Optothermal stability

Optothermal stability describes the rate at which an optical element distorts due to a changing thermal environment. A changing thermal environment can cause an optic to bend due to either 1) thermal gradients on the optic and a non-zero coefficient of thermal expansion, or 2) coefficient of thermal expansion gradients in an optic and a temperature change. [1] Therefore, optothermal stability is an issue for optics that are present in a changing thermal environment. For example, a space telescope will have variable heat loads 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 low frequency (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 optical systems which require a high level of stability such as those that use a coronagraph.[2]

Material Characterization

Material characterization numbers have been mathematically derived to 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.[3] 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:

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

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See also

References
  1. Brooks, Thomas (August 6, 2017). "Modeling the Extremely Lightweight Zerodur Mirror (ELZM) Thermal Soak Test". SPIE Proceedings. 10398: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170009042.pdf.
  2. Brooks, Thomas (August 6, 2017). "Predictive thermal control applied to HabEx". SPIE Proceedings. 10398: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170009034.pdf.
  3. Brooks, Thomas (September 23, 2015). "Advanced Mirror Technology Development (AMTD) thermal trade studies". SPIE Proceedings. 9577: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019495.pdf.
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