The Sun has long since set, but the Moon keeps its memory alive. In the moonlight, we traverse this short path up to the top of the mountain each night from one of the small houses where we stay for this 10-night astronomical observing stint. Tonight’s drive offers a clear reminder that the path to new knowledge is as winding and uncertain as the roadway to the heavens.

Most nights, clear and dark skies prevail, and the Dark Energy Camera (DECam) has a clean view to the objects in the sky it aims to see. The signals from stars and galaxies arrive unfettered, uninterrupted. Occasionally, however, clouds block light from the celestial sphere, the Moon outshines it and a turbulent atmosphere redirects it. What’s more, the atmosphere itself is constantly at work, emitting light from across the electromagnetic spectrum.

The atmosphere and Moon represent very important sources of noise when observing, especially when incredibly distant and faint objects are the intended targets: DECam is designed to see light from galaxies that are more than 15 billion light-years away. The goal is to get as much signal as possible, while minimizing the effects of all the noise sources, like those mentioned above.

Clouds, like those seen in this week’s image (20-second integration time), block light from stars and galaxies. Less light means less signal. On some nights, the Moon is too bright for the sensitive detectors in DECam, and we have to point the Blanco Telescope away from the moon. Some of the light from the Moon still bounces around the layers of the atmosphere and trickles into the Blanco field of view. Too much scattered light from the Moon or other sources adds to the noise and obscures the signal. Turbulence in the atmosphere deflects light from the objects we seek: multiple layers of air with different temperatures, moving at different speeds heavily disrupt light paths. Consider how the light of a straw is refracted when it goes into a glass of water. This happens in our atmosphere many many times over.

Over the years, astronomers, engineers and climate scientists have worked more and more closely to understand how weather and climate impact astronomical observations. While we’ve come quite far, and we will be able to do exquisite dark energy science at the Blanco, we know there is more road to pave.

Written by: Det. B. Nord [FNAL]
Image Credit: Det. B. Nord [FNAL]

As day gives way to night, our star plummets into the pacific. We refuel our brains for a night of work and then watch the sun scorch the horizon into darkness. This is our nightly ritual.

After dinner, our crew heads back to the telescope. Some of us take a car up the roads, while others make their way up the winding paths through the clay and dirt. Like clockwork, we pass a family of zorros (“foxes”), who often wait outside the kitchen for tasty scraps. There are more mouths to feed now: this past spring, a new litter of pups appeared. Occasionally, a few viscachas (rabbit-like rodents in the chinchilla family) graze on the rare sprig of fauna in the dry mountaintops and then rest on warm rocks in the fading sunlight.

The Dark Energy Survey (DES) observes during these summer months, and the community has priority access to the instrument during the remainder of the year. DES runs optimally during the dry summer (in the southern hemisphere, lasting from December to February) to avoid atmospheric water absorbing and scattering light from the higher-wavelength portions of the electromagnetic spectrum. We desperately need that light to see older, more distant cosmic structures.

On especially dry evenings, a green flash can be seen in the moments before the last of the sun falls below the horizon. Earth’s prismatic atmosphere scatters the suns rays and splits the light by color. As the sun drops, the spread-out spectrum rolls vertically across our eyes, quickly from red to orange and very very briefly through green.

Celestial objects that DES observes set just like the sun does; between the beginning of night and the time a galaxy has fallen below the horizon there is very little time—from minutes to hours.   If the Universe were a year old, humanity has existed for about 20 seconds.  We have but mere fractions of a moment to take snapshots of these galaxies and stars that have lived for millions and billions of years and that reside millions of light-years away. Utterly ephemeral, the green flash reminds us of the difficulty of our endeavor, of the challenge of catching light from billions of objects so distant in time and space from us.