Vignetting is the term used to describe a situation in an optical system where the field of view is not fully illuminated at the edges. In real-world optics, this is actually very common. Fortunately, it’s not always a problem with visual or photographic observations. Vignetting is caused by a reduction of light through the optics from rays of light that don’t come straight in through your scope. These off-axis rays are either absorbed or bent away from the optical axis and/or miss a secondary mirror, dimming the image at its edges. It can also be caused by baffling or field stops inside the optical tube actually blocking the light from other parts of the optical system. Eyepiece drawtubes can also cause vignetting.
For visual work, vignetting is typically noticeable when using wide-angle eyepieces and low magnifications; it manifests as a dimming around the outer edge of the field. In photographs, prime focus imaging with full-frame 35 mm or larger films or large-format CCD chips will usually show darkening at the corners.
If vignetting is bothersome, switch to a higher-power eyepiece or an eyepiece with a smaller apparent field of view. Remove any accessories which can introduce vignetting, like a focal reducer. Using 2 in eyepieces on smaller scopes may show vignetting, so try 1.25 in eyepieces instead. For photography, try using a smaller format imager or film and removing focal reducers. For digiscoping, zoom in with either the eyepiece or the camera’s optical zoom.