IC List of Deep-Sky Objects
In the annals of astronomy, numerous catalogs have been created over the centuries to aid observers in their exploration of the night sky. Among these, the Index Catalogue (IC), which supplements the renowned New General Catalogue (NGC), stands out for its depth and comprehensiveness. The Index Catalogue contains 5,386* celestial objects, making it a substantial addition to the NGC. These objects, like those of the NGC, include a variety of galaxies, nebulae, and star clusters. In practice, the two catalogues are often used together, and objects are referenced by their IC or NGC number as appropriate.
* Although there are 5,386 objects in the IC, there are a number of multi-part and duplicate ojects, which brings the number of unique IC entries in the database to over 5,500.
The Index Catalogue was published in two parts by Danish astronomer John Louis Emil Dreyer, the first in 1895 and the second in 1908. The purpose of the IC was to supplement the NGC with new discoveries and corrections to the original catalogue. Dreyer was meticulous in his approach, sifting through astronomical observations from around the world and integrating new deep-sky objects into the Index Catalogue.
Though the NGC is perhaps more commonly cited, the Index Catalogue is integral to any comprehensive study or observation of deep-sky objects. The IC includes a wealth of objects not included in the NGC and corrected many errors and inaccuracies present in the original NGC. The IC entries have been widely incorporated into the databases used by digital astronomy tools and software.