The Orion pentaprism Degree diagonal is a non-inverting, non-reversing 1.25" diagonal with a Degree viewing angle. Unlike a star diagonal, it does not produce a mirror-reversed image, which can make navigation and referring to star Maps while observing with your telescope difficult. But Unlike a correct-image diagonal, there is no image degradation caused by the ridge line where prisms intersect, which especially affects high-power viewing. It is the only diagonal that can be used with the Orion premium linear binoviewer to render an upright, non-mirror-reversed image through a telescope. So we highly recommend it for viewing with the premium linear binoviewer, especially for terrestrial applications for which a correctly oriented view is a must. The pentaprism diagonal consists of a pentagonal-shaped prism made of bk-7 glass that is multi-coated on its two transmitting surfaces and dielectric coated on its two reflective surfaces, for maximum light transmission. The pentaprism is housed in as aluminum body that features a non-marring 1.25" compression ring collar, and a tapered 1.25" barrel that is threaded for filters. The Orion pentaprism diagonal has an optical path length (back focus Requirement) of 120mm, and a generous clear aperture of 29mm. One nice feature of the pentaprism diagonal is that it causes no vignetting, even when used with long focal length, wide-angle eyepieces. When used in combination with just an eyepiece, the pentaprism diagonal will render an upside-down image, but not mirror reversed as you get with a standard star diagonal. That can be advantageous for telescopic viewing with star charts, since you need only rotate the star chart to match the view in the eyepiece. While the pentaprism is the ideal diagonal to use with the premium linear binoviewer, Note that it may not provide a correctly oriented view with other binoviewers. Try this premium quality Orion pentaprism diagonal. It's the "best and brightest" diagonal for the job!.