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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 chromatic aberration.  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 image scale (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 millimeters.

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 (see above).  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 compromise.

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 applications.

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 primary mirror, 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.

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