I’m an astronomer working at the University of Hawaii’s Institute for Astronomy.  My main areas of research are astronomical instrumentation in the area of adaptive optics and atmospheric optical turbulence.

UH88ASM – A new adaptive secondary mirror for the University of Hawaii 2.2-meter telescope Why?

• Including the wavefront correction of an adaptive optics system in the telescope optics has two key advantages over more conventional approaches.   First, placing the adaptive element on either the primary or secondary mirrors maintains the thermal emissivity of the telescope.  This is particularly important for observations at thermal infrared wavelengths (e.g. > ~2 microns) where the thermal background is the dominant background source.   A conventional adaptive optics system must enclose, cool, and maintain the optical relay to avoid introducing additional background.   Second, with the advent of adaptive optics systems that provide larger fields of view, including the adaptive element in the telescope optics simplifies the optical design/implementation.   In some cases (e.g. ground-layer adaptive optics) the optical design/fabrication become the limiting factor for the end performance.
• Adaptive secondary mirrors exist on several telescopes.   However, their performance, cost, and complexity is rooted in the technology deployed for their actuators.   In particular, the choice of voice coil actuators (~1N force) leads to thin optical shells, a tight internal control servo/sensor to position the actuators, and a large amount of thermal energy dissipated at the secondary.   These lead to reliability and scaling issues which make large systems challenging and expensive.

What?

• Our NSF ATI funded (AST-1910552) project uses an innovative hybrid variable reluctance (HVR) actuator technology developed by TNO which provides a 10x gain in the force output and a 40x increase in electrical efficiency.   The larger force output is used to make the thin optical shell thicker and hence fragile while not sacrificing dynamic range/stroke of the system.   The thickness of the shell is about 3.5mm thick (very similar to your car windshield thickness).  The higher force output also makes the response of the system linear which negates the need for a separate sensor/control servo to set the position of the actuator.   This, combined with the increase in electrical efficiency of the variable reluctance actuators, reduces the thermal energy dissipated near the secondary by more than an order of magnitude.
• The goal of this project is to demonstrate the approach by deploying an adaptive secondary mirror on the University of Hawaii 2.2-meter telescope.   This 50 year old telescope is an ideal test bed with its ability to mount/use either of two secondary mirrors and its Rayleigh laser guide star AO system (RoboAO) and its wide-field ground-layer adaptive optics system (`imaka).

NASA IRTF-ASM-1 : the first HVR-actuator based ASM is NASA IRTF-ASM-1

• 36 TNO HVR “UH88-style” actuators, 245mm diameter hyperbolic shell 3.3mm thick.
• Delivered to Hawaii Feb 2024 to be integrated with the UH AO wavefront sensor and real-time control system
• On IRTF, closed-loop in SCAO mode on the first night of our first engineering run (April 2024)

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