TURNED DOWN EDGE

Turned down edges are a common problem for mirror-based telescopes. Due to the nature of the polishing machines the outer rim of the mirrors are not correctly polished. The name turned-down-edge relates to the fact that the mirror has a slope on the edge that is lower than the perfect shape. Although a turned-down-edge cannot be changed on the mirror, a possible solution is masking the outer-edge of a mirror. In succesive turns more of the outer-edge is covered by a non-permanent mask (cardboard, nothing sticking to the mirror !) until the TDE-effect dissappears. Your telescopes effective aperture decreases but the quality increases.