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
FF - Field Flattener - a lens designed to reduce field
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
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
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.
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
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
Focal Plane - In a telescope or eyepiece, this is where
the image comes to its sharpest reproduction of the image entering the
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.
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
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
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
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
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.