Optothermal stability describes the rate at which an optical element distorts due to a changing thermal environment.
[1] Therefore, optothermal stability is an issue for optics that are present in a changing thermal environment.
Optothermal stability can be important when measuring the surface figure of optics, because thermal changes are typically low frequency (diurnal or HVAC cycling) which makes it difficult to use measurement averaging (commonly used for other error types) to remove errors.
Because added weight is undesirable for non-thermal reasons, especially in spaceflight applications, both MOS and OS are defined below:
Where ρ, cp, α are density, specific heat, and the coefficient of thermal expansion respectively.