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Billions of Pixels Map the Universe

Photonics Spectra
Dec 2002
Sally B. Patterson

Astronomy was once the purview of kings and their most learned advisers. Over the centuries, it has become increasingly accessible to a wider population interested in taking a nocturnal trek with telescope in hand or in visiting a planetarium.


A wide-field view looking toward the core of our Milky Way galaxy has the constellations annotated. This view is composed of several thousand images.


Now the night skies can be studied in the classroom or even from an armchair with a simulation program from Main Sequence Software Inc. called Desktop Universe.

The system comprises some 20,000 CCD images joined into one seamless sky map and accessed via software. It allows one to navigate any part of the sky from any vantage point on Earth, to locate and zoom in on phenomena of interest and to set up "tours" of sites visited for later display.

The project, which took more than five years from concept to commercialization, involved numerous expeditions to remote locations in Australia and Arizona to record data from both hemispheres and during different seasons. Collaborators Douglas B. George and Peter Ceravolo required a system that could produce an image with minimum distortion, a flat field, consistent star visibility from center to corner and no vignetting. Ceravolo designed and fabricated a system with oversize lenses to provide full illumination and an f-Theta distortion characteristic. This, George said, was more suitable to constructing the mosaic than a standard tan-theta would have been. Another consideration was maintaining a consistent spot pattern across the image plane so that stars would appear the same size and shape throughout.

appsPixels2.gif
A close-up of the Eta Carinae nebula in the southern Milky Way shows a region of active star formation. Individual objects are identified by overlaid outlines and catalog designations.

For the imager, the partners used a customized AP9E cooled CCD camera with 3072 x 2048 pixels from Apogee Instruments Inc. One parameter of the lens design was that the rear element had to be fairly close to the focal plane, so they modified the front plate to accept the two rear lens elements inside the camera and relocated the shutter to the outside of the camera.

It was a "huge effort" to stitch together a continuous mosaic of more than 125 billion pixels of recorded data, George said. Processing was done using MaxIm DL software from Diffraction Limited, in combination with custom code. Each image had to be aligned by comparing it with star catalogs and using pattern matching to identify stars and calculate the exact pixel locations. Because they had been taken under various sky conditions, the images also had to be processed for consistency from frame to frame to avoid a patchwork effect. "The goal was to achieve a uniform black background, as if atmosphere was not present," he explained.

The vast skyscape differs from previous planetarium programs in that it displays not just plotted diagrams, but actual sky images, and the user can zoom in to view faint 14th-magnitude stars as well as galaxies, clusters and nebulae. Overlays and data boxes provide full information on all objects of interest.

As a next step, George hopes to add a library of even deeper images -- perhaps taken by satellite -- for items of particular significance.


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