Differential Image Motion Monitor (DIMM) Tower

The challenge to viewing and imaging celestial objects from Earth’s surface is that Earth’s atmosphere distorts light from space. When you look up at the stars and see them twinkle, you’re experiencing this phenomenon; light from stars is (generally) constant, but the light that reaches your eyes has been pushed around by turbulence in Earth’s atmosphere. That turbulence is caused by the interaction of varying temperature and density layers between Earth’s surface and space. Twinkling stars might be pretty to look at, but they’re pretty annoying for scientists who want a crisp, clear view of the objects they’re studying.

While it’s not possible to eliminate the effects of Earth’s atmosphere on the images taken by a ground-based, wide-area survey telescope like Rubin Observatory, there are methods to help us predict and understand those effects. Near the main Rubin Observatory building on the summit of Cerro Pachón stands a white tower, and on that tower we've mounted an instrument called a Differential Image Motion Monitor, or DIMM. The DIMM will operate at night when Rubin Observatory is observing, monitoring turbulence in the atmosphere above the telescope. Data from the DIMM will be sent to Rubin Observatory’s Engineering and Facility Database, where it will be integrated with other data from Rubin Observatory facility-monitoring systems. Over time, information collected by the DIMM and other environmental sensors around the facility will be used to develop a computational fluid dynamics model of the atmosphere. By taking data on different points in the sky throughout the night we can determine, with increasing accuracy as we continue to refine the model, how the air above the summit is behaving. Another, portable DIMM will also be used to measure the turbulence from different locations around the immediate area of the telescope. These data in particular will help determine how the topography of the summit area influences atmospheric turbulence, and will inform the Rubin Observatory team on how to configure the telescope to optimize Rubin Observatory image quality in various environmental conditions.

The two DIMMs used at Rubin Observatory were manufactured in Germany, and are exact replicas of each other. When they arrived in Chile, they were tested together to confirm that they both perform identically. The DIMMs and Rubin Observatory operate independently of each other; the DIMM takes its data using stars that will be too bright for Rubin Observatory to image, and the DIMM has much less sky coverage than Rubin Observatory. Compared to the Rubin Observatory telescope and camera, the DIMM is a very simple instrument. But the purpose it serves is important: helping us better collect, understand, and interpret the data produced by Rubin Observatory.