Filters - A Neutral Density Filter using polarized glass. Some polarizing filters use two polar filters that can be rotated to produce variable amounts of filtering - these are known as Variable Polarizing Filters.

Porro Prism - A erecting prism system that corrects the inverted image of binoculars and refracting and catadioptric telescopes by simply rotating the field 180ˇă and leaving its left-to-right image orientation intact.

Positive Lens Projection Photography Projection photography in which a positive lens magnifies the image formed by the telescope objective and projects the image into the camera. Also called Eyepiece Projection Photography.

Power A measurement of magnification. Power can be calculated by dividing the focal length of a telescope by the focal length of an eyepiece.

Powermate: a telenegative lens by TeleVue. They bear some similarity to Galilean telescopes. The Powermate eliminates (or nearly eliminates) the effect that a traditional Barlow lens has on eye relief (see Barlow lens). The distance between the element and the ocular does not increase image scale in the same way as with traditional Barlows.

Presbyopia - "Old Vision". What happens when, as we age, the lens in the eye stiffens up and the muscles in the eye can no longer force the lens into a shape allowing focus on near objects. People who hear this word for the first time are usually getting their first reading glasses. It's a sign of wisdom

Primary - the large mirror or lens in a telescope that forms the primary agent for the gathering of light.

Prime Focus Photography A photographic technique that substitutes the telescope objective for the lens of the camera, focusing the image formed by the objective directly onto the film. Also called Direct Objective Photography.

Prism Diagonal: a diagonal assembly where the light is diverted 90 degrees by the use of a prism, rather than a mirror. The feat is achieved by taking advantage of the critical angle to induce total internal reflection. Some prism types can transmit as much light as a good mirror diagonal, but all introduce some chromatic aberration and other aberrations depending on the focal ratio of the telescope. These effects are more pronounced in fast (short f/ratio) optical systems. In addition, in fast systems, the critical angle may not be met for positions on the prism that are inside the observable field of view. A tilt error in a prism diagonal (i.e., miscollimation of the diagonal) will result in a ray deviation of twice the tilt, as is expected. Additional problems arise because a prism diagonal has 3 optical surfaces. Typically, only found in lower-priced diagonals.

Prominence An explosion of hot gas that erupts from the Sun's surface. Solar prominences are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere as the charged gas streams past the Earth.

Protostar Dense regions of molecular clouds where stars are forming.

Pulsar A spinning neutron star that emits energy from a spinning part of its surface. This energy is received as pulses as the star rotates, often thousandths of a second.

Pyrex A Trade Name for a glass having a low coefficient of expansion due to high borosilicate content.

Q

Quadrature A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth.

Quasar An unusually bright object found in the remote areas of the universe. Quasars release incredible amounts of energy and are among the oldest and farthest objects in the known universe. They may be the nuclei of ancient, active galaxies. There is some evidence this is a phase in the creation of super-massive black holes that form in the centers of galaxies.

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ASTRONOMICAL BOOKS

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Agena Astro Products - The Great Atlas of the Sky, Jubilee Edition
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Agena Astro Products - The Great Atlas of the Sky, Jubilee Edition
Item No. 786-20000

Agena Astro Products BOOKS The Great Atlas of the Sky by P.Brych
 
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Sun Observer's Guide, Firefly Books
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Sun Observer's Guide, Firefly Books
Item No. 896-23211
Manuf. No. Firefly 978-1-55297-941-9

A practical reference for how and why to view our nearest star. This book can start you making worthwhile observations.
 
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The History of Astronomy, Firefly Books
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The History of Astronomy, Firefly Books
Item No. 896-23210
Manuf. No. Firefly 978-1-55407-537-9

This extraordinary book traces humans' interaction with the endless wonders of the night sky.
 
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Stargazing with a Telescope, Firefly Books
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Stargazing with a Telescope, Firefly Books
Item No. 896-23208
Manuf. No. Firefly 978-1-55407-577-5

Stargazing with a Telescope is a practical guide that demystifies the process of buying and using a telescope.
 
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Stargazing with Binoculars, Firefly Books
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Stargazing with Binoculars, Firefly Books
Item No. 896-23207
Manuf. No. Firefly 978-1-55407-368-9

A practical, concise beginner's guide to viewing the night sky through binoculars
 
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300 Astronomical Objects: A Visual Reference to the Universe, Firefly Books
300 Astronomical Objects: A Visual Reference to the Universe, Firefly Books
Item No. 896-23206
Manuf. No. Firefly 978-1-55407-175-3

A handy and comprehensive reference to the 300 most interesting celestial objects. This book provides a tour through the galaxy.
 
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Moon Observer's Guide, Firefly Books
Moon Observer's Guide, Firefly Books
Item No. 896-23203
Manuf. No. Firefly 978-1-55297-888-7

Moon Observer's Guide offers practical guidance to amateur astronomers viewing Earth's only natural satellite.
 
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Night Sky Atlas: The Moon, Planets, Stars and Deep Sky Objects, Firefly Book
Night Sky Atlas: The Moon, Planets, Stars and Deep Sky Objects, Firefly Book
Item No. 896-10719
Manuf. No. Firefly 978-1-55407-026-8

This Firefly book is a very nice introduction to astronomy for beginners.
 
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Software Bisque TheSkyX Professional Edition for Mac
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Software Bisque TheSkyX Professional Edition for Mac
Item No. 773-55161
Manuf. No. Software Bisque SKYX_PRO_MAC

TheSkyX Professional Edition for Mac is the next major release in TheSky's 25+ year history
 
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Software Bisque TheSkyX Professional Edition for Windows
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Software Bisque TheSkyX Professional Edition for Windows
Item No. 773-55160
Manuf. No. Software Bisque SKYX_PRO_WIN

TheSkyX Professional for Windows will get you charged up about your passion. Explore, engage, enjoy while getting the most from your observing sessions.
 
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Sky & Telescope - Sky Atlas 2000.0 Companion, 2nd Edition
Sky & Telescope - Sky Atlas 2000.0 Companion, 2nd Edition
Item No. 846-10899
Manuf. No. Sky Publishing 46956

This essential reference features an alphabetical listing of every deep sky object plotted in Sky Atlas 2000.00.
 
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Sky & Telescope - SKY ATLAS 2000.0 DESK UNLAMINATED
Sky & Telescope - SKY ATLAS 2000.0 DESK UNLAMINATED
Item No. 846-10898
Manuf. No. Sky Publishing 46883

Includes close-up charts of such areas as the celestial poles and the Virgo-Coma galaxy region, as well as an acetate coordinate-grid overlay for determining accurate positions
 
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Sky & Telescope - SKY ATLAS 2000.0 FIELD LAMINATED
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Sky & Telescope - SKY ATLAS 2000.0 FIELD LAMINATED
Item No. 846-10895
Manuf. No. Sky Publishing 46921

The standard against which all other star atlases are measured, each version of Sky Atlas 2000.0 contains 26 charts covering the whole sky and showing 81,312 single, multiple, and variable stars of magnitude of 8.5 and brighter and 2,700 deep-sky objects
 
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Sky & Telescope: SKY ATLAS 2000.0 FIELD UNLAMINATED
Sky & Telescope: SKY ATLAS 2000.0 FIELD UNLAMINATED
Item No. 846-10890
Manuf. No. Sky Publishing 46891

Field Version: Stars and deep sky objects are white on a black background. Unbound and printed on heavy paper
 
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Sky & Telescope - Astronomy Library Sky Atlas 2000.0 Deluxe Edition
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Sky & Telescope - Astronomy Library Sky Atlas 2000.0 Deluxe Edition
Item No. 846-10880
Manuf. No. Sky Publishing 46875

Sky Atlas 2000.0 Deluxe Edition contains 26 charts showing close to 85,000 objects (stars and deep sky objects) down to magnitude 8.5
 
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