The Tarantula Nebula

A Stellar Example of High Dynamic Range Imaging

The Tarantula Nebula isn't just an ordinary cloud of gas and dust in outer space. Scientists consider it one of the most valuable sites for observing the birth, evolution and death of stars. "The Tarantula Nebula is in a galaxy quite close to our Milky Way,” explains astronomer Dr. Robert Fosbury of the European Space Agency. "So all of its stars are the same distance from Earth—157,000 light years, give or take just a few percent. The fact that they're the same distance is very important, because astronomers then can use the relative brightnesses of stars within this nebula as key data in studying stellar evolution."

Improved Detail With HDRI

But until recently, none of the images of this nebula showed its many forms of stars over a very wide field in any meaningful detail. That changed in 2006, when Dr. Fosbury and a team of colleagues used a photographic technique called High Dynamic Range Imaging (HDRI) to create the most detailed image of the Tarantula Nebula region ever produced. Hailed as one of the ten best astronomical images of the year, it was one of the first of its kind made using HDRI, which requires combining a large number of images shot at various exposures through a series of color filters. "You can look at many astronomical images and their bright parts are just white," says Dr. Fosbury. "But for this image, we were very careful to maintain the dynamic range, and that really gives it a special character."

To create the image, Dr. Fosbury and his team used dual processor Power Mac G5 computers along with imaging tools and techniques he helped pioneer.

His team also used Macs to stage the photo shoot, in conjunction with the observatory. "We ran a UNIX-based software application provided by the La Silla observatory in Chile, " says Dr. Fosbury. The process was relatively straightforward—we specified a position on the sky, the filter, the exposure time, and the observatory staff did the rest, using the Wide Field Imager on their 2.2m Max-Planck-ESO telescope."

Building the Image Using FITS Liberator

It took nearly eleven hours to shoot roughly 100 images that made up the final picture. "Constructing the final image is quite a complicated, computer-intensive process," explains Dr. Fosbury. "We import the data into our Macs using an Adobe Photoshop plug-in we developed called FITS Liberator. We use it to build up and colorize the different filter images as a mosaic in 16-bit Photoshop layers," he says. "We work to get the right white point and color balance. Finally, we intensity-stretch the different regions to show the maximum detail while avoiding saturation. In the language of HDRI, this is known as ‘tone mapping.’

Once complete, the European Southern Observatory released the large color image to the public, along with a package of calibrated, science-grade images for use by scientists and educators. According to Dr. Fosbury, scientists commonly use the large color image as a 'finding map' to select specific things they want to investigate. "The general public also loves to use images like this one for their computer desktop. It's very popular," he says.

What's more, says Dr. Fosbury, anyone can access the observatory’s data to build their own images, using Photoshop and the free FITS Liberator plug-in they provide. "We have about 60,000 users worldwide, including amateurs who sift through our public archives, find all the bits of the sky imaged in different filters, and create their own wonderful color images," he says.

Macs Provide "A Tremendous Advantage"

Dr. Fosbury credits Macs for their role in the intensive and complex processing required by High Dynamic Range Imaging, and has seen Mac OS X become the dominant computing platform in the field of astronomy over the past several years. "There's a tremendous advantage over other platforms, and a real explosion in the use of Macs by scientific researchers," he says. "A lot of UNIX software is used in astronomy and in science in general, and many scientists have moved to Macs because they can run UNIX software under Mac OS X and do other work on the Mac at the same time. I was one of the pioneers—I bought the first Macintosh for the European Southern Observatory in 1986 and we've been using them in increasing numbers ever since."