While the Messier catalogue is used by amateur astronomers as a list of deep-sky objects for observation, Moore noted that Messier's list was not compiled for that purpose and excluded many of the sky's brightest deep-sky objects,[1] such as the Hyades, the Double Cluster (NGC 869 and NGC 884), and the Sculptor Galaxy (NGC 253). The Messier catalogue was actually compiled as a list of known objects that might be confused with comets. Moore also observed that since Messier compiled his list from observations in Paris, it did not include bright deep-sky objects visible in the Southern Hemisphere, such as Omega Centauri, Centaurus A, the Jewel Box, and 47 Tucanae.[1][2] Moore compiled a list of 109 objects to match the commonly accepted number of Messier objects (he excluded M110[3]), and the list was published in Sky & Telescope in December 1995.[3]
Moore used his other surname – Caldwell – to name the list, since the initial of "Moore" is already used for the Messier catalogue.[1][4] Entries in the catalogue are designated with a "C" and the catalogue number (1 to 109).
Unlike objects in the Messier catalogue, which are listed roughly in the order of discovery by Messier and his colleagues,[5] the Caldwell catalogue is ordered by declination, with C1 being the most northerly and C109 being the most southerly,[1] although two objects (NGC 4244 and the Hyades) are listed out of sequence.[1] Other errors in the original list have since been corrected: it incorrectly identified the S Norma Cluster (NGC 6087) as NGC 6067 and incorrectly labelled the Lambda Centauri Cluster (IC 2944) as the Gamma Centauri Cluster.[1]
The Caldwell Catalogue has generated controversy in the amateur astronomy community for several reasons.[6]
Moore did not discover any of the objects in his catalogue which are often very well known objects and not 'neglected' as claimed by Moore.
Its presentation as a catalogue with distinct designations, rather than a list, potentially may cause confusion amongst amateur astronomers as the 'C' Designation is not commonly used.
The list was promoted as an extension of the Messier Catalogue, however the objects are often arbitrary with many easily viewable objects omitted while some objects not readily available to visual observers are included.
Caldwell advocates, however, see the catalog as a useful list of some of the brightest and best known non-Messier deep-sky objects. Thus, advocates dismiss any "controversy" as being fabricated by older amateurs simply not able or willing to memorize the new designations despite every telescope database using the Caldwell IDs as the primary designation for over 25 years. NASA/Hubble also lists the 109 objects by their Caldwell number.
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