Michael D. Wheeler
With few exceptions, charge-coupled devices have become the preferred method for imaging the universe. There has been one problem: Atmospheric changes such as high winds cause images to blur.
To create clearer, more visible images, John Tonry and Barry Burke from MIT's Lincoln Laboratory decided to put a new twist on the design of a conventional CCD.
In conventional chips, individual pixels in a CCD array act like
buckets collecting rainwater in a thunderstorm. Once the exposure is complete, the signals collected by each pixel are transferred one after another, to an amplifier.
The MIT design, dubbed an orthogonal-transfer CCD, allows the charge to travel in four directions rather than in a single column, as in conventional CCDs. Working with the imaging array is an adjacent storage area on the CCD chip called a frame store.
It acquires an image from a bright "guide star" and sends that data to a data acquisition system. After the guide star's speed and direction are calculated, that information is transferred to a clock that controls the main imaging array. The clock then instructs the imaging array to shift the charges at a certain rate and direction. Thus, the CCD shifts the charge in step with the wandering star, allowing the shift to occur repeatedly before an exposure ends. The result: clearer, crisper images of stars.
"If you look at a typical star, the image dances around at fairly high speeds and the eye perceives this as a twinkle. [We have tried] to take out that random image motion," said Burke.
A prototype of the new CCD was tested on the University of Michigan-Dartmouth College MIT 2.4-m reflector in Arizona, improving the image resolution from 0.73 arc sec without tracking to 0.50 arcs with the orthogonal transfer CCD. One of the first prototypes featured a resolution of 512 3 512 pixels, and another chip is planned with a resolution of 2048 3 4096.
Burke said the new CCD could be an inexpensive way to gain some of the improvements that expensive adaptive optics systems provide for satellites and telescopes, though he said the most sophisticated and expensive adaptive optics still do a better job. Besides compensating for atmospheric turbulence, he said, the orthogonal CCD improves signal-to-noise ratio.