If optics in an optical system bend, deform, or move relative to each other, then wavefront errror is produced and the image becomes blurry. Further, for astonomical telescopes with a small field of view, a small motion of the optics could move the intended target off of the instrument's detector. These things are undesirable, therefore, an optical design engineer must make certain that the optics do not bend or move. One source of motion and deformation is a change in the thermal state of the optical system, and athermalization can minimize the effects of this source.
Perfect athermalization can be achieved by constructing the entire telescope out of the same material or out of a zero coefficient of thermal expansion (CTE) material. This is difficult to do in practice for two reasons: 1) no materials are ideal for both the mirror and the metering structure and 2) zero CTE materials are expensive and often brittle. This simple method of athermalization has some clear drawbacks that make it untenable for most optical system designs. More complex engineering designs can achieve athermalization through the use of switch-backs. A switch-back can be placed in the metering structure which causes the net expansion of the assembly to be zero.