超大型拼接CCD相机焦平面平坦度的测量
Measuring the flatness of focal plane for very large mosaic CCD camera
摘要：
大型拼接多CCD相机是现代数字巡天的关键设备。DECam是一个非常红敏感的520万像素相机设计的传入暗能量调查（DES）。它由62个4k $\times$ 2k和12个2k x 2k 250微米厚的全耗尽CCD组成，焦平面直径为44厘米，视场为2.2平方度。它将连接到CTIO的布兰科4米望远镜。从2011年开始，DES将在5年内覆盖5000平方度的南银河帽的5个彩色波段（g，r，i，z，Y）。为了实现限制暗能量演化的科学目标，对DECam的设计提出了严格的要求。其中，焦平面的平坦度需要控制在60微米的包络内，才能达到规定的PSF变化极限。在173 K的高真空杜瓦瓶中，焦平面的平面度要达到这样的精度是非常具有挑战性的。我们开发了两种基于图像的技术来测量焦平面的平坦度。通过在焦平面上成像一个规则的点网格，CCD沿光轴的偏移量被转换为焦平面上不同位置处的网格间距的变化。在提取图案并比较间距的变化之后，我们可以高精度地测量平面度。在方法1中，规则点保持在高亚微米精度，并覆盖整个焦平面。在方法2中，对网格的精度要求不高。取而代之的是，我们使用一个精确的XY平台在整个焦平面上移动图案，并比较不同CCD成像时的间距变化。模拟和实际测量结果表明，这两种方法都能很好地实现我们的目的，并且与直接光学测量结果吻合良好。
英文摘要：
Large mosaic multiCCD camera is the key instrument for modern digital sky survey. DECam is an extremely red sensitive 520 Megapixel camera designed for the incoming Dark Energy Survey (DES). It is consist of sixty two 4k$\times$2k and twelve 2k x 2k 250-micron thick fully-depleted CCDs, with a focal plane of 44 cm in diameter and a field of view of 2.2 square degree. It will be attached to the Blanco 4-meter telescope at CTIO. The DES will cover 5000 square-degrees of the southern galactic cap in 5 color bands (g, r, i, z, Y) in 5 years starting from 2011. To achieve the science goal of constraining the Dark Energy evolution, stringent requirements are laid down for the design of DECam. Among them, the flatness of the focal plane needs to be controlled within a 60-micron envelope in order to achieve the specified PSF variation limit. It is very challenging to measure the flatness of the focal plane to such precision when it is placed in a high vacuum dewar at 173 K. We developed two image based techniques to measure the flatness of the focal plane. By imaging a regular grid of dots on the focal plane, the CCD offset along the optical axis is converted to the variation the grid spacings at different positions on the focal plane. After extracting the patterns and comparing the change in spacings, we can measure the flatness to high precision. In method 1, the regular dots are kept in high sub micron precision and cover the whole focal plane. In method 2, no high precision for the grid is required. Instead, we use a precise XY stage moves the pattern across the whole focal plane and comparing the variations of the spacing when it is imaged by different CCDs. Simulation and real measurements show that the two methods work very well for our purpose, and are in good agreement with the direct optical measurements.
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