The design of high-performance quality lightweight mirrors necessitated by payload requirements must be shown to be capable of resisting environmental-load-induced distortion. Such loading can include thermal gradients in the presence of flux loading, or extreme thermal soak in the cryogenic environments demanded by IR systems. Additionally, for aircraft systems, the optics may be subject to a changing gravitational vector, causing performance error. For orbital systems, gravitational error is a major concern as well, as it is necessary to perform meaningful ground tests prior to the zero-g release condition. These mirrors must exhibit excellent stiffness and thermal expansion characteristics, particularly in a passive system, and often in an active system as the mirror size increases but the aerial density requirement does not. To meet the stringent requirements, analyses for mirrors of various sizes, both solid and lightweight, are presented to show the effects of material properties and inhomogeneities on performance characteristics in the presence of a thermal and gravitational environment. Included is the effect of kinematic mount location and coefficient of thermal expansion uncertainties. Passive and active focus performance is compared, and design points are indicated for actively controlled deformable mirror requirements.