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90 degree diagonal mirror
The 90 degree diagonal mirror is placed between the eyepiece and the eyepiece tube. This way, the light is bent 90 degrees, allowing easy viewing from the top. The image is upright, yet it is still mirrored. A 90 degree diagonal mirror is essential for Schmidt-Cassegrain & Maksutov Cassegrain telescopes.
Every telescope shows the image flipped upside down. Using an erecting prism also involves using an eyepiece. First the erecting prism is placed in the eyepiece, after which the eyepiece is shoved into the lens.
An Amici prism or roof prism is a normal rectangular equilateral prism where, instead of the biggest side, the two sides have been perpendicularly ground to each other. The prism actually consists of two combined triangular prisms. An Amici prism provides an upright image in an astronomical telescope. The prism cannot be used in combination with another optical instrument (90 degree diagonal mirror). Please bear in mind that using this prism does involve some loss of light.