May 4, 2008

On The Sky Atlas And The NGS-Palomar Observatory Sky Survey

The Palomar Observatory Sky Survey was sponsored by the National Geographic Society. Over ten years, between 1948 and 1958, astronomers at Cal Tech's Palomar Observatory used a 48-inch Schmidt Telescope to create the most advanced sky survey ever, a comprehensive portrait of most of the visible universe as seen from the Northern Hemisphere. [Actually, the survey detected objects of a magnitude of +22, one million times fainter than the limits of human vision.]

When completed, the 935 pairs of glass plates, one filtered red, and one blue, were compiled into the Sky Atlas, which was published over the years on paper, glass, and film.


I first found out about the NGS-POSS about ten years ago I was visiting a physicist friend at her university office. There was a reception in the department library, and we wandered off to explore, and ended up in a rear stairwell, where she showed me a large, grey, metal flatfile on a landing. Inside each drawer was a stack of large, slightly curled photographs. They were printed in the negative; instead of glowing stars on a black sky, each page was light silver, flecked with linty black specks. There was a string of numbers--right ascension and declination, it turned out--on the top of each print.

Two print versions of the POSS were published, one 14x17" format in the 1950's and 1960's, and another, 14x14", in the 1970's. The version I saw was the former. Paper turned out to be the least useful format for astronomical analysis; glass and film were much better, and since the POSS plates were scanned into the Digitized Sky Survey in the 1990's, the print version of the Sky Atlas is obsolete by several generations.

So besides finding an extant copy gathering dust and taking up space in a university physics department somewhere, the logical [sic] thing to do is to print an entirely new set from the original plates.

Two sets of original plates were created, one to work from, and the other to be "stored, unused, in the dome of the 200-inch telescope on Palomar Mountain, where it will be available in the even that the plates in Pasadena should be damaged or destroyed by some catastrophe."

At least that's where they were in 1963 when R.L. Minkowski and G.O. Abell wrote about the NGS-POSS for K. Aa. Strand's Basic Astronomical Data, Vol. III of Stars and Stellar Systems. They described in some detail the characteristics of the Survey, as well as the making of the prints. [Strand's compendium also includes descriptions of earlier sky surveys, which I might get to later.] After the jump, some excerpts from Minkowski & Abell's paper, pp.481-6, on the production of the Sky Atlas

2. THE TELESCOPE The 48-inch telescope is the standard Schmidt type (Harrington 1952). The spherical mirror is 72 inches in diameter and has a radius of curvature of 241 inches.


The photographic plates employed are 14 inches square and only 1mm thick, so that they can be curved in the plateholder into a 121-inch radius along the focal surface, which is concentric with the spherical mirror. Under good seeing conditions the smallest stellar images have a diameter of about 30 u, determined by the resolution of the photographic emulsions used... The total field covered on a plate is 6.6 degrees square. The non-vignetted field of the telescope, determined by the respective sizes of the mirror and correcting plate, is 5.4 degrees in diameter.


The centers of the 935 fields of the survey and its southern extension lie along declination circles at 6-degree intervals from +90 degrees to -30 degrees, inclusive, and are so spaced in right ascension that adjacent photographs overlap along a zone at least 0.6 degrees wide on all four edges...

Each field was photographed on both blue- and red-sensitive photographic emulsions. The two exposures were taken in immediate succession. The order of the exposures, however, was arbitrary, generally being dictated by convenience or efficiency in arranging the observing schedule.

All exposures were made on photometrically clear nights in the absence of moonlight...

Each pair of photographs of a field was inspected for quality. Plates were rejected because of meteorologic conditions, such as clouds, haze, excessive sky brightness, bad seeing; for misleading or too numerous emulsion defects; or for operational difficulties, such as poor focus, breakage, star trails due to improper alignment of the polar axis of the telescope, and airplane trails. Rejection of one member of a pair automatically caused rejection of the other. Many fields had to photographed several times before acceptable pairs of plates could be obtained. Altogether, 1620 pairs of plates had to be exposed to obtain the 935 pairs included in the survey.

In a 1995 survey of sky surveys made using Schmidt telescopes, the Royal Observatory, Edinburgh's D.H. Morgan noted that survey plates made after 1980 have more than twice the contamination rate of satellite and fast-moving object trails than plates made before 1980. Whatever its technical shortcomings, the POSS is unique for capturing the last view of the earth's sky before it filled with man-made objects.
No known reproduction method can reproduce on paper the full density range obtained on a photographic plate. For the Sky Atlas, it was decided that the reproduction of the faintest details at the limit of the plates should be achieved, even at the expense of inadequate reproduction of bright objects, because the 48-inch Schmidt telescope permits the observation of faint stars, galaxies, and nebulosities that are beyond the limits of smaller instruments. Satisfactory preservation of the plate limit is possible only on the negative reproductions; this was one reason for their choice for the production of the Atlas.

Experiments with various methods of reproduction showed that only photographic copies could preserve the faintest stellar images with diameters of about 0.03 mm. At least one intermediate step--the production of a positive--was necessary to avoid the risk of damaging the plates in printing large numbers of copies directly from the originals. Since positive reproduction would have necessitated the making of a duplicate negative as a second intermediate step, negative reproduction was to be preferred, for each intermediate step entails some deterioration of image quality.

Separate contact printers were used for the production of the glass positives and the paper prints. The source of illumination in the positive printer was a projection lamp at a distance of 16 feet. A close approximation to illumination by a distant point source was thus obtained, which minimized the loss of definition because of imperfect contact. Diffuse illumination was used in the paper printer. Both printers were of the vacuum type, in which the plate to be copied and the duplication plate or paper were kept in nearly perfect contact by evacuation of the air between them.

After extensive tests and trials, Eastman Kodak Finegrain Positive glass plates were selected for the positives, and Eastman Kodak Unicontrast double-weight paper for the negative prints. The positive plates were processed 3 minutes with agitation in standard formula 16D developer to a photographic contrast of gamma 1.8. The paper reproductions were processed in standard formula D-72 developer. The contrast of the paper is gamma 2.0, practically independent of development.

The prints were fixed for 3 minutes in each of two fixing baths they were then washed with agitation until a starch-iodine test indicated that every trace of the sodium thiosulfate from the fixing bath had been removed. They were finally treated with commercial print-flattening solution that minimizes the danger of the emulsion cracking with handling. It is believed that every possible care has been taken to insure a long life for the prints.

The original prints show considerable variation in background density, which depends on the sky brightness. It was found that the most satisfactory reproduction could be obtained by printing all positives to a density of 1.4, without regard to the densities of the originals. This was achieved by measuring the background density of the originals and adjusting the exposure time for the positives with rigid control of the illumination and carefully standardized development. Originals with background densities below 0.3 or above 0.9 were found to give inferior results and were therefore rejected.

The paper prints were found to be most satisfactory with a background density of 0.2 or slightly higher. ...

The National Geographic Society Palomar Observatory Sky Atlas is distributed as a set of 1870 unbound photographic prints on double-weight paper of 14 x 17-inch size. Each print as a margin of 1 inch on one side and 2 inches on the other, with the wider margin on the west edge of photographs in the blue and on the east edge of those in the red.

In one corner, usually the north-following one, of each print are given the plate number with a letter "O" or "E," denoting, respectively, whether the original exposure was on the Eastman 103a-O or 103a-E emulsion, the date, and the 1950 coordinates of the field center. The 1855 co-ordinates of the field center have been stamped on the back of each print.

A catalogue distributed with the Atlas gives for each exposure the date, sidereal and Pacific Standard times, hour angle, exposure time, and seeing.

Throughout the entire program, every effort was made to maintain high standards of quality.


The Sky Survey was made financially possible by grants from the National Geographic Society. The society provided the photographic materials and special equipment required,the salaries of the personnel employed full or part time on the survey, and the production of the two contact positives on glass of each survey photograph.

National Geographic Society - Palomar Observatory Sky Survey [wikipedia]

Since 2001 here at, I've been blogging about the creative process—my own and those of people who interest me. That mostly involves filmmaking, art, writing, research, and the making thereof.

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first published: May 4, 2008.

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