The circumstance in which a detector’s resolution elements, or pixels, are too big to resolve the detail in the spectrum or image under study. Undersampling of astronomical images can occur when the atmospheric seeing conditions are exceptionally good. If almost all the light in a star’s image lies within one pixel of a CCD, then the image appears square. The image is said to be undersampled, and it is not then possible to obtain accurate estimates of the star’s image size or its position. See also aliasing.