English: Like many avid stargazers, an Englishman named Sir Patrick Moore developed an interest in astronomy at a very young age, which blossomed into a lifelong obsession. He became a prolific amateur astronomer and writer, gaining prominence as the presenter of the BBC documentary series The Sky at Night and earning knighthood in 2001 for “services to the popularization of science and to broadcasting.” In 1982, he came up with a complementary list to the popular Messier catalog of night-sky objects to entice fellow amateur astronomers to experience more of the wonders of the night sky.

Since Messier objects are designated using the letter “M” and their number in the catalog, Moore decided not to use his last name for his catalog to avoid confusion. Instead, he called it the Caldwell catalog, applying a surname that was used in previous generations of his family. Therefore, the objects in Moore's catalog are designated with the letter “C” and a number.

Unlike the Messier catalog, which only features objects that were visible from astronomer Charles Messier’s viewing location in Europe, the Caldwell catalog includes celestial bodies that are found in both the northern and southern skies. The catalog consists of 46 star clusters, 35 galaxies and 28 nebulas, adding up to 109 objects, similar to the Messier catalog. Moore intentionally avoided including any of the Messier objects in his catalog, hoping to expand his fellow amateur astronomers’ cosmic horizons. From nearby clouds of gas and dust that are left over from dying stars to remote galaxies that formed billions of years ago, the Caldwell catalog is brimming with surprising celestial treats.

While the Hubble Space Telescope has not taken images of every object in the Caldwell catalog, it has observed 95 of them as of late 2019. Processed images for 56 Caldwell objects are included here. More images will be added to Hubble’s catalog of Caldwell objects in the future.

Some of Hubble’s images are close-ups of a particularly interesting region of an object rather than capturing the whole thing. That’s because Hubble provides high-resolution views but of relatively small regions of the sky. Sometimes the entire astronomical object doesn’t fit in Hubble’s view, and the scientists taking the observations don’t always need to view the full object for their studies.

Other Hubble images of the Caldwell objects have an unusual staircase-like shape where an edge of the picture appears cut off or missing. These images were taken using the Wide Field and Planetary Camera 2 (WFPC2), which was in operation between 1994 and 2009. WFPC2 was made up of four light detectors with overlapping fields of view, one of which gave a higher magnification than the other three. When the four images were combined together into one picture, the high-magnification image needed to be reduced in size in order for the image to align properly. This produced an image with a layout that looked like steps.

Today, Hubble has two primary cameras to capture images of the cosmos. Called the Advanced Camera for Surveys (ACS) and the Wide Field Camera 3 (WFC3), they work together to provide superb wide-field imaging over a broad range of wavelengths. The telescope’s visible-light observations allow us to view cosmic objects in the wavelengths of light we see with our own eyes, but in a much greater level of detail. Infrared observations extend our vision, detecting lower-energy light than our eyes can see and peering through shrouds of dust to image some of the faintest and farthest objects yet discovered. Hubble’s ultraviolet vision extends our view in the opposite direction, opening a window on the evolving universe and allowing us to glimpse some of the more violent events in the cosmos.

With its advanced capabilities, Hubble has transformed our understanding of the Caldwell objects, providing scientists with key insights about the nature of the universe. We have learned more about what happens when stars die and have seen what happens when entire galaxies collide and merge. Hubble has allowed scientists to trace the growth of galaxies and stars, forming a more complete picture of our evolving universe.

While Hubble provides images in exquisite detail, the Caldwell objects can be observed using modest ground-based telescopes, though some are more challenging targets than others. The catalog features many deep-sky objects that are bright enough to be seen with binoculars and a few that are visible to the naked eye. Regardless of the viewing instrument, the Caldwell objects are rich in history, brimming with science and fun to observe.

The following are some of Hubble’s best images of the Caldwell objects taken thus far. This collection has been assembled for amateur astronomers to compare what they see to what Hubble sees, allowing them to see finer details in each of the objects.

For Hubble's Caldwell catalog site and information on how to find these objects in the night sky, visit:

www.nasa.gov/content/goddard/hubble-s-caldwell-catalog

Credit: South Downs Planetarium (creativecommons.org/licenses/by-sa/3.0/deed.en)