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How does a Newtonian reflector direct light to an observer’s eyepiece?
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Telescope
Aerial view of the Keck Observatory's twin domes, which are opened to reveal the telescopes. Keck II is on the left and Keck I on the right.
In a Newtonian reflector, light enters the telescope tube and is collected by a concave primary mirror at the base, which reflects the light toward the prime focus near the upper end of the tube.1
To prevent the observer from blocking the incoming light with their head while viewing, Isaac Newton introduced a small, flat secondary mirror placed at a 45° angle inside the prime focus. This secondary mirror intercepts the converging light and redirects it to the side of the telescope tube, where the eyepiece is positioned for the observer. This design is highly efficient, as the amount of light lost to the secondary mirror is negligible compared to the total light-gathering power of the primary mirror.12