When you gaze towards a clear moonless night sky, the stars appear as points of light – most are colorless. There are a few exceptions, however: Mars, Aldebaran and the star at the heart of the constellation Scorpius, Antares, can be seen to have a very slight reddish hue. Through a small telescope, star and planetary colors become more apparent but galaxies and nebulas remain un-pigmented and monochromatic. These objects begin to take on a greenish ting when viewed through very large telescopes but rarely show the rainbow of hues seen in many deep space pictures, like the one shown here.
This begs the question that is often asked of astrophotographers: are those the real colors or did you make them up?
The human eye’s retina contains two types of photoreceptors called rods and cones. There are about 120 million rods compared to approximately 7 million cones. Rods are more sensitive to light but only cones detect color. This is why we can make out objects that surround us, in dimly lit situations, but we cannot discern their hue. Light is comprised of three primary colors, red, blue and green. Of these, the cones in our eyes are most sensitive to the later, which makes some evolutionary sense if your ancestor’s survival was dependant upon discerning plants.
Astronomical telescopes are essentially used for two purposes: 1) to help separate distant but closely spaced objects and 2) to collect a lot of light. The amount of light collected by even the world’s largest telescopes is still insufficient for the cones in our eyes to detect color in faint nebula and galaxies other than green. Therefore, the full color of distant astronomical places, other than stars and planets, is something that still eludes direct observation. It should be noted, however, that there have been some rare claims of seeing other colors by a few observers who may simply have eyes with more color sensitivity.
But film and digital cameras do not have this type of color bias. Film emulsion contains crystals that are sensitive to each of the three primary colors of light and color digital cameras place microscopic red, green or blue filters on top of their pixels. Manufacturers use various schemes to place these filters, it should be noted, but here’s the point: only a portion of the pixels in any color digital camera are dedicated to one color. Regardless, this enables cameras to detect color much more efficiently than human eyes. Digital astronomical cameras go one-step further- they use every pixel for each color.
Cameras specifically designed for taking deep space images are unsurpassed for detecting very faint light but they only produce results in black and white. To create a full color picture, astronomers, both professional and amateur, place a red, green or blue filter in front of the camera so that every pixel is limited to detecting one specific color reflecting or shining from the astro-subject. This, by the way, is a very time consuming process. To create a full color picture, the astronomer digitally combines separate red, green and blues images using commercially available software like Photoshop. Thus, the colors seen in deep space objects taken through a camera are very real and, unless mis-handled during processing, they are also accurate.
One of the most colorful night sky locations, seen here, is located in the constellation of Scorpius, just to the north of its brightest star, Antares. This scene is a riot of colors and can best be seen in the full size image.
We are looking toward the heart of our galaxy and his picture captures a menagerie of space objects and places as we gaze into the distance. For example, there are three globular clusters. M80 is at the top and M4 is toward the bottom. Between them, to the upper left of M4, is NGC6144. The dark threads that swirl about are vast clouds of dust that absorb light and therefore appear as shadows. The bright clouds are also made of dust but these reflect light from nearby stars. Antares is just below the bottom of the image and provides the appearance of the sun at dawn.
This kaleidoscopic picture was produced by Steve Crouch using a 7-inch telescope that was specially designed for taking wide-angle photographs. Steve took this image from his home observatory located in Canberra, Australian Capital Territory, Australia during the month of June, 2006. Steve uses an 11 mega-pixel astronomical camera.
Written by R. Jay GaBany