Vishnu Anand Muruganandan, University of Canterbury; Andrew Lambert, UNSW; Jake Liu, University of Canterbury; Richard Clare, University of Canterbury; Steve Weddell, University of Canterbury
Keywords: Adaptive optics, natural stars, atmospheric turbulence, tip-tilt correction system, partially resolved imaging and image metrics for extended object.
Abstract:
Artificial satellite images captured by ground-based telescopes are distorted due to atmospheric turbulence and motion blur. The major constituents of distortion are tilt aberrations and the solution to partially improve the resolution is to develop a tilt mirror control system on ground-based telescopes. A real-time tilt mirror control system measures and corrects for tip-tilt aberrations in optical wavefronts. An adaptive optics tip and tilt correction system was implemented on the Boller & Chivens optical telescope (aperture: 61 cm) at the University of Canterbury Mount John Observatory (UCMJO), which enhanced the Full Width at Half Maximum (FWHM) by 16.7% with good seeing, and 6.1% when operating under poor seeing conditions.
The closed-loop tip-tilt correction system contains Andor iXon and FLIR Grasshopper camera, a piezo-electric (tip-tilt) mirror, and a controller driver from Physik Instrumente. The iXon is a high-sensitivity camera, which images the background faint stars with a limiting magnitude ranging from 4 to 9. The grasshopper is a high-speed camera that images artificial satellites in the foreground with a visible magnitude of up to 5. The light from the telescope is proportioned for each camera using the equally distributed beam splitter and re-focused on the image plane. The iXon, tilt mirror, and control driver are in a closed-loop system. The Grasshopper and tilt mirror is an open-loop system.
The telescope is operated at a sidereal rate and pointed to a background star that has the expected close proximity to the trajectory of the satellite. Image processing performed in the iXon estimates the centroid displacement of the star due to the atmospheric turbulence and the displaced centroid coordinates in two axes are fedback to the controller and driver, which actuates the mirror to compensate the tip-tilt aberration. While the tilt mirror in a closed loop system compensates the atmospheric distortion, the high-speed grasshopper camera simultaneously captures images of a partially resolved artificial satellite in real-time when the satellite is in the Field of View.
The real-time tilt correction system is applied on the sky, and the International Space Station is imaged and partially resolved using a background star. The grasshopper requires a short exposure of 0.2 to 0.4 ms to image the artificial satellite in Low Earth Orbit without motion blur. This imposes a limitation on our hardware, the current update rate of the tip-tilt correction system is 50 Hz. In this paper, the methodology, operations, and results of our real-time tip-tilt correction system using a natural star are detailed, and possibilities to enhance the update rate of the system are discussed.
Date of Conference: September 19-22, 2023
Track: SDA Systems & Instrumentation