The National Geographic Society-Palomar Sky Survey was completed in 1956. For seven years the 48-inch Schmidt telescope had surveyed the sky north of = -27°. The 1758 highest-quality plates that were finally accepted penetrated about three times deeper into space than any previous survey had ever reached. Astronomers are still studying and cataloguing the information contained in this survey, and will continue to do so for many years to come. One of the first astronomers to use the prints of the Sky Survey for a systematic study was Professor Vorontsov-Velyaminov of the Sternberg Astronomical Institute in Moscow (2). In 1959 he published positions, with copies of Sky Survey pictures, of 355 peculiar and interacting galaxies that he had discovered on those prints. The publication of this list enabled the undertaking of one kind of project for which the 48- and 200-inch telescopes on Palomar Mountain were originally designed. The fast-focal-ratio, wide-field Schmidt telescope was intended to survey objects of interest. The maximum light-gathering power and resolution of the 200-inch could then be turned individually on the most interesting objects.
When selected members of Vorontsov-Velyaminov's catalogue were photographed with the 200-inch, some turned out to be much more interesting than on the smaller-scale plates, while others turned out to be less interesting or ordinary. After some preliminary experience with the 200-inch scale, it soon became possible to inspect the Vorontsov-Velyaminov objects first on the Survey prints to cull out the less interesting objects. In the process of inspecting these objects and checking their positions, other very unusual galaxies were noticed on the same Survey prints and included in the 200-inch program. This demonstrated that not all the important objects had been catalogued, and efforts were made to compile from other sources a more complete list of candidates for peculiar galaxies. One additional source of peculiar-galaxy candidates was the set of notes which A.G. Wilson had made upon inspecting the original Sky Survey plates as they were taken. These were kindly put at my disposal. Another list of peculiar objects was given me by E. Herzog, who has carefully searched the Survey plates for such objects. Thornton Page contributed peculiar objects he knew and a list of peculiar galaxies which C.A. Wirtanen had compiled from the Lick Position Survey. Holmberg's pairs of galaxies were inspected. Special objects were also contributed by W.W. Morgan, F. Zwicky, Charles Kowal, and Gibson Reaves. Finally, the plates of Minkowski and Baade, which are stored at the Mount Wilson Observatory, were searched for peculiar objects. None of these lists, including my own, had very much overlap with each other. The conclusion seems to be that, aside from the brighter and therefore well-known peculiar galaxies, the fainter peculiars have not been fully catalogued, and that the fainter peculiar galaxies pictured in this Atlas represent only a sample of that group.
At first the photographs with the 200-inch were made with various plate and filter combinations to discover in which wavelengths the peculiar features would show best. Although red wavelengths sometimes showed features better, in general, the filaments, connections, and faint outer features were more conspicuous on blue-sensitive (Eastman Kodak 103a-O) plates. At that time, however, the sky was becoming so dark because of sunspot minimum, that it was possible to reach fainter limiting magnitudes by exposing blue plates for sixty to seventy minutes. To make the project possible in terms of available observing time, the band-pass was widened by using 103a-D plates and including the visual as well as blue wavelengths in a limiting exposure of the order of thirty minutes. Finally, it became clear that the night sky emission line at 5577 was contributing appreciably to the brightness of the night sky background, and the emulsion was changed to 103a-J from there until the conclusion of the project. The 103a-J plates registered light roughly between the 3600 half-transmission point of the f/3.67 corrector lens of the 200- inch telescope and the 5400 photographic emulsion cutoff. That, in general, is the region of maximum contrast for galaxies (10), and the very deep exposures made here (to densities of 0.7 to 1.2 for sky background), the very dark night skies, and the 20 percent increases in development time give, on the average, a set of photographs that show fainter stars - and particularly fainter surface brightness features - than previously detected in galaxy subjects. The reproduction of these prints in the Mount Wilson and Palomar photographic laboratory by William Miller was a difficult job which was carefully controlled so that almost all the original features on the plates, even the faintest, are reproduced in the Atlas.
Whenever possible, poor-seeing plates were repeated under better seeing conditions, so that the final Atlas contains only plates taken with seeing 2 or better. The star images on the plates taken with the 200-inch presented in this Atlas are therefore generally between 1" and 2" diameter. Search of the Observatory plate records located some of the prospective Atlas galaxies which had been already photographed. I am grateful to Zwicky, Sandage, and Baum for allowing me to reproduce some of the photographs of these objects, and they are credited under the listed plate numbers in Table 1. Most of the 338 photographs shown in the Atlas are from plates taken with the 200-inch telescope. Occasionally a very large object is shown in a print from a 48-inch telescope plate (designated PS) in order to emphasize its correct sequence in the order of forms.
Because so many of the physical processes pictured are not understood, no rigorous attempt at classification has been made. The galaxies have been grouped empirically, putting together all the objects that look alike. Special emphasis is on the form of the galaxies or the nature of the peculiarity and the gradual change of the peculiarity from object to object. Sometimes an object will belong in more than one category, and then it is cross-referenced in Table 1 or shown under different magnification in different sections of the Atlas. The schematic plan of arrangement of the different kinds of galaxies is shown in Diagram 1. The largest class involved peculiar spiral galaxies (Nos. 1 - 102). The largest subclass of peculiar spirals are spirals with companions attached to spiral arms (Nos. 102-145. Of course, there is overlap, and in the very interesting group ranging from Nos. 91-114 it is impossible to say whether the E is attached to the spiral galaxy or vice versa. The third major group (Nos. 146-268) involves galaxies or groups of objects that are not primarily classifiable as either E's or spirals, or whose most outstanding peculiarity does not fall in the first two major categories. In the fourth major category (Nos. 269-327), group character is the most important consideration. Six objects classifiable only as miscellaneous are shown at the end (Nos. 332-338).
When possible, information has been referenced in the literature regarding magnitude, redshift velocities, and any known spectral peculiarities. Table 2 lists all the objects in this Atlas in order of right ascension and gives known redshift velocities. In Table 3 coincidences of Atlas objects with catalogued radio sources are noted and referenced. With the exception of bright radio sources such as Fornax A, Atlas objects were not selected because they were radio sources although Minkowski's plates were generally taken in search of radio source identifications. In many cases, however, nothing more is known about an object than what is shown in the Atlas. An important task in the future will be to undertake photometric and spectroscopic observations of these objects. Then, when distances, absolute magnitudes, and spectral characteristics are known, a more meaningful classification and interpretation of the objects in this Atlas can take place.