seeing disk

A "seeing disk" is a test pattern used in astronomy to evaluate the quality of a telescope's optics. It features a circular pattern that helps astronomers determine how well the telescope can resolve fine details. When observed through the telescope, a clear, well-defined seeing disk indicates good atmospheric conditions and proper focusing, allowing for sharper images. Conversely, a blurry or fuzzy seeing disk suggests atmospheric disturbances or optical issues, reducing image clarity. This assessment helps astronomers optimize their observations and understand the quality of their viewing conditions.