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DEFINITION OF TERMS (GLOSSARY): F - H

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F

Faculae - Calcium clouds in the Sun's chromosphere, seen in projection against the photosphere in white light and detectable most easily near the Solar limb.

Fan - Used in reflecting telescopes to remove the primary mirror's thermal boundary layer and for keeping the mirror closer to ambient temperature.

FF - Field Flattener - a lens designed to reduce field curvature.

FF/FR - Field Flattener/Focal length Reducer. A lens added to a telescope to reduce the focal length and to concentrate the light from the field of view onto a smaller area. Field flattening usually comes as a side aspect of the design. It is most important for photography. See also FR.

Field Curvature - Curvature of a telescope's focal surface inherent in certain telescope designs. In this aberration, the image does not fall on a flat plane. Thus, the focus changes from the center to the edge of the field of view. As the image is viewed, it appears sharp and crisp either at the center or at the edges of the field of view but not both, i.e. it causes a flat surface to be imaged onto a curved surface rather than a plane. The focal plane of the system is not a plane, but a curved surface, in this case. Common commercial Schmidt-Cassegrain telescopes exhibit field curvature on a radius about equal to that of a basketball. So long as the field of view is small, this doesn't deviate from flat by a significant enough amount to cause problems in focusing. It can be positive or negative (in most scopes it is positive). It can be seen in the edge of the field not being in focus at the same time the center of the field is in focus. Sometimes a compromise position for focus is achievable if the viewer can accommodate the necessary refocusing require by the eye. This is more often a problem in eyepieces than in the telescopes themselves.

Field Lens: in an eyepiece, the element or group which is farthest from the user's eye.

Field of View (FOV) The area visible through an optical device (the angular separation between opposite edges of the visual field).

Figure - the shape of the surface of an optical element. It's quality is often described by a specific fraction of the wavelength of visible light.

Filament A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun.

Finder A small, wide-field telescope attached to a larger telescope. The finder is used to help point the larger telescope to the desired viewing location.

Finder Telescope - A small auxiliary telescope used to find objects for viewing through a larger telescope to which it is attached. See Finder.

Fireball An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some can even be accompanied by a sonic boom.

Five-element Plossl - a 2:1:2 lens design that was described by Carl Zeiss in the 1950s and nearly perfected by the Japanese optical firm Masuyama. Today, it is made by several manufacturers and represents a design that is an excellent compromise between orthoscopy and field width. Though it is not a Pl?ssl at all, it is marketed as such due to the popularity of the Pl?ssl type.

FL - Focal Length

Flares A brief, sudden brightening in the sun's atmosphere that accompanies a burst of radiation from a sunspot.

Floaters - little bits of protein condensations in the vitreous humor of the eye. There is no cure for them other than laser vaporization or the replacement of the fluid in the eye. They interfere with vision though a telescope when the object viewed is bright and the exit pupil of the telescope is very small (typically, this occurs at high powers).

Fluorite: Today, this is often a type of "ED" glass, discovered by Ernst Abbe to have a low refractive index and low dispersion, as well as an abnormal partial dispersion. Chemically CaF2. Historically, natural fluorspar crystals were used, and applications were limited to microscopes. Development of artificial crystals was necessary to produce elements large enough for use in telescope objectives. Fluorite is about 1/4 as hard as typical glass, and it is consequently delicate and very sensitive to temperature changes. It is considered rather difficult to polish. Fluorite will be degraded by long term contact with water, acids, and other atmospheric contaminants, but in telescope objectives the coatings should protect the surface. Like lanthanum glass elements, fluorite has had a good deal of attention by marketing hypesters, and like lanthanum glass, it is not a mysterious substance, but provides optical designers with important latitude in improving optical systems. Because glass types of similar refractive index are now available, fluorite is passing out of fashion and may soon disappear from the market.

Focal Length The effective distance from a telescope objective to the focal plane. This distance is not apparent in compound telescopes.

Focal Plane - In a telescope or eyepiece, this is where the image comes to its sharpest reproduction of the image entering the optics.

Focal Point - for a point source, this is the point on the focal plane of the smallest image size.

Focal Ratio - The ratio of a telescope's effective focal length to its aperture.

Fovea centralis - the center of the retina, wherein all the cones of the eye are concentrated (our source of color vision)

FR or f/ - Focal Ratio. FR also Focal length reducer. See also FF/FR.

Fraunhoffer doublet: an achromatic telescope objective in the positive crown element forward configuration. The interior radii are unequal, and the air-spaced objective is separated by only a small amount: An aplanatic refracting telescope objective known for its wide coma-free field.

G

Galactic Nuclei - the centers of galaxies where the densest congregation of stars occurs. They are often quite small. The larger concentration of stars in the center of a galaxy is called the Core.

Galaxy A large grouping of stars. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains a few hundred billion stars. Some galaxies are so distant their light takes millions or billions of years to reach Earth.

Galilean Moons The name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius.

Gamma-ray The highest energy, shortest wavelength form of electromagnetic radiation.

GEM - German Equatorial Mount. See also EQ

Geosynchronous Orbit An orbit in which a satellite's orbital velocity is matched to the rotational velocity of the planet. A spacecraft in geosynchronous orbit appears to hang motionless above one position of a planet's surface.

German Equatorial Mount (GEM) A common type of equatorial mount. As opposed to an English equatorial mount, here the OTA is offset from the centre of the mount and is balanced by means of counterweights on the other side and an interconnecting shaft. The shaft rotates at 90 degrees to the mount's polar axis (i.e. the shaft points constantly at 0 degrees declination but can move through all hours of right ascension), and the OTA can rotate about the shaft's axis so as to point at any angle of declination..

Giant Molecular Cloud (GMC) Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to produce thousands of stars and are frequently the sites of new star formation. Also referred to as HII regions.

Globular Cluster A tight, spherical grouping of hundreds of thousands of stars. Globular clusters are composed of older stars, and are usually found in spherical halos around the central regions of a galaxy. They are believed to be the first star clusters formed when a galaxy forms.

GOTO Self-pointing capability in a computerized telescope mount.

GRS (Great Red Spot) A large high pressure storm system on Jupiter akin to a hurricane on earth. It is almost three times the diameter of the earth, produces winds of up to 400km/h and has been raging for at least 400 years. It transits the face of Jupiter every 9+ hours. It is the largest of Jupiter's storms. The color is believed to be due to oxides of chemicals brought up from deep within Jupiter's atmosphere. The color ranges, with time, from deep red to very pale pinkish-white.

Gravity A physical force (of mass in space-time) that causes two bodies to attract each other.

Group: a set of lenses, typically cemented together, in an optical system. A group of two elements is usually an achromatic doublet, but not always. A single-element group is simply one lens standing on its own in a larger optical system. For example, an Abbe orthoscopic is a two-group eyepiece, composed of a three-element group field lens, and a single-element eye lens.

GSO (Guan Sheng Optical) Maker of telescopes and eyepieces in Taiwan

H

Hat Trick - The Hat Trick is a technique in astrophotography to reduce vibrations due to opening a closing the camera shutter. The technique if as follows: 1) Place a dark piece of cardboard or a hat in front of the telescope. 2) Using a locking shutter cable open the shutter. 3) Wait a few seconds for the vibrations to subside. 4) Remove the cover or hat to start the exposure. 5) When the exposure is complete pace the cover or hat in front of the telescope. 6) Close the camera shutter

HC - Hand Controller - the small remote control box that controls the telescope by means of pushing the buttons on the controller and reading a screen on the controller.

Houghton Camera - A high-speed catadioptric telescope that is free of coma and spherical aberration.

Hour Angle - The Right Ascension indicated by the numbers on a setting circle of an equatorial mounting.

Hubble's Law The law of physics that states that the farther a galaxy is from us, the faster it is moving away from us.

Huyghenian - a simple two-lens eyepiece invented by the 17th century astronomer Christian Huyghens. It is not designed for full correction of color or aberrations and is not recommended for use in today's telescopes.

Hydrogen-Alpha Filter - A type of Solar Filter System consisting of an Energy Rejection Filter and a tunable bandpass filter. These filter system are used to view details of the Solar chromosphere like Solar Prominences.

Hydrogen-Beta (H-Beta) Filter - A type of nebular filter that isolates the hydrogen-beta line alone.

Hyperopia - Far-sightedness. You can see distant objects well, but not nearer ones.

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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.
 
Our Price: $29.95
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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
 
Our Price: $349.00
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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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