A | B |
C | D |
E | F |
G | H |
I | J | K | L |
M | N |
O | P | Q |
R | S |
| U | V | W | X | Y |
False Color - Achromatic
refractors suffer from chromatic aberration, where different
wavelengths of light do not focus to the
same point. Specifically, red and blue light focus to one point while green
focuses to another. Since the human eye predominantly sees green, focusing
for this color results in a purple halo (out-of-focus red and blue light) around
bright objects like planets and stars. This is false color.
Apochromatic refractors are designed to eliminate false color.
Field Curvature - Most telescope do not inherently
have a flat focal plane (see below). This means light from off-axis (the edge of the
field of view) focuses closer to or father from the objective
or primary mirror than does on-axis
light. Visually this is not usually a problem (except in wide-angle
binoculars), but for imaging having a telescope free of field curvature is
preferable. Eliminating field curvature requires a specialized optical
design, although curvature can be reduced in other designs using accessories
such as field flatteners.
Field Stop - The limiting diameter in an eyepiece,
it determines the field of view. In wide field, low power eyepieces, the
field stop must be large, hence the existence of 2" eyepieces, and the fact that
there is an upper limit to field of view with such eyepieces.
Field Rotation - An alt-azimuth mount will suffer from
field rotation, where the image appears to rotate about the center of the image,
despite the telescope tracking. This is due to the fact that the telescope
must track in two axes while Earth rotates in only one (which is not aligned
with the telescope's axes). For visual observing, this is no problem,
but it precludes the possibility of long-exposure astrophotography. For
this, an equatorial mount is necessary.
Field of View - The angular width of the image. The
larger the field of view, the more sky you can see. Field of view is a
function of magnification and eyepiece type (or camera). Field of view is
normally measured in degrees.
Finderscope - Small scope mounted on top of the main
instrument for locating objects. Finderscopes have a much wider field of
view than a telescope, making finding objects much easier.
Flexure - Differential flexure is
the movement of optical or mechanical components relative to one another.
For example, the optical axis of a telescope might flex relative to the mount
axes, due to the mirror shifting, affecting the pointing accuracy. Or a
telescope piggybacked on another scope might flex
relative to the main scope. Likewise, mechanical components can flex
relative to each other, causing pointing or tracking inaccuracies.
Fluorite - Fluorite is a synthetic glass (actually a
crystal) that has unusual optical properties making it ideal for use in
refracting telescopes and camera lenses to reduce
Fluorite has very low dispersion, similar to
ED glass. Fluorite has the disadvantage of being very expensive.
Focal Length - This is the optical
length of a telescope, binocular, or camera lens. Eyepieces also have
focal lengths. A telescope's focal length
determines magnification (in combination with an eyepiece) or
(with a camera). Long-focal-length telescopes provide higher magnification
with a given eyepiece or camera than a short-focal-length scope.
Magnification is determined by dividing the focal length of the telescope by the
focal length of the eyepiece. Focal length is almost always quoted in
Focal Plane - The focal plane is where a telescope reaches
focus. This is not necessarily the same distance from the
objective or primary
mirror for all points in the field of view, meaning it is possible for the
focal plane to be curved. In fact, most telescopes have inherent field curvature
Only specialized designs have a flat focal plane.
Focal Ratio - This is the ratio between an optical
system's focal length and aperture. For example, a 100mm aperture
telescope with a focal length of 1000mm would have a focal ratio of 10.
This is usually written f/10. Focal ratio determines photographic speed,
so an f/5 telescope or lens requires shorter exposure times than an f/10 optic.
Therefore higher focal ratios are called slower, and lower focal ratios
are called faster. Also, all things otherwise being equal, slower
focal ratios produce less aberrations, but in many designs, a faster focal ratio
produces a more compact telescope, meaning there is always some sort of
Focal Reducer - A focal reducer is a lens that is
used to decrease the focal ratio of a telescope, allowing for a wider field of
view and, more importantly, a faster speed for photography. Focal reducers
allow a telescope to be used for both high-magnification and wide-field
Focuser - This is the part of a telescope that moves
to allow focusing for observers with different eyesight or for various accessories.
Newtonians and refractors have external focusers that usually use a
rack-and-pinion assembly to move the focuser drawtube in and out. Most
Schmidt-Cassegrains (SCTs) use internal focusing systems that move the
but accessory external focusers are sometimes added to SCTs.
Fork Mount - This type of mount uses two arms to
support the telescope tube. This is how most Schmidt-Cassegrain telescopes
are mounted. Alt-azimuth fork mounts are very
convenient for visual observing but require an equatorial
wedge for photographing. Due to the long
moment arm of the forks, fork mounts are not as stable as similar-sized
German equatorial mounts, but their portability and
ease of use makes them very popular.
Fully Multi-Coated Optics - Glass does not transmit
all of the light that strikes it. Coatings are used to increase the light
transmission in a refractive optical system. Multi-coated optics have
multiple layers of coatings on the glass surface. Fully multi-coated
optics have multiple coatings on every air-to-glass surface. These are the
best type of coatings used in binoculars.