天頂鏡

天頂鏡（英語：Star diagonals）是在望遠鏡中使用的有角度的鏡子或稜鏡，允許從垂直於物鏡光軸的方向觀察。 當望遠鏡指向或靠近天頂（即直接在頭頂方向）時，它允許人觀察得更方便和舒適。 此外，天頂鏡生成的圖像上下不顛倒，但左右顛倒。

天頂鏡的種類

天頂鏡有1.25英寸和2英寸兩種直徑。2英寸的天頂鏡能用於更大的長焦距低倍率2英寸桶式目鏡，以獲得更大的視野。天頂鏡的價格範圍從低至30美元到數百美元不等。

反射式天頂鏡

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These diagonals (often called Star diagonals) use a mirror set at a 45° angle inside the diagonal to turn the telescope's image at a 90° angle to the rear cell. Mirror diagonals produce an image in the eyepiece that is correctly oriented vertically, but is reversed left-to-right horizontally. This causes image reversal, the view in the eyepiece is flipped left-right. The major advantage to mirror diagonals is that they cost less to produce to a high degree of optical accuracy compared to a prism and that they do not introduce any color errors to the image. The major disadvantage of mirror diagonals is that unless the reflective coating is properly applied they can scatter light rendering lower image contrast compared to a 90-degree prism. Also they deteriorate with age as the reflective surface oxidizes. The newer Dielectric mirrors have largely solved the deterioration problem, and if properly made the Dielectric mirrors scatter less light compared to conventional mirrors. With short focal length instruments a mirror diagonal is preferred over a prism.

稜鏡式天頂鏡

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A prism diagonal uses either a simple 90-degree angle prism, pentaprism or an Amici roof prism rather than a mirror to bend the light path.

On longer focal ratio telescopes a well-made 90-degree prism diagonal is the optimum choice to deliver the highest image contrast short of using the telescope without a diagonal entirely. However prisms seem to be falling out of favor probably due to marketing forces which have been favoring short focal length instruments which tend to function better with a mirror diagonal. In some special cases however, the color dispersion effects of a prism diagonal can be used to advantage to improve the performance of undercorrected refractor objectives (regardless of focal length) by shifting the spherical and color correction of the objective closer to the design optimum. The natural color dispersion properties (overcorrection) of the prism works to lessen or nullify the undercorrection of the objective lens.

On the other hand, a well-made conventional 90-degree prism star diagonal can transmit as much or more light as a mirror, and do so with higher image contrast since there is no possibility of light scattering from a reflective metallic surface as in a mirror diagonal. Also a prism will never degrade over time as a mirror will since there is no reflective metal coating to degrade from oxidation. However prism diagonals may introduce chromatic aberration when used with short focal-length scopes although this is not a problem with the popular Schmidt-Cassegrain and Maksutov Cassegrain telescopes, which have long focal lengths.

Pentaprism

A pentaprism provides the same inverted image orientation as viewing without a diagonal would. A simple 90-degree angle prism provides the same "flipped" or mirror reversed image as a mirror diagonal. Pentaprism diagonals are extremely difficult to find.

Amici prism

主條目：Amici roof prism

An Amici prism is a type of roof prism which splits the image in two parts and thus allows an upright image without left-right mirroring. This means that what is seen in the eyepiece is the same as what is seen when looking at the sky, or a star chart or lunar map.

The disadvantage of typical "correct image" Amici roof prism diagonals is that because the light path bounces around through a piece of glass, the total amount of light transmitted is less and the multiple reflections required can introduce optical aberrations. At higher magnifications (> 100x) brighter objects have a bright line through the object viewed. Therefore, most Amici roof prisms are more appropriate for low power viewing or for use in spotting scopes for terrestrial rather than astronomical use. But with low-power usage with a rich field, the field can easily be compared with star charts as it is no mirror image.

They are available in two types: with 90º angle, just like an ordinary star diagonal and with a 45º angle. Such prisms are often used in spotting scopes for terrestrial viewing, mostly with 45º angle. Such telescopes rarely use magnifications over 60x.

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