Thermo-Optical Metrology for Exoplanet observatories

Exoplanet exploration requires a large attenuation of starlight to observe the planets around it. To do so, telescopes include occulters, also known as coronographs. The quality of the observations depends on both the telescope and the coronograph performance, and the wavefront errors must be kept exceptionally low over long time scales. This is so because the telescope and coronograph stabilities are crucial to maintain deep contrast over long time scales (up to days) such that dim Earth-like planets can be spectroscopically characterized. The wavefront stability requirement is 10-100 pm over time scales of minutes. Exquisite wavefront control is also necessary in future segmented space telescopes and it is expected that laser-interferometric truss systems will be able to provide the necessary resolution in displacement metrology to achieve demanding stabilities of 10 pm RMS over long integration times of thousands of seconds. The proposed project will focus on the demonstration of highly compact miniaturized laser-interferometric truss systems, capable of achieving sensitivities at pm and below over measurement time scales of seconds to hours. We will use a highly integrated quasi-monolithic optical head, and we will also develop a simplified data acquisition and processing system to readout each unit across the large segmented telescope. In addition, we will work the stability improvement of telescope structures and coronograph instruments by means of achieving thermal stability below the mK. The achievable thermal stability depends upon several variables such as the thermal stability of the environment, and the passive thermal shielding of the instrument. While passive shields can provide large attenuation at high-frequency, they become inefficient over long-time scales. Thus, thermally stabilized environments are achieved by an optimal combination of passive and active thermal disturbance rejection systems. The active control, as with any other feedback control, relies primarily on the sensor since it sets the ultimate stability limit. In thermal controls of optical systems such as telescopes or optical benches, the actuators are another crucial element since they are slow and uni-directional, i.e., in many cases an actuator can only apply heat but not extract it. Such limitations make the control asymmetric, and in many cases gain-limited due to the lack of bandwidth. In this project, we propose the development of i) a temperature measurement subsystem capable of measuring temperature fluctuations at the 1-10 micro-K/sqrt(Hz) for frequencies as low as 1 mHz (1,000 s), which corresponds to rms values of 30 nK for averaging time of 1,000 s; ii) a passive thermal shield optimized for telescopes required in future exoplanet exploration missions; iii) a multi-zone active thermal control to improve the low-frequency thermal stability, and iv) a detailed thermal model to infer temperature stability of points were thermal sensors are not present. The latter is a key point since temperature sensors cannot be deployed at every location in the instruments, and thus a thermal map, inferred from thermal modelling and measurements, is necessary. The team will be led by Drs. Felipe Guzman as PI and Moritz Mehmet as Co-I, and the Laboratory of Space Systems and Optomechanics at the University of Arizona, which has demonstrated expertise in the development of compact laser-interferometric systems for scientific space missions. In addition, Dr. Jose Sanjuan at Texas A&M University will serve as Co-I and will lead the development of the temperature measurement systems. Both Dr. Guzman and Dr. Sanjuan have ample experience in ultra-stable optical systems in highly stable thermal environments from their participation in the LISA Pathfinder, LISA, and GRACE follow-on missions, among other ongoing efforts requiring these types of measurement systems. Dr. Mehmet has extensive experience in the development of stable laser systems.

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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