the primary mirror is the part of a reflecting telescope that forms the image.
what parts of a reflecting telescope form the image?
primary mirror
a reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. the reflecting telescope was invented in the 17th century, by isaac newton, as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. almost all of the major telescopes used in astronomy research are reflectors. reflecting telescopes come in many design variations and may employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. since reflecting telescopes use mirrors, the design is sometimes referred to as a "catoptric" telescope.
reflecting telescope
in 1666 isaac newton, based on his theories of refraction and color, perceived that the faults of the refracting telescope were due more to a lens's varying refraction of light of different colors than to a lens's imperfect shape. he concluded that light could not be refracted through a lens without causing chromatic aberrations, although he incorrectly concluded from some rough experiments that "all" refracting substances would diverge the prismatic colors in a constant proportion to their mean refraction. from these experiments newton concluded that no improvement could be made in the refracting telescope. newton's experiments with mirrors showed that they did not suffer from the chromatic errors of lenses, for all colors of light the angle of incidence reflected in a mirror was equal to the angle of reflection, so as a proof to his theories newton set out to build a reflecting telescope. newton completed his first telescope in 1668 and it is the earliest known functional reflecting telescope. after much experiment, he chose an alloy (speculum metal) of tin and copper as the most suitable material for his objective mirror. he later devised means for grinding and polishing them, but chose a spherical shape for his mirror instead of a parabola to simplify construction. he added to his reflector what is the hallmark of the design of a ""newtonian telescope"", a secondary "diagonal" mirror near the primary mirror's focus to reflect the image at 90° angle to an eyepiece mounted on the side of the telescope. this unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. he also made all the tube, mount, and fittings. newton's first compact reflecting telescope had a mirror diameter of 1.3 inches and a focal ratio of f/5. with it he found that he could see the four galilean moons of jupiter and the crescent phase of the planet venus. encouraged by this success, he made a second telescope with a magnifying power of 38x which he presented to the royal society of london in december 1672. this type of telescope is still called a newtonian telescope.
history of the telescope