The Dark Energy Survey, in its search for distant cosmic secrets, needs many nights of clear sky. Unfortunately, on this night, a river of clouds flowed overhead. We often look so far away to the faintest objects, that a wisping of cottony clouds moving over the Blanco telescope requires extra attention. In these cases, we might change our observing strategy to look at parts of the sky that require less detail.
As we sift through the tons of sediment and the terabytes of data, occasionally the clouds get so heavy and consuming that we must close the telescope dome to preserve the instrument. In these cases, we still use the time wisely: our Dark Energy Calibrations (DECal) team has developed a method to precisely measure and characterize the flow of light through the telescope at every relevant wavelength, and to monitor for any changes in that flow over the years of the survey.
As we fish for light in the sky, we cast a broad net. In the river of dreams, we sift through sediment for celestial gold.