Automated searching of Stardust interstellar foils

The Al foils lining the aerogel tiles of the Stardust interstellar tray represent approximately 13% of the total collecting area, about 15,300mm ². Although the flux is poorly constrained, fewer than 100 impacts are expected in all the Al foils on the collector, and most of these are likely to be less than 1μm in diameter. Secondary electron (SE) images of the foils at a resolution of approximately 50nm per pixel are being collected during the Stardust Interstellar Preliminary Examination, resulting in more than two million images that will eventually need to be searched for impact craters. The unknown and complicated nature of 3-dimensional interstellar tracks in aerogel necessitated the use of a massively distributed human search to locate only a few interstellar tracks. The 2-dimensional nature of the SE images makes the problem of searching for craters tractable for algorithmic approaches. Using templates of craters from cometary impacts into Stardust foils, we present a computer algorithm for the identification of impact craters in the Stardust interstellar foils using normalized cross-correlation and template matching. We address the speed, sensitivity, and false-positive rate of the algorithm. The search algorithm can be adapted for use in other applications. The program is freely available for download at http://jake.ssl.berkeley.edu:8000/groups/westphalgroup/wiki/14e52/ISPE_SEM_Crater_Search.html.