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Ecliptic - The plane of Earth's orbit around the sun, projected onto the celestial sphere.  From our perspective, the sun follows the ecliptic around the sky during the course of one year.  Since most solar system objects (planets, asteroids, and our moon) are also confined to a similar plane, these objects appear to follow the path of the ecliptic as well.

ED Glass - Nothing to do with Bob Dole or Raphael Palmero, ED stands for extra-low dispersion.  Glass refracts light of different wavelengths by different amounts, which results in the familiar prism effect of light being dispersed into a rainbow of colors.  ED glass reduces how much the different wavelengths of light are dispersed, allowing for better chromatic aberration correction in refractors, camera lenses, and binoculars.  Some companies will use similar terms such as SD (super-low dispersion) and UD (ultra-low dispersion).

Encoders - Encoders read the position of a telescope are are used to feed back position data to a computer in a goto telescope or digital setting circles system.

Enhanced Coatings - Glass does not transmit all of the light that strikes it, and mirrors do not reflect 100% of the light that bounces off them.  Enhanced coatings are used to either increase the light transmission in a refractive system or the reflectivity of a reflecting system.  Enhanced coating technology allows almost 100% transmission through or reflection from a single surface.  Since there are many surfaces in a normal optical system, good coatings are critical to image brightness.

Equatorial Mount - An equatorial mount is one in which one axis (the polar axis) is aligned with Earth's rotation axis.  (In the northern hemisphere this is done by aligning the mount with the north celestial pole, which coincides almost exactly with Polaris, the north star.)  This allows the mount to track Earth's rotation using a simple clock drive and eliminates any field rotation.  It is, however, more cumbersome than an alt-azimuth mount.  See also German Equatorial Mount.

Exit Pupil - The diameter of the beam of light coming out of an eyepiece in a telescope or binocular.

Eye Lens - This is the lens in an eyepiece closest to the observer's eye.  Typically larger eye lenses offer an easier view, although eye relief (see below) is an important factor as well.

Eye Relief - This is the distance from the eye lens (see above) in an eyepiece to the observer's eye.  Long eye relief allows observers with eyeglasses to easily observe.  Short-eye-relief eyepieces can be difficult to view through, even for observer's without glasses.

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