http://www.lasertechn.com/index.php/toroidal-mirrors in the figure, what does S’s – for the sagittal plane means? the distance between the image point and the sagittal plane ?? I see in some papers,they use toroidal mirror to collimate the beam from a point source, how does this happen and why? some material say toroidal mirror have two focus point, where is it? any thing help would be appreciated best wishes! Thank you!