Arcfinder: An algorithm for the automatic detection of gravitational arcs

We present an efficient algorithm designed for and capable of detecting elongated, thin features such as lines and curves in astronomical images, and its application to the automatic detection of gravitational arcs. The algorithm is sufficiently robust to detect such features even if their surface brightness is near the pixel noise in the image, yet the amount of spurious detections is low. The algorithm subdivides the image into a grid of overlapping cells which are iteratively shifted towards a local centre of brightness in their immediate neighbourhood. It then computes the ellipticity for each cell, and combines cells with correlated ellipticities into objects. These are combined to graphs in a next step, which are then further processed to determine properties of the detected objects. We demonstrate the operation and the efficiency of the algorithm applying it to HST images of galaxy clusters known to contain gravitational arcs. The algorithm completes the analysis of an image with 3000x3000 pixels in about 4 seconds on an ordinary desktop PC. We discuss further applications, the method's remaining problems and possible approaches to their solution.Comment: 12 pages, 12 figure

Location Is (Not) Everything: Re-Assessing Shanghai’s Rise, 1840s -1860s

While Shanghai’s pre-war history (1842-1937) is thought to be already ‘quite well understood’ thanks to an inordinately large number of studies, other Chinese urban centres received less attention. Consequently, a number of Western scholars have recently shifted their gaze elsewhere in search of other Chinese articulations of modernity. Yet a thorough examination of the history of other Chinese cities cannot replace a continual robust engagement with Shanghai. This is not least because the vast array of materials available at the Shanghai Municipal Archives and Zikawei Library has been systematically catalogued only over the last few years. They are indispensable to understanding the city’s rise to prominence and its preponderant position within China’s economic modernisation process. Mainly drawing on rare early editions of the North-China Herald held at the Zikawei Library, this article highlights one aspect of Shanghai’s early treaty-port development; it reprises the conventional wisdom positing that, because of its perceived advantageous location, Shanghai had been almost deterministically poised to become China’s gateway to the outside world following the First Opium War (1839-1842). Instead, it argues that location was significant but not sufficient of itself in delivering Shanghai’s economic take-off