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Secondary Mirror - This is the smaller second mirror in a reflecting or catadioptric telescope design. In a Newtonian this is a flat mirror used to bend the light path out to the side of the optical tube. In a Cassegrain design, the secondary mirror is curved and provides additional magnification and optical correction and reflects the light back through a hole in the primary mirror.

Seeing - This is a measure of the steadiness of the atmosphere. Poor seeing can result in blurry images, especially at high magnifications. Seeing is often confused with transparency, the clarity of the atmosphere, but the two are often mutually exclusive: the best seeing often comes on nights of poor transparency. Seeing is the single most important factor in planetary observing; everything else (including the telescope) is secondary to atmospheric conditions.

Setting Circles - Setting circles are found on equatorial mounts. They allow the right ascension and declination to be determined. Very few observers use setting circles to navigate, preferring the easier method of star hopping. Of course, many new telescopes are computerized, eliminating the need for setting circles altogether.

Sidereal - This is the rate at which the stars appear to rotate around Earth. It is actually the time it takes Earth to spin once, and it is different from how long it takes for the sun to reach its highest point on two consecutive days. Therefore a solar day and a sidereal day are different. This is due to the fact that Earth is orbiting the sun and it takes a bit longer for the sun to return to a given position than for a star (because Earth has to rotate an extra bit to make up for having moved 1/365th of the way around the sun). A solar day is 24 hours, and a sidereal day is 23 hours 56 minutes 4 seconds. Telescopes normally track at the sidereal rate in order to follow the stars, but some have solar and lunar tracking rates to account for the different apparent motion of these objects.

Skyglow - This is light pollution, manmade or natural, which lights up the sky and reduces the visibility of faint celestial objects. Artificial sky glow (city lights) can be reduced with the use of a light pollution filter (or by leaving the city). Natural skyglow is caused by aurora, which can occur at a low, almost invisible level in addition to their more familiar spectacular displays.

Slewing - The automated motion of a goto telescope is called slewing.

Slow-Motion Controls - Manual adjustment knobs on a telescope mount (usually an equatorial mount) for fine positioning of the telescope in each axis.

South Celestial Pole - This is the point on the celestial sphere directly above Earth's south pole. The Earth's rotation axis is aimed at this point in the sky, so during the course of the night, while the rest of the sky appears to turn, this spot in the sky remains in the same position. Unlike its northern counterpart, no bright star is located near the south celestial pole.

Spherical Aberration - An optical aberration. Spherical aberration occurs when converging light beams from a spherical mirror or lens do not focus at the same distance along the optical axis. To eliminate or minimize spherical aberration, multiple optical elements or an aspheric optical element (or both) must be used.

Spider - This is the set of vanes that supports the secondary mirror in reflecting telescopes such as Newtonians and Ritchey-Chrétiens. It causes diffraction spikes that form the cross-shaped patterns seen on bright stars in some astrophotos (and can be seen visually around very bright objects). Most spiders consist of 4 vanes, but 3-vane spiders are sometimes seen. Spiders using curved vanes are sometimes used to reduce the prominence of the diffraction spikes.

Spot Diagram - A spot diagram plots the size, shape, and distribution of light from a point source, as imaged by a telescope. Spot diagrams are simulations of what a given optical system would produce under ideal conditions. They allow analysis of optical aberrations and comparisons of different optical designs. Sometimes the size of spot diagrams are quoted to show the quality of an optical system (smaller spots are better). Spots are measured in microns.

Supernova - A supernovae is an exploding star. When a star like the sun dies, it will gently shed its outer atmosphere, creating a planetary nebula. A much more massive star, on the other hand, will actually explode, blasting its material outward at a tremendous velocity. The light given off by a supernova can briefly outshine the billions of other stars in a galaxy. The supernova remnant can often be seen through a telescope as a cloud of gas, or nebula, and can be quite colorful in photographs. Supernovas are often seen in other galaxies, but one has not occurred in our own Milky Way since 1604, just before the invention of the telescope. Examples of supernova remnants include the Crab Nebula and Veil Nebula.