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O

Objective - This is the main lens in a refractive optical system such as a refractor or binocular.  The aperture in such a system is determined by the diameter of the objective lens.  An objective lens consists of at least two glass elements, but can contain three or more elements in more complex optical systems.

Open Cluster - A type of deep-sky object, an open star cluster is a grouping of tens to hundreds of stars.  Open clusters tend to be bright objects and are among the most beautiful sights in a telescope.  An open cluster is distinct in appearance from a globular cluster in that the stars are scattered and distinct, whereas they are tightly grouped into a ball in a globular cluster.  They are also different in that open clusters tend to be groups of young stars (millions of years old) while globular clusters are comprised of much older stars (billions of years old).  Examples of open clusters include the Pleiades and the Beehive Cluster.

Optical Tube Assembly (OTA) - The actual telescope itself, less the mount, fork arms, tripod, etc.  OTAs are often sold separately for observers who already own a mount.