Orion Telescopes & Binoculars 1.25-inch Pentaprism Diagonal

The Orion Pentaprism 90-Degree Diagonal ($199.99US) is a non-inverting, non-reversing 1.25-inch diagonal. Unlike typical star diagonals, it does not produce a mirror-reversed image, which can make navigation and referring to star maps difficult while observing with your telescope.

But unlike common correct-image diagonals, 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, which is especially advantageous when using the BinoViewer for terrestrial applications for which a correctly-oriented view is a must.

As its name implies, the Pentaprism diagonal uses a pentagonal-shaped prism (BK-7 glass), which is multi-coated on its two transmitting surfaces and dielectric coated on its two reflective surfaces, for maximum light transmission to the eyepiece. The pentaprism is housed in a black-anodized aluminum body that features a non-marring 1.25-inch compression ring collar with a large knurled thumbscrew, and a tapered 1.25-inch barrel that is threaded for use of filters.

The Orion Pentaprism diagonal has an optical path length (backfocus requirement) of 120 mm, and a generous clear aperture of 29 mm. 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.

A mirror-reversed eyepiece view, on the other hand, requires some mental gymnastics when referring to a star map. While the Pentaprism is the ideal diagonal to use with the Orion Premium Linear BinoViewer, note that it may not provide a correctly oriented view with other binoviewers, which unlike the Premium Linear BinoViewer typically do not incorporate an image erecting lens system.

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