View source for Physics:Optothermal stability

'''Optothermal stability''' describes the rate at which an optical element distorts due to a changing [[Physics:Heat|thermal]] environment. A changing thermal environment can cause an optic to bend due to either 1) changing thermal gradients on the optic and a non-zero coefficient of thermal expansion, or 2) coefficient of thermal expansion gradients in an optic and an average temperature change. <ref name="Brooks Zerodur Thermal Stability">{{cite journal|last1=Brooks|first1=Thomas|title=Modeling the Extremely Lightweight Zerodur Mirror (ELZM) Thermal Soak Test|journal=SPIE Proceedings|date=August 6, 2017|volume=10398|page=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170009042.pdf}}</ref> Therefore, optothermal stability is an issue for optics that are present in a changing thermal environment. For example, a space telescope will experience variable heat loads from changes in spacecraft attitude, solar flux, planetary [[Earth:Albedo|albedo]], and planetary [[Physics:Infrared|infrared]] emissions. 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. Also, optothermal stability is important for optical systems which require a high level of stability such as those that use a [[Astronomy:Coronagraph|coronagraph]].<ref name="Brooks HabEx Thermal Stability">{{cite journal|last1=Brooks|first1=Thomas|title=Predictive thermal control applied to HabEx|journal=SPIE Proceedings|date=August 6, 2017|volume=10398|page=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20170009034.pdf}}</ref>
== 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|wavefront]] stability (dynamic) and [[Wavefront|wavefront]] error (static). A higher Massive Optothermal Stability (MOS) and Optothermal Stability (OS) number will result in greater stability.<ref name="Brooks Thermal Stability">{{cite journal|last1=Brooks|first1=Thomas|title=Advanced Mirror Technology Development (AMTD) thermal trade studies|journal=SPIE Proceedings|date=September 23, 2015|volume=9577|page=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20150019495.pdf}}</ref> 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:

<math>MOS = {{\rho c_p} \over \alpha}</math>

<math>OS = {c_p \over \alpha}</math>

Where ρ, c<sub>p</sub>, α are [[Physics:Density|density]], [[Physics:Specific heat|specific heat]], and the coefficient of thermal expansion respectively.

==See also==
*[[Physics:Athermalization|Athermalization]]

==References==
{{Reflist}}

[[Category:Optics|Thermal stability]]
[[Category:Temperature]]

{{Sourceattribution|Optothermal stability}}