High Resolution Visible Light Imaging of Large Aperture Telescopes

The deformable mirror used in adaptive optics can instantly correct the static wavefront aberrations and atmospheric turbulence wavefront disturbances of the optical system by changing its surface. This enables the optical system to automatically adapt to changes in the environment and maintain optimal performance. It is widely used in high-resolution astronomical observations, laser atmospheric transmission, and biomedical imaging. Traditional astronomical adaptive optical systems are usually installed on a platform independent of telescopes, mainly composed of special deformable mirrors, tilt mirrors, wavefront sensors, and relay optical components. Due to the presence of a large number of optical components and the long optical path, the system has problems such as large volume, large static aberration, and low light energy utilization. Therefore, this architecture is not conducive to measuring and correcting the wavefront of weak stars at high spatial and temporal frequencies.

(a) Sketch of PDSM-241. (b) Actuator layout (light aperture: 270mm). (c) The self correcting aberration of PDSM-241. Image source: Opto Electronic Advances (2023). DOI: 10.29026/oa.2023.230039

Deformable secondary mirror (DSM), which refers to the transformation of a telescope's secondary mirror into a deformable mirror for wavefront correction, was first proposed by American astronomer Beckers as a means of addressing these defects. This concept enables deep integration of telescopes and adaptive optical systems. Subsequently, many well-known large aperture ground-based observatories such as MMT, LBT, Magellan, VLT, etc. have successfully utilized Voice Coil Deformable Secondary Mirror (VCDSM), demonstrating the feasibility of DSM technology. At the same time, the Institute of Optoelectronics Technology has initiated research on piezoelectric DSM (PDSM) technology. The researchers subsequently developed the first 73 unit PDSM prototype and successfully installed it on a 1.8 meter telescope for astronomical observation in 2016.

Practice has proven that PDSM technology is practical for astronomical observations. Compared to VCDSM, PDSM is more compact and does not require any additional cooling systems, internal control electronics, or actuator position sensors. This article introduces the new 241 unit PDSM developed by the Institute of Optoelectronics Technology and its application on the 1.8-meter adaptive telescope at the Lijiang Tianwen Observatory, supported by a key project of the National Natural Science Foundation of China. The PDSM-241 is equipped with a quartz reflector with a diameter of 320 millimeters and a light aperture of approximately 270 millimeters. It is driven by 241 piezoelectric actuators to change its surface for wavefront correction. The self corrected image difference of the PDSM-241 is approximately 10 nm.

The structure of the Lijiang 1.8-meter adaptive telescope adopts a combined wavefront correction device, which combines PDSM-241 with a six dimensional displacement station to achieve long range, high-precision tracking and high-order wavefront aberration correction. The main mirror of the 1.8 meter telescope reflects the distorted stellar beam due to atmospheric turbulence, and then corrects tilt and higher-order wavefront aberrations through PDSM-241 and a six dimensional displacement station. Finally, the third mirror reflects the beam of light onto the wavefront sensor and high-resolution imaging camera at the Nasmyth focal point. The Lijiang 1.8-meter adaptive telescope obtained high-resolution stellar images using efficient closed-loop correction of PDSM-241. The visible light R-band (center wavelength 640 nm) image is displayed, with an imaging resolution of 1.25 times the diffraction limit and an imaging Strehl ratio (SR) close to 0.5.

This research aims to meet the needs of high integration and resolution for large aperture optical telescopes, and has made remarkable progress in the development of high-performance piezoelectric deformable secondary mirrors and astronomical observation applications. This further simplifies the structure of large aperture high-resolution optical telescopes, improves imaging resolution, and has significant application value in astronomy. The research results have been published in the journal Opto Electronic Advances.

Source: China Optical Journal Network