30. Describe the working of a prism binocular with a suitable diagram. Discuss its advantages over an ordinary astronomical telescope.
for 5 marks
Answers
Answer:
Explanation:
Prism binoculars
Optical prisms added to the design are another way to turn the image right way up, usually in a Porro prism or roof-prisms design.[4]
Porro prism binoculars
Double Porro prism design
Porro prism binoculars
Porro prism binoculars are named after Italian optician Ignazio Porro who patented this image erecting system in 1854, which was later refined by makers like the Carl Zeiss company in the 1890s.[1] Binoculars of this type use a pair of Porro prisms in a Z-shaped configuration to erect the image. This results in binoculars that are wide, with objective lenses that are well separated and offset from the eyepieces, giving a better sensation of depth. Porro prism designs have the added benefit of folding the optical path so that the physical length of the binoculars is less than the focal length of the objective.
Roof-prisms binoculars
Abbe-Koenig "roof" prism design
Binoculars with Schmidt-Pechan "roof" prisms
Binoculars using roof prisms may have appeared as early as the 1870s in a design by Achille Victor Emile Daubresse.[5][6] In 1897 Moritz Hensoldt began marketing roof prism binoculars. Most roof prism binoculars use either the Abbe-Koenig prism (named after Ernst Karl Abbe and Albert Koenig and patented by Carl Zeiss in 1905) or the Schmidt-Pechan prism (invented in 1899) designs to erect the image and fold the optical path. They have objective lenses that are approximately in line with the eyepieces.
Roof-prisms designs create an instrument that is narrower and more compact than Porro prisms. There is also a difference in image brightness. Porro-prism binoculars will inherently produce a brighter image than Schmidt-Pechan roof-prism binoculars of the same magnification, objective size, and optical quality, because this roof-prism design employs silvered surfaces that reduce light transmission by 12% to 15%. Roof-prisms designs also require tighter tolerances for alignment of their optical elements (collimation). This adds to their expense since the design requires them to use fixed elements that need to be set at a high degree of collimation at the factory. Porro prisms binoculars occasionally need their prism sets to be re-aligned to bring them into collimation. The fixed alignment in roof-prism designs means the binoculars normally will not need re-collimation.[7]