Aperture Diameter of the light inlet of a telescope. Apochromat Sort of refractor, with primary lens corrected for 3 wavelengths. Very expensive, but marvelous images. When you rotate an axis of a gear box, then you will often experiment that when you invert the rotation direction, the output axis does not change immediately its rotation direction. There is some play on the system, called backlash.
Every mechanic system has backlash, but it only becomes a problem when it becomes noticeable.
Baffle Sort of diaphragm in the light path, used for enhancing the image contrast by reducing the incidence of unwanted scattered light. Common barlow interfaces multiply the focal length of the primary by 2 or 3, resulting in 2 or 3 times more power with the same eyepiece. Barn door mount Sort of telescope mount, mostly used for photographic cameras. Bartelize, to This verb refers to Mel Bartels' motorized Dobson, and means motorize the alt-azimuth telescope with steppers and a laptop. Blueing Coating a lens with an anti-reflective coating.