Objective Mirror
In a reflecting telescope the principle mirror (reflecting surface) that forms an image of the object viewed
Oblate Spheroid ...
Which depend on the size of the objective mirror or lens?
How many times brighter will a 60-centimeter telescope make a 10-second exposure image than a 12-centimeter telescope?
prime focus: The point at which the objective mirror forms an image in a reflecting telescope.
primeval atmosphere: Earth's first air, composed of gases from the solar nebula.
Telescope devised by Cassegrain in which an auxiliary convex mirror reflects the magnified image, upside down, through a hole in the center of the main objective mirror - i.e., through the end of the telescope itself.
(a) Telescope devised - but never constructed - by James Gregory, in which an auxiliary concave mirror reflects the magnified image, the right way up, through a hole in the centre of the main objective mirror, i.e.
cassegrain focus - An optical arrangement in a reflecting telescope in which light is reflected by a second mirror to a point behind the objective mirror.
For every action there is an equal and opposite reaction. Newton also improved the design of the refracting telescope (using an objective mirror, instead of a lens), and it is now called a Newtonian telescope.
While certainly not intended to be held to the eyes of a viewer, it uses two telescopes to view the same object, giving additional information due to the larger field of view that results from the separation of the objective mirrors.
 See also: Telescope, Astronomy, Objective, Light, Diameter
  
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