Cluster and Parallel Fields: Two for the Price of One

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.

A picture of the galaxy cluster field and the parallel field

This image illustrates the “footprints” of the Wide Field Camera 3 (WFC3) infrared detector, in red, and the visible-light Advanced Camera for Surveys (ACS), in blue. An instrument’s footprint is the area on the sky it can observe in one pointing. These adjacent observations are taken in tandem. In six months, the cameras will swap places, with each observing the other’s previous location.

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.

Light paths from fields to Hubble instruments

This diagram shows the light paths that originate with the galaxy cluster field and the neighboring parallel field. The light from the galaxy cluster field (red) is imaged with the Hubble’s Wide Field Camera 3 (WFC3) infrared detector, while the light from the parallel field (blue) is imaged with the visible-light Advanced Camera for Surveys (ACS). Hubble’s entire field of view is shown on the left side of the diagram. It includes the “footprints” of ACS (red) and WFC3 (blue), as well as those of the fine guidance sensors (FGSs), which are the three, white wedges on the outside, and everything in between them.

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.

Detailed view of galaxy cluster Abell 2744

This diagram shows a detailed view of galaxy cluster Abell 2744, one of two massive galaxy clusters being imaged in the first year of Frontier Fields.

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.

Hubble's field of view and the footprints of its instruments

This illustration shows the “footprints” of all the instruments in Hubble’s field of view. These include the fine guidance sensors (FGSs), the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), the Space Telescope Imaging Spectrograph (STIS), the Cosmic Origins Spectrograph (COS), the Wide Field Camera 3 (WFC3), and the Advanced Camera for Surveys (ACS), which includes the Solar Blind Channel (SBC). WFC3 and ACS are the two instruments involved in the Frontier Fields program.