How It Sees

More than just a pretty picture
by Jeff Hester, Arizona State University

Many people have gazed in awe at the Hubble Space Telescope image (left) taken of the Eagle Nebula, a spectacular star-forming region about 6,500 light-years away. Indeed, I'm proud to say—since I "took" the picture with my colleague Paul Scowen at Arizona State University—that the image is arguably the most famous Hubble photograph ever taken. But few laypeople realize just how much effort goes into preparing such images sent down from our orbiting eye on the universe.

In this feature, I'll walk you through the various steps it took to assemble this image from the raw data, then end with just a bit about what the image tells us about the "Pillars of Creation," as this fascinating star nursery has been dubbed.

But first a little background: The Eagle Nebula image was taken on April 1, 1995, by the Hubble's Wide Field and Planetary Camera 2. Like a digital camera, the WFPC2 uses charge-coupled devices (CCDs) rather than film to record images. CCDs have an array of light-sensitive elements called pixels that turn light falling on them into electrical signals. As we'll see, all the hard work of preparing the final image involves knowing how to interpret each of those signals, removing the "fingerprints" of the camera itself, and turning the signals into measurements of how bright the light is at specific points in the sky.

The WFPC2's field of view is about 1600 by 1600 pixels, which makes it roughly equivalent to a 2.5 megapixel digital camera. While the final image is not true color, it is closer to what your eye would see if you went there than the picture you would get with color film.

Before we begin fashioning the image, I need to reveal why the final picture seems to have had a big bite taken out of its upper-right quadrant.

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