局地大气温度梯度对环形拼接望远镜主动控制中光学测量的影响

Effects of a Local Atmospheric Temperature Gradient on Optical Measurement for the Active Control of a Segmented-Ring Telescope

环形拼接望远镜是一种独特的大型拼接望远镜方案。由于其主镜具有全空间频率覆盖能力,环形望远镜拥有完备的干涉成像能力,因此它也被称做环形干涉望远镜。环形主镜中空的特点使它需要额外倾斜测量才能实现主动控制的闭环。为了实现环形望远镜在可见光波段的衍射极限成像,主动控制对倾斜测量的精度提出了极高的要求。为此,一种基于内参考光源Shack-Hartmann波前传感器的倾斜探测器被提出。光学测量具有极高的精度,但对环境干扰极为敏感。其中局地大气的温度梯度引起的折射率梯度是最重要的误差源之一,它能够导致测量的系统误差。这个误差在主镜上表现为各种低阶像差,严重降低了环形望远镜的成像质量。从梯度介质中光线传播方程出发,理论上建立了温度梯度,望远镜姿态与测量误差间的关系。结果发现当假设测量光路中的温度梯度与地面垂直时,该系统误差具有显著的各向异性的特点。根据上述函数关系,可以通过环境温度的测量和控制对该误差的补偿和抑制。结果显示,对于30 m口径的极大环形拼接望远镜,为了满足衍射极限成像对面形的要求,测量光路中的温度梯度需控制在0.003 K/m以内。而采取通过测温进行误差补偿时,光路中需要布置多个温度探头,且测量的精度要在0.1 K以下。

A proposed segmented-ring telescope is a unique solution for giant telescopes. Because a ring configuration can cover baselines smaller than its outer diameter in all directions, such a telescope can have a complete interferomometry imaging capability, and it is named as a Ring Interferomety Telescope. Due to the absence of reflection in the circular region enclosed by the inner rim of the ring, additional tip-tilt sensing is needed for the closed-loop active control of the telescope. For realizing diffraction-limited imaging in the visible wavelength range, tip-tilt sensing of extremely high accuracies is needed for the active control of the system. We hereby propose an optical-metrology method using classical Shack-Hartmann wavefront sensors and reference light sources inside the telescope for accurate tip-tilt sensing. Theoretically the proposed method has a very high accuracy, but it is very sensitive to environmental disturbance. The refractive-index gradient caused by a local atmospheric temperature gradient is among the most important error sources of the method. It contributes to the systematic measurement error by causing low-order aberrations on the primary-mirror surface. Starting from the equation of optical path in non-uniform media, we derived the theoretical dependence of the systematic measurement error on the temperature gradient and the attitude of the telescope. We have found that theoretically the error is anisotropic if the temperature gradient is in the vertical direction. Using the theoretical analysis we propose a method of compensating or reducing the error by controlling the environmental temperature. Our results show that for an 30m Ring Interferometry Telescope to achieve diffraction-limited imaging by using the error compensating/reducing method, the temperature gradient must be controlled within 0,003K/m and the precision of temperature measurement needed for the control must be better than 0.1 K.