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A few years back, I was playing around with ways of presenting multiwavelength data for my thesis. As an experiment, I made some three-color images of Messier 42, more commonly known as the Great Nebula of Orion. Different-sized versions of the images below are available by clicking on the links; the full-sized images are 1024x1024 pixels -- just right for computer desktop backgrounds or greeting cards!
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Each image is a ``true color'' composite of three monochromatic images, in which the red, green, and blue intensities each correspond to the intensity of one of the original images. This approach mimics the behavior of the human eye, which assembles color vision from cones in the retina that are separately sensitive to red, green, or blue light (color television operates on a similar principle). In this case, the three basic colors are assigned as follows:
color filter name wavelength width component revealed RED Hydrogen Alpha 6575 Å 14 Å ionized gas GREEN Oxygen [III] 5013 Å 14 Å strongly ionized gas BLUE Broadband Blue 4324 Å 1156 Å dust and starlight
The observations were made in the early morning of 1994 December 6 with the 0.6-meter Curtis Schmidt Telescope at CTIO. Multiple CCD exposures were taken in each filter, with total integration times of 10 seconds in blue continuum and 20 seconds in each of the two emission line filters.
The narrow-band filters are redshifted a few Ångstroms from the rest wavelengths of the two lines in order to match the doppler shift of the Large Magellanic Cloud, the real subject of my observing run; I found myself with a little free time in the morning twilight when I couldn't take any useful LMC photometry, so I took these M42 shots for fun. Fortunately, the emission lines in M42 were bright enough enough to show up in the passband wings of the H-alpha and O[III] filters.
At any rate, the general intent was to achieve a color balance near that of a person's eye, by ordering the images by wavelength and fudging the intensity scalings a bit (which should be logarithmic for human vision).
To really do this right would require appropriate R, G and B filters, which I did not have. As a result, the images on the left are probably too red -- our eyes are most sensitive in the green, whereas the CCD used here is more red-sensitive. This is also a problem with traditional color photography, which is why many beautiful pictures of bright red nebulae turn out not to look that way to a human observer. To me, M42 has always seemed to be a pale greenish gray. I have tried enhancing the green component some in the images on the right, with limited success.
The HST image is assembled from different filters than mine: red is Nitrogen [II] at 6583 Å, green is H-alpha at 6563 Å, and blue is O [III] at 5007 Å. These wavelengths correspond respectively to red, red, and green in actual color terms, leading to a rather different, though dramatic, appearance of the HST composite image.
The Orion nebula is a popular target for amateurs and professionals alike. Some good general information, images, and links can be found at the SEDS M42 site.