What does vignetting mean?
Light coming out of the eyepiece creates a cone. The light striking a properly-aligned surface. In the context of astronomical imaging, vignetting occurs when this illuminated area does not fully cover the light detecting surface (the CCD chip in a digital camera). Because the image does not completely cover the CCD chip, the result is a circular image that is smaller than the camera’s field of view. This effect, which causes a dark area surrounding a smaller circular image, is called vignetting. The cone of light narrows as distance from the surface of the eyepiece increases. Thus, as a camera lens is moved closer to the eyepiece, the cone of light intersected has a larger diameter, resulting in a larger illuminated area reaching the CCD chip. In most cases, to avoid vignetting, the digital camera should be placed as close to the eyepiece as possible. However, when using eyepieces with relatively long eye relief it is actually possible to reduce the field of view by having the camera
