A Newtonian telescope with a focal ratio, lower than or equal to f/6, is said to have a highly significant coma for visual or photographic use. The third order tangential coma is given by the formula 3θ / 16F², θ stands for the angle off-axis to the image formed (in radians), and F stands for the focal ratio.This flare is linearly increasing with field angle and is inversely proportional to the square of the mirror focal ratio (the ratio between the mirror focal length and the mirror diameter). There is so much observable flare on-axis. (Images of stars on the edge of the field of view appears to have a “comet-like” shape). Coma is referred to as an off-axis aberration due to which images tend to flare inwards in the optical axis direction. The Newtonian telescope, like any other reflector, is subjected to coma.Image Source: Szőcs Tamás Tamasflex, Newton01, CC BY-SA 3.0 Disadvantages of Newtonian telescope The magnification and focus take place within the eyepiece. The eyepiece is generally located at around 90 degrees from the secondary mirror in the tube. This secondary mirror then further reflects the image towards the eyepiece. The light (image) captured by the primary mirror is then redirected/reflected towards the flat secondary mirror. The lower end of the telescopic design has a parabolic or spherical primary mirror used to capture light emitted by the celestial bodies. The telescope was designed to capture light from the top of the tube that pointed towards the sky. This mirror reflected a huge amount of light and was comparably less costly than the silver used during that time. Initially, Sir Isaac Newton created mirrors on his own by mixing copper and tin on the back. Nowadays, these telescopes are found in several design variations and sometimes incorporate additional optical elements to enhance image quality or mechanically improve the image position. The Newtonian telescope is popular because of its simple yet effective design lauded by amateur telescope makers. This design uses a primary concave mirror and a secondary flat diagonal mirror. The Newtonian reflecting telescope (also known as the Newtonian reflector) is the oldest reflecting telescope variant developed by Sir Isaac Newton in the year 1668.
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