天顶镜（英语：Star diagonals）是在望远镜中使用的有角度的镜子或棱镜，允许从垂直于物镜光轴的方向观察。 当望远镜指向或靠近天顶（即直接在头顶方向）时，它允许人观察得更方便和舒适。 此外，天顶镜生成的图像上下不颠倒，但左右颠倒。
天顶镜有1.25英寸和2英寸两种直径。2英寸的天顶镜能用于更大的长焦距低倍率2英寸桶式目镜，以获得更大的视野。天顶镜的价格范围从低至30美元到数百美元不等。
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.
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.
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.
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.
