Binoviewers

A binoviewer is an optical device designed to enable binocular viewing through a single objective.

In contrast to binoculars, it does not allow stereoscopic viewing, as both images are produced by the same objective and do not differ except for aberrations induced by the binoviewer itself. However, binocular viewing allows the brain's visual system to make use of binocular summation, resulting in a more relaxed viewing experience and an increased ability of detecting faint details.

A binoviewer consists of a beam splitter which splits the image provided by the objective into two identical (but fainter) copies, and a system of prisms or mirrors that relay the images to a pair of identical eyepieces. In order to get an exact match of the two images for the eyes to combine comfortably it is important to use exactly identical eyepieces.

All binoviewers add considerable distance through which the light from the objective lens must travel. This additional length means that, to achieve focus, you need to adjust the focuser to move the eyepieces in a binoviewer closer to the objective of the telescope. In many cases, especially with Newtonians and refractors, this is impossible because the focuser does not have enough in-travel. So most binoviewers come equipped with or need an optional additional optical element, called by various manufacturers the optical corrector assembly (OCA), the optical corrector system (OCS), or the glass path corrector (GPC) to allow you to bring your binoviewers to focus. Essentially this is a barlow lens and typically magnifies 1.6x or 2x.

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