Astronomy has always relied on optical instruments, from the early Galilean telescope to fiber-fed spectrographs, so much so that astronomy and modern optics are called astrophotonics. Since astronomical observations depend on electromagnetic radiation, optical instruments are critical for advancing our understanding of the universe.
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Astronomical instruments must operate efficiently across a wide range of daytime or night-time temperatures.1 Solving complex astronomical problems requires instruments with extreme characteristics, highly dependent on temperature.2 Thermal analysis is, therefore, pivotal for optimizing these instruments, ensuring precision in extreme conditions.1
Science of Thermal Optical Analysis
Temperature variations influence the functioning of optical systems due to thermoelastic (changes in the position and dimensions of optical elements) and thermo-optic (changes in the refractive index of optical materials) effects.
Thermoelastic effects in optical components result from the expansion and contraction of materials due to thermal strains induced by temperature variations. These appear as changes in the optical element’s thickness, diameter, radii of curvature, and surface deformations.1
Due to the thermo-optic effect, as light travels through an optical component at varying temperatures, its path length changes, resulting in optical path differences over the aperture and giving rise to transmissive optics.
Thus, thermal optical analysis, including both thermoelastic and thermos-optic analyses, becomes crucial in optical engineering. Integrated optomechanical analysis involves representing both thermoelastic and thermo-optic errors in an optical model to predict the device’s performance under complex thermal loads.1
All modes of heat transfer (conduction, convection, and radiation) are considered for the analysis of precision optical systems.
Radiation is the most critical aspect in astronomical applications where no air is present for convection. Convection analysis is equally important for ground-based observatories.
Even minor temperature variations can generate significant internal stress in an optical system and degrade its optical performance. Hence, thermal optical analysis is a critical aspect of astronomical instruments being developed to explore the frontier of space.1
Materials and Design for Thermal Stability
Glass ceramics like Zerodur (a Li2O-Al2O3-SiO2 composite) are used as mirror substrates for astronomical telescopes due to their extremely low thermal expansion coefficient, which makes them highly stable against temperature changes. The high reproducibility and homogeneity of such materials also make them suitable for application in large astronomical telescopes.1
In the case of telescopes operating in the ultraviolet-visible spectrum range, high-temperature gradients can cause the mirror to deform thermally. Thus, materials with ultra-low thermal expansion coefficients are required for their fabrication. For example, the mirror with a diameter of 2.4 m for the Hubble telescope is made of specially designed glass with a near-zero thermal expansion coefficient.2
Passive and active thermal control techniques are employed to maintain temperatures within operational limits, ensuring satisfactory performance and structural integrity. A thermal control system provides cooling, heating, insulation, or shading to ensure the system optics remain within the operational temperature range.1
A system for providing thermal mode (SPTM) is often used to thermally stabilize radiation detectors in astronomical instruments. It comprises both active and passive levels.
The SPTM passive level reduces temperature oscillations due to solar radiation, while the active level contains a heater and a cooler, which smooths the remaining temperature oscillations of the detector in the cold and warm orbits, respectively.2
Case Studies: Overcoming Thermal Challenges in Optical Systems
One of the most complex cryogenic tests executed by NASA was the Optical Telescope Element and Integrated Science Instrument Module (OTIS) Cryo-Vacuum (CV) test for the James Webb Space Telescope (JWST).
Due to its size, it was not possible to test the whole JWST observatory at a single facility. Therefore, the system-level testing (for payload, ground support equipment, and surrounding test chamber) was correlated using thermal models to achieve observatory-level verification.
The simulations, involving cooling down and warming up in a flight-like environment, were able to predict payload performance at cryo-stable conditions.3 The successful operation of JWST with unprecedented capabilities demonstrates the efficacy of such a complex optical thermal analysis.
A structural-thermal-optical performance (STOP) analysis was recently used for the subsystems of the under-construction European Extremely Large Telescope, which will be the world's largest optical or near-infrared telescope once completed.
The team used an indigenously developed software called Sensitizer to carry out optical thermal analysis at different stages of the telescope, such as phasing and diagnostic station and pre-focal station.4
Thermal optical analysis is a crucial aspect of giant projects involving tremendous efforts and resources.
Innovations in Optical Thermal Analysis
Along with the physical influence of temperature on astronomical instruments, experimental and mathematical modeling of their thermal systems also presents challenges, such as accurately replicating external thermal factors for precise modeling of the instruments' thermal modes. Innovations are therefore required to increase the efficiency of modeling techniques in optical thermal analysis.
Since the different components of an instrument are manufactured in various countries, it becomes necessary to accommodate the thermal modes of each component into a single model.2
Thermal models have been developed to design and predict temperature distribution over optical systems. These models can be used for thermoelastic and thermos-optic analyses. Computational fluid dynamics, finite-difference methods, and finite elemental modeling are some of the commonly used optical thermal analysis techniques.
The greater the target accuracy of the system, the more intensive the computational analysis needs to be.1 Thermal models, such as the JWST OTIS CV test, help with test and runtime scheduling and ensure hardware safety.
It was a comprehensive test of the integrated telescope system for its optical, structural, and thermal performance. Beyond optical system design and testing, these models can accurately predict the temperatures that can be expected during the flight, thereby facilitating the performance prediction of the instrument.3
The Future of Astronomical Exploration Instrumentation
Astronomical instruments working in different spectrum ranges encounter various challenges. For instance, microwave and infrared telescopes require their temperature to be kept close to absolute zero.
However, current cryogenic technologies have not yet achieved such low temperatures for large mirrors used in telescopes.2 Thus, continued investment in materials research is essential for the next generation of optical technologies.
For complex systems like astronomical instruments that demand high performance in extreme environments, integrated STOP analysis is required as a thermal management strategy.1
Thermal management in optical systems could be revolutionized through potential breakthroughs in material science, such as artificial intelligence that facilitates the design and simulation of novel materials with targeted properties and predicts their performance in operational environments.
Such advancements will enhance the performance and durability of astronomical instruments and expand human understanding of the universe.
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References and Further Reading
1. Ahmad, A. (2018). Handbook of Optomechanical Engineering. CRC Press, Taylor & Francis Group. doi.org/10.4324/9781315153247
2. Semena, NP. (2018). The Importance of Thermal Modes of Astrophysical Instruments in Solving Problems of Extra-Atmospheric Astronomy. Cosmic Research. doi.org/10.1134/s0010952518040032
3. Yang, K., Glazer, S.D., Thomson, SR., Feinberg, LD., Burt, W., Comber, BJ., Ousley, W., Franck, R. (2018). Thermal Model Performance for the James Webb Space Telescope OTIS Cryo-Vacuum Test. [Online] NASA. Available at: https://ntrs.nasa.gov/citations/20180004589
4. Holzlöhner, R., Aglaé Kellerer, Ulrich Lampater, Lewis, S., Zanoni, C. (2022). Structural, thermal, and optical performance analysis applied to subsystems of the European Extremely Large Telescope. Journal of Astronomical Telescopes, Instruments, and Systems. doi.org/10.1117/1.jatis.8.2.021504
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