Conjugate Plane Photometry: Reducing Scintillation Noise in Ground-Based Astronomical Photometry

闪烁是指我们观察到的星星的“闪烁”现象。它是由地球大气中的高空湍流的光学效应造成的，在任何给定的时间，这种效应都会将一部分星光转向地面的接收孔径(例如望远镜或眼睛)或从接收孔径(例如望远镜或眼睛)中转移出来。这会导致观测到的总光强的快速波动。对于大型望远镜，这些波动是在孔径范围内平均的，因此通过大型望远镜看到的闪烁程度比肉眼要小得多。然而，对于某些类型的望远镜观测，这些微小的残留强度波动是至关重要的：特别是对于快速测量快速变化的现象，例如从地球上看，当太阳系外行星从其母星后面经过时，测量到的光强度降低。这种系外行星日食造成的亮度下降通常很小(远低于0.1%)，因为这颗行星只贡献了系统总光线的一小部分。然而，对这种日食的准确观测可以提供关于这颗行星及其围绕母星的轨道的丰富信息。对于地面观测，闪烁噪声限制了测定日食“光曲线”的精确度，因此，到目前为止，最好的数据通常是通过天基望远镜获得的，高于地球大气的影响。在这里，我们建议开发一种新的技术，称为“共轭平面测光”，以有效地“不闪烁”地面望远镜的光度观测。在这种方法中，高空湍流层被聚焦到仪器光学系统内的圆形掩模上。掩模的孔径比望远镜本身的直径略小。这种面罩阻挡了靠近望远镜光圈边缘的那部分星光，这部分光可能被高湍流度转向进入或流出望远镜，导致闪烁。由于闪烁引起的强度波动的大小将大大减小，因此使用地面望远镜进行更准确的光度观测成为可能。我们提出的研究试图通过开发一个原型仪器并将其部署在大型望远镜上来展示共轭平面测光技术的全部潜力。这项工作的最终目标将是证明，可以从地面对系外行星日食等现象进行光度观测，其精度接近天基望远镜可能达到的水平。

Scintillation is the phenomenon that we observe as the 'twinkling' of stars. It results from the optical effect of high-altitude turbulence in the Earth's atmosphere which, at any given time, diverts a fraction of the starlight into or out of the receiving aperture (e.g. telescope or eye) at the ground. This results in a rapid fluctuation of the total light intensity observed. For a large telescope, these fluctuations are averaged over the aperture area, so that the degree of 'twinkling' seen through a large telescope is much less than for the naked eye. However, for some types of telescope observations, these small residual intensity fluctuations are of critical importance: in particular for high-speed measurements of rapidly changing phenomena, such as the reduction in the measured light intensity which results when an extra-solar planet passes behind its parent star, as seen from Earth. The dip in brightness resulting from such an exoplanet eclipse is usually very small (much less than 0.1%), since the planet contributes only a small fraction of the total light from the system. However, accurate observations of such eclipses can provide a wealth of information about the planet and its orbit around the parent star. For observations from the ground, scintillation noise limits the accuracy with which the eclipse 'light curve' can be determined so that, until now, the best data have typically been obtained with space-based telescopes, above the effects of the Earth's atmosphere. Here we propose to develop a new technique, known as 'Conjugate Plane Photometry' to effectively 'un-twinkle' photometric observations with ground-based telescopes. In this method, the high-altitude turbulent layer is focused onto a circular mask within the optical system of the instrument. The mask aperture is slightly smaller than the diameter of the telescope itself. This mask rejects that part of the starlight, close to the edge of the telescope aperture, that could have been diverted by the high turbulence into or out of the telescope, causing scintillation. The size of intensity fluctuations due to scintillation are then greatly reduced, so that more accurate photometric observations become possible with ground-based telescopes. Our proposed research seeks to demonstrate the full potential of the conjugate-plane photometry technique by developing a prototype instrument and deploying it on a large telescope. The ultimate goal of the work will be to demonstrate that photometric observations of phenomena such as exoplanet eclipses can be made from the ground with an accuracy approaching that which is possible with space-based telescopes.