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A New Camera — and Eye — on the Sky

EuroPhotonics
Jul 2008
BELFAST -- By the end of the year, the world’s biggest digital camera will be snapping pictures of the night sky, assembling the most comprehensive astronomical catalog ever produced. Dubbed PS1, the system also will help try to keep humans from going the way of the dinosaurs.

Alan Fitzsimmons, a professor of astronomy at Queen’s University, noted that an object 10 km across ended the dinosaurs’ reign when it hit the Earth. However, an object one-tenth that diameter is big enough to cause global climate change shortly after impact. “We think there may be just over a thousand asteroids that are 1 km across that come close to the Earth,” he said. “There are probably 50,000 that are maybe 150 m across or larger.”

A majority of the near-Earth climate changers have been spotted, but perhaps 30 per cent remain undetected. The smaller objects have yet to be extensively surveyed. Thus, to avoid catastrophe and to advance science, there is a need to map what is in the sky.

Enter PS1, a single 1.8-m-diameter telescope and 1.4-gigapixel camera prototype of the four-times-larger PS4, which is scheduled to deploy in 2012. Because PS1 has the world’s largest and most advanced digital camera, astronomers will be able to scan the entire sky in less than a week and to detect anything brighter than the 23rd magnitude — millions of times fainter than can be seen with the naked eye.

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The Pan-STARRS telescope uses a gigapixel camera. Photo by Richard Wainscoat.


Queen’s University is part of the consortium supporting PS1, a name derived from Pan-STARRS or Panoramic Survey Telescope and Rapid Response System. Other institutions involved in PS1 include the University of Edinburgh and Durham University, both in the UK, the Max Planck Society institutes in Garching and Heidelberg, Germany, and the University of Hawaii. Other universities and astronomical organizations in Asia and in the US round out the group.

PS1 is located at Haleakala in Maui, Hawaii. It uses a 1.8-m concave primary mirror to gather light from a region about six widths of the full moon. That light ends up on a CCD detector cooled to about –80 °C, said University of Hawaii astronomy professor and Pan-STARRS principal investigator Nick Kaiser.

The detector, which covers about 40 cm2, is a mosaic like other astronomical sensors. It is actually an array of arrays in that each physical unit, which is 5 cm on a side, is further subdivided into an 8 × 8 grid of individually addressable cells, Kaiser explained.

He said this improves manufacturability and increases speed, allowing a complete readout in a few seconds. Such speed is needed because a whole sky survey consists of many, many exposures. Another benefit is the ability to handle bright stars without saturation.

The detectors contain a new design, an orthogonal transfer CCD, which enables rapid compensation for image fluctuations. The effect is similar to a tip-tilt adaptive optics system, except that it is entirely electronic. Another twist is that the CCDs are deep-depletion devices, thanks to 75-μm-thick active layers; therefore, they are optimized to work in the red. They also can sense into the near-infrared, up to ∼1 μm.

Once PS1 gets rolling, Queen’s University’s Fitzsimmons predicts the discovery of millions of new asteroids, hundreds of comets and thousands of trans-Neptunian objects. Some of those asteroids will bear watching, but most of the gigabytes churned out by the system every minute will have less earth-shattering applications.

“We’ll be using the data coming out of PS1 for science,” Kaiser noted.


GLOSSARY
digital camera
A camera that converts a collected image into pixels that are black or white digital or shades of gray. The digital data may then be manipulated to enhance or otherwise modify the resulting viewed image.
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