Optothermal stability

Optothermal stability describes the rate at which an optical element distorts due to a changing thermal environment. Therefore, optothermal stability is an issue for optics that are present in a changing thermal environment. Optothermal stability is important for objects in orbit which have varying heating levels from changes in spacecraft attitude, solar flux, planetary albedo, and planetary infrared emissions. Optothermal stability is important when measuring surface figure of optics because thermal changes are typically of a low frequency nature (diurnal or HVAC cycling) which makes it difficult to use the same methods used to remove errors due to vibrational disturbances. Also, optothermal stability is important for certain optical systems such as those that use a coronagraph which requires a high level of stability.

Material Characterization

Material characterization numbers have been mathematically derived which describe the rate at which a material deforms due to an external thermal input. It is important to note the distinction between wavefront stability (dynamic) and wavefront error (static). A higher Massive Optothermal Stability (MOS) and Optothermal Stability (OS) number will result in greater stability. As shown in the equation, MOS increases with density. Because added weight is undesirable for non-thermal reasons, especially in spaceflight applications, both MOS and OS are defined below:

$MOS = {{\rho c_p} \over \alpha}$

$OS = {c_p \over \alpha}$

Where ρ, c_p, α are density, specific heat, and the coefficient of thermal expansion respectively.

See also

• Athermalization