Previous section: CCD image processing . ... Figure 6: A small region from a CCD image showing various bad columns. Some pixels are damaged: they cannot transfer the electrons when the image is read out. ... Most telescopes have a 4-legged spider, which produces the typical cross-shaped diffraction pattern visible around the brightest stars. 1-legged spider produces a bar through the star, and some telescopes (like the Keck) have 6-legged spider, so the star images exhibit a 6-legged pattern. ...