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Rediscovering the color of the Crab nebula.

FOR MANY YEARS we have made celestial color pictures at the Anglo-Australian Observatory by combining separate black-and-white images taken in red, green, and blue light. With this in mind we remembered the classic montage of Crab-nebula photographs in four colors distributed by Mount Wilson Observatory. Since the information in those images had never been combined in a single picture, we set out to do so in the form of a color photograph.

Ever since it was first noted by Welshman John Bevis in 1731, the Crab nebula has been an object of increasing interest to astronomers. It was rediscovered by Charles Messier in August 1758 and appears as the first entry in his famous catalog. In the 1840s Lord Rosse saw it as a "closely crowded cluster with branches streaming off from the oval boundary, like claws," which gave rise to the popular name, the Crab.

The earliest photographs of the nebula failed to show its most striking feature -- the extended radial system of filaments that more than anything else hints at the violence of the supernova explosion that produced the Crab. The supernova itself was visible for almost two years and caught the attention of Chinese astronomers in 1054.

For many years one of us (Malin) had hoped to photograph the Crab with the Anglo-Australian Telescope. Unfortunately, the nebula's 31 |degrees~ north declination makes it a difficult object for a telescope at 31 |degrees~ south. No such problem had faced Walter Baade, who took the original Mount Wilson montage with the 100-inch telescope in the early 1940s.

WALTER BAADE

Baade moved from Germany to the United States in 1931 to take up a permanent staff position at Mount Wilson. During World War II he was classified as an enemy alien because of his origins and was confined to the Pasadena area with special permission to work at the observatory. This irksome restriction was to prove a blessing.

At this time the number of active astronomers was depleted, for many were applying their scientific talents to the war effort. Baade suddenly found himself with generous amounts of telescope time, a luxury he soon exploited. As an added boon, nighttime blackouts were in force along the western seaboard of the United States in reaction to Japan's bombing of Pearl Harbor. For the first time in many years the skies over Mount Wilson were very dark!

During 1941 and 1942 Baade took plates of the Crab nebula with the 100-inch telescope, using combinations of photographic emulsions and filters that isolated well-defined bands of the spectrum. He discussed these plates in a 1942 Astrophysical Journal paper in which he concluded, "The line spectrum of the Crab nebula originates in the outer envelope of filaments, the continuum in the inner amorphous mass. It is hardly necessary to point out that on account of these features the Crab nebula is a unique object among the galactic nebulae."

We were unable to locate Baade's original plates. Fortunately, four of the original plates were enlarged onto an 8-by-10-inch glass positive from which negative copies were made. Lettering was added to show the wavelength coverage of each image. Contact prints from this montage have since appeared in many publications. Richard Black kindly lent us the glass copy.

CREATING A COLORED CRAB

The montage contained more than enough information for our purposes; indeed, we could make a series of color pictures by combining any three of the four images.

The contrast and sky-background density of the four images appeared well matched and skillfully adjusted, so we could make satisfactory black-and-white contact prints on a normal grade of photographic paper. This careful matching of background and contrast is essential to the first step in our process. An error at this stage can give the sky an odd color that is difficult to correct without some very fancy photographic footwork.

The montage negative was almost perfect in that respect, though its contrast was rather low. A contact copy made on high-contrast film corrected that situation and provided our individual positive color separations. The positives were then projected, by means of an enlarger, one at a time and in register onto a sheet of tungsten-balanced color-negative film. The red-light image was passed through a red filter, yellow through green, and blue through blue. The exposure ratios were chosen based on many years of experience with similar plates.

The images in the Crab montage had originally been enlarged to slightly different scales, but this problem was easily corrected during the superimposition process. Also, the original images and lettering were not identically aligned. Although the orientation was corrected easily, the lettering appeared untidy; also, a colored border remained around the picture that could only be removed by cropping unusually close to the nebula. Despite these shortcomings, we constructed an eye-catching color photograph of the Crab that must be among the deepest ever made. To our knowledge it is the only three-color picture ever derived from the thousands of plates taken with the venerable Mount Wilson 100-inch telescope.

A NEW THREE-COLOR IMAGE

Our search for the original 100-inch plates of the Crab produced an unexpected result when Robert Brucato of Palomar Observatory located many exposures of the nebula made with the Hale 200-inch telescope. There were 64 of them, taken between 1955 and 1967 with a wide range of emulsions and filters. Careful sifting found three plates that had the potential to make a color picture, though their exposures were not well matched, nor were they as deep as we would have wished. All deep images we found had been exposed in very poor seeing. We suspect that all the best originals taken at either Mount Wilson or Palomar have been scattered throughout the astronomical community and are effectively lost, a very sad state of affairs. (We welcome hearing from anyone with information to the contrary.)

Nonetheless, we created a pleasing color image (see page 43) from what was available. The red-light plate, somewhat underexposed, was copied at a higher contrast than those in blue and green light through a mask to reduce oversaturation toward the nebula's center. Despite this maneuvering, we believe the image is a good impression of what we might see of the Crab if our eyes were sensitive to color at very low light levels.

A POLARIZATION IMAGE

The greatest treasures among the 200-inch plates looked at first the least interesting. These were taken through continuum filters designed to eliminate the filamentary red emission lines that dominate the Crab's fine detail. This highly polarized continuum radiation is produced by electrons spiraling along magnetic field lines and is known as synchrotron radiation. We see it as a bluish haze in color pictures.

In the box of Palomar plates were several sets made by photographing the continuum radiation through polarizing filters rotated to different angles. Most of them were well-matched quartets at four different polarization angles 45 |degrees~ apart. Although these images contain much information, it is difficult to appreciate the subtle differences among them when they are simply placed side by side, as is normally the case.

One of us (Malin) had learned to extract interesting photographic data from another supernova, SN 1987 A in the Large Magellanic Cloud. The technique of image subtraction had worked well with the supernova's light echo (S&T: January 1990, page 22), and now here was a similar challenge.

Image subtraction works when a positive copy of one plate is used to cancel the negative of another when they are aligned. Canceling one negative with its own positive leaves only the saturated images of the brighter stars. But if the positive is aligned with any similar but not identical negative, the difference between the two is highlighted.

We obtained three of these curious negative-positive pictures from a set of four similar plates. They contained contributions from all four originals. Next we combined the three derivatives into a color picture. Since there is no color information inherent in the originals, it did not matter which images were exposed with red, green, or blue light, though we decided that the neutral gray background looked rather smart. Because no color dominated, it also indicated that the different images were well matched.

More remains to be done with this fascinating collection of plates, including a full analysis of the images we already have. The pictures reproduced here reveal that there is plenty more to be discovered about the Crab nebula and that old photographic plates are a fruitful source of such information.

David Malin is a photographic scientist at the Anglo-Australian Observatory. Jay M. Pasachoff is a well-known textbook author and professor of astronomy at Williams College.
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Author:Malin, David; Pasachoff, Jay M.
Publication:Sky & Telescope
Date:Jul 1, 1993
Words:1437
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