Magnetic fluid deformable mirror for correcting off-axis aberrations of liquid mirror telescope

摘要

Liquid mirror telescope (LMT) is a viable kind of telescope with a thin rotating mercury layer, which can generate excellent parabolic surface under the constant pull of gravity and a centrifugal acceleration. The cost of LMT is normally smaller than 1% of the cost of a traditional glass mirror telescope. However, the LMT cannot be tilted to observe larger field of sky due to the liquid property so that the field of view of LMT is very limit. In order to observe a larger field of the sky with LMT, the large off-axis aberrations must be compensated. Since the aberrations of a parabolic mirror increase rapidly with the increase of field angle, the classical correctors used in adaptive optics (AO) systems cannot correct the large off-axis aberrations of LMT when the field angles are significantly greater than 1 degree. In this paper, the magnetic fluid deformable mirror (MFDM) has been proposed as a new perspective to wavefront correction technology, which could produce a large stroke to compensate the large off-axis aberrations of LMT. The designed MFDM has a radius of 95 mm with 1141 actuators, which is able to correct the large off-axis aberrations of a 3-m f/4 LMT and permits the LMT to be operated at 10 degree off-axis observation from the zenith. The type and the order of off-axis aberrations generated by the liquid mirror telescope are first studied analytically and then the 3-m f/4 LMT operated at 10 degree off-axis observation is simulated in ZEMAX, where the off-axis aberrations and the Zernike coefficients of those aberrations are obtained from the simulation result. The required surface shape of MFDM can be calculated from the obtained Zernike coefficients of off-axis aberrations. Since the shape of the magnetic fluid surface is controlled by the combined magnetic field generated by a Maxwell coil and an array of micro-electromagnetic coils, the Maxwell coil and micro-electromagnetic coils are hence optimally designed to generate the required magnetic field. The correction performance of MFDM for the large off-axis aberrations of LMT is finally co-simulated by MATLAB, COMSOL and ZEMAX software. The simulation results show that the off-axis aberrations can be compensated with the designed MFDM and the Zernike coefficients of wavefront are substantially reduced.