Hubble is doing double-duty as it peers into the distant universe to observe the Frontier Fields. While one of the telescope’s cameras looks at a massive cluster of galaxies, another camera will simultaneously view an adjacent patch of sky. This second region is called a “parallel field”—a seemingly sparse portion of sky that will provide a deep look into the early universe.
Many people are familiar with Hubble’s deep field images, where the telescope stared at what appeared to be relatively empty areas of the sky for long periods of time. Instead of a vast sea of blackness, what astronomers saw in these long exposures were thousands upon thousands of galaxies of all shapes and sizes.
But these deep fields covered just a small fraction of the area of the full moon on the sky. Do they really reflect what our universe looks like, or are these unusual regions? The truth is, astronomers just don’t know. That’s why they are adding to their knowledge by studying the six “parallel” deep fields in various locations across the sky. Whether or not they find similar galaxy-rich regions, they will learn something interesting about our universe.
Hubble simultaneously uses the Advanced Camera for Surveys (ACS) for visible-light imaging, and Wide Field Camera 3 (WFC3) for its infrared vision. So the infrared camera could observe the cluster while the visible-light camera focuses on the parallel field.
Six months later, the Earth will be at the opposite side of the sky in its yearly orbit around the Sun. The telescope, which is powered by solar arrays, has also pivoted 180 degrees to achieve the optimal orientation to illuminate the arrays. This opposite rotation means the cameras will effectively “swap places,” with each camera now observing the other’s previous location. Now the visible-light camera views the cluster while the infrared camera images the parallel field.
For each of the six cluster and six parallel fields, astronomers will have both infrared and visible-light observations. This will allow them to create more detailed, overlapping and complete images.
Over three years, 840 orbits will be devoted to these 12 fields. That’s about 2 million seconds of Hubble time. These data are taken in 15-20 minute exposures, and they come down to the ground as digital files. These images are then stitched together to create mosaics. The resulting views will give us new insight into the early universe.