What is a Star Diagonal? When using a refractor or cassegrain style telescope a unique problem can sometimes occur. Depending on the telescope's position, it might be very uncomfortable for the viewer to look straight through the eyepiece! For that reason, a simple piece of equipment was invented: the diagonal. The star diagonal is easily recognizable. It contains an angled mirror or prism that allows for comfortable viewing, especially if the telescope is pointed at or near the zenith. At the same time, it can erect the image, making it appear "right side up", but it also reverses left to right. Let's discuss the two styles.
The first design is the mirror diagonal. Many star diagonals use a high quality mirror set at a 45° angle inside the diagonal. This allows the telescope's image to appear at a 90° angle to the rear cell where it is then magnified by the eyepiece. The image in your eyepiece will be correctly oriented vertically, but reversed left-to-right horizontally. At first, this change in direction can be slightly confusing when comparing the image to a star chart, but a high quality mirror diagonal provides very little light loss in the optical path.
The second design is the prism diagonal. Inside the angled body, an Amici roof prism bends the light path in the same fashion as binoculars. Because it is not reflecting, but bending, this design allows it to be "image erecting" or "image correcting" both horizontally and vertically. Because the eyepiece image is oriented the same as looking at a star chart or lunar map, it does simple things. However, unless a high quality prism is used, it can reduce the total amount of light transmitted to the eyepiece and the eye. Some prism diagonals may also introduce chromatic aberration when used with short focal-length telescopes. Before you believe prism diagonals to be a bad decision, however, they are an excellent choice for viewing terrestrial subjects where orientation matters the most and a small amount of light lost is no problem.
Because most diagonals supplied with average telescopes are not exceptionally high quality, you may wish to upgrade your star diagonal. Even the finest of eyepieces are no better than the diagonal in which they are placed. For a telescope with a 2" focuser adapted down to a 1.25" diagonal, the change to a full 2" star diagonal will be nothing short of amazing! Here at OPT, we stock the finest quality star diagonals from leading manufacturers, including star diagonals exclusively for binoviewers. When you browse through our selection in 1.25" Star Diagonals, you'll find a wide variety of offerings from Meade, Celestron, Orion, Takahashi, TeleVue, Stellarvue, and William Optics. The 2" Star Diagonals section is home to OPT, Meade, Celestron, William Optics, Takahashi, Televue, and more. You'll also find the Denkmeier Power X, which allows for multiple magnifications within the diagonal and models which allow filters to be threaded directly to the diagonal body instead of the eyepiece. If you specifically want Image Erect Diagonals and Adapters, check out our offerings from William Optics, Meade, Celestron, Stellarvue, and TeleVue.