As a by-product of high-precision, ultra-deep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly point-like. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mu_alpha = 3.56 +/- 0.04 mas/yr and mu_delta = -17.34 +/- 0.04 mas/yr. We note that the formal statistical errors quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way, and find that the cluster has made frequent passages through the Galactic disk.
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机译：作为使用哈勃太空望远镜在银河系球状星团NGC 6397中进行高精度，超深恒星测光的副产品，我们能够测量大量背景星系，其图像几乎是点状的。这些提供了史无前例的准确性的银河系参考系，相对于此，我们可以测量为球状星团确定的最准确的绝对固有运动。我们发现mu_alpha = 3.56 +/- 0.04 mas / yr和mu_delta = -17.34 +/- 0.04 mas / yr。我们注意到，为NGC 6397的正确运动而引用的形式统计误差并不包括可能不可避免的系统误差源，例如星团旋转。这些极不可能超过百分之几。我们使用这一新的适当运动来计算NGC 6397的UVW空间速度及其绕银河系的轨道，并发现星团已频繁通过银河系盘。
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