Contactless sub-millimeter displacement measurements

摘要

Weather effects on foldable domes, as used at the DOT and GREGOR, are investigated, in particular the correlation betweenthe wind field and the stresses caused to both metal framework and tent clothing. Camera systems measure contactless thedisplacement of several dome points. The stresses follow from the measured deformation pattern. The cameras placednear the dome floor do not disturb telescope operations. In the set-ups of DOT and GREGOR, these cameras are up to 8meters away from the measured points and must be able to detect displacements of less than 0.1 mm. The cameras havea FireWire (IEEE1394) interface to eliminate the need for frame grabbers. Each camera captures 15 images of 640 x 480pixels per second. All data is processed on-site in real-time. In order to get the best estimate for the displacement within theconstraints of available processing power, all image processing is done in Fourier-space, with all convolution operationsbeing pre-computed once. A sub-pixel estimate of the peak of the correlation function is made. This enables to process theimages of four cameras using only one commodity PC with a dual-core processor, and achieve an effective sensitivity ofup to 0.01 mm. The deformation measurements are well correlated to the simultaneous wind measurements. The resultsare of high interest to upscaling the dome design (ELTs and solar telescopes).