B

Back Focus - Back focus is the distance from some telescope component to the focal plane, or point where the telescope reaches focus. For example, a Schmidt-Cassegrain might be said to have a back focus of 100mm from the rear cell of the telescope to the focal plane. This would allow 100mm of room for accessories such as a camera, diagonal, etc. A telescope with a short back focus may not accommodate certain accessories. Back focus is also sometimes measured from an internal component such as the primary mirror.

Backlash - When a set of gears reverses direction and there is some small gap between the gears, there is a moment of hesitation while the gears catch up and mesh again. This is backlash and is most often seen in telescope mount drive gears and in lower-quality rack-and-pinion focusers. With drive gears, excessive backlash can make guided astrophotography very difficult (or impossible). Worm gears are more precise since they have less backlash than spur gears. Focusers with backlash can make precise focusing difficult. Crayford focusers do not use a rack-and-pinion design, so they eliminate backlash.

Barlow Lens - This lens acts to increase the magnification of a telescope. Normally a Barlow lens fits between an eyepiece and the telescope and doubles the power of the eyepiece. Barlows are also available in other magnifications and can be used with cameras in addition to eyepieces.

Binary Star - A binary star is a pairing of two stars in orbit around each other. Double stars are a pairing of stars seen through a telescope, but a double star could consist of two stars aligned along the same line of sight by chance. A true binary star is one in which the stars are bound by gravity. Some binary stars may be too close to each other to resolve with a telescope but can be detected by changes in the light coming from the star. In addition to simple binaries, some star systems can contain three or more stars, bound together by gravity. Examples of binary stars include Mizar and Algol.

Binoviewer - A binocular viewer, or binoviewer, is used on a telescope to allow observing with both eyes. A binoviewer splits the light between two eyepieces (thereby dimming the view) but can allow more comfortable viewing for some observers. Two of the same focal length eyepieces are required with a binoviewer.

Broadband Filter - A broadband filter is one which passes most of the visible spectrum of light, but blocks certain wavelengths that are associated with light pollution sources. Broadband filters therefore can block light pollution. They are used both visually and for imaging. They work for any deep-sky viewing are more general in their application than narrowband filters which work only on nebulae.