Practical atmospheric transparency measurements from all-sky images

Abstract

Our experience operating the telescopes of Las Cumbres Observatory (LCOGT) has shown us the utility of using the attenuation of starlight to determine the transparency of the atmosphere over a telescope site. To measure the attenuation, we have applied an algorithm developed by the IACT group at Technische Universität Dortmund to images from all-sky cameras. The algorithm compares a smoothed visibility map, based on the stars extracted from the image, against an ideal visibility map, created from a catalog of stars for the same time and location. The result of the comparison is a cloud map, which we convert into a value for transparency by multiplying by a weighting scheme with three concentric zones. Because the telescopes are usually pointed to high altitudes (low airmasses), pixels within 51 deg of the zenith receive full weight. The final transparencies are used to determine when the sky conditions are satisfactory for photometry (and the domes should be open) and when they are unsatisfactory (and the domes should be closed). We completed the rollout of the transparency algorithm to all LCOGT sites in April 2025, and since then, observations in cloudy skies have been reduced. We recommend that operations personnel at other observatories where all-sky cameras are part of the weather apparatus use and adapt the algorithm, which we provide open source in a Github repository.