Tank - A second variation relative to Brierley turns the previous design inside out, thereby reducing the problem of alignment to the very much simpler task of polishing two sides of the same mirror (50) parallel. In this figure, the disk is shown edge on with the flat mirrors (91, 92). The penalty is the addition of folding mirrors, but it is much easier to adjust two static mirrors than even one moving mirror. The two general problems with this design are that optical access to both sides of the disk must be maintained and tilted disks with parallel sides undergo serious deformation unless they are quite thick. It is particularly difficult to mount a disk of this design because of the hole through the center. This design, however, has a substantial advantage relative to a solid disk design; by passing the beams through the hole in the center of the disk, a much shorter path in each arm is achieved so that a larger beam divergence can be tolerated.