Multiwavelength Astronomy

Photo of Neil Gehrels

Gamma Ray Tools, Neil Gehrels

Imaging with CCDs

It wasn’t until the 1990s that we began using the detectors with imaging systems. The mechanism for detecting gamma rays is still the same, but now we also have very small, solid-state detectors using a new material called Cadmium Zinc Telluride (CZT) that’s like a charge-coupled device (CCD) in the camera of your cell phone or digital camera. A CCD is sort of a reusable piece of film. It’s composed of picture elements, or pixels.

Credit: NASA/Goddard Space Flight Center

Swift Detector Module: On the right you can see the CZT detectors that form one 8 x 16 array.
Credit: NASA/Goddard Space Flight Center


The Chandra Advanced CCD Imaging Spectrometer (ACIS): The Advanced CCD Imaging Spectrometer is an instrument on the Chandra X-Ray Observatory that consists of an array of charged coupled devices. ACIS is especially useful because it can make X-ray images, and at the same time, measure the energy of each incoming X-ray, so that scientists can make pictures of objects using only X-rays produced by a single chemical element.

When the camera takes an exposure, these pixels collect light that is converted to an electric charge when it hits the surface. A photographic image can record about 5% of the light that reaches the film. Modern CCDs can record up to 90% of the light. Virtually all the astronomical images you see today are produced by CCD detectors or cameras.

With CZT, we could make the detectors smaller and more compact, so instead of having seven of them like we did in the balloon instrument I told you about, we can have an array of thousands of pixels, each one converting light energy into electrons.

Even now when we have these fine detectors it’s still tricky to get a gamma-ray image that’s as good as what you get with other wavebands. With optical astronomy you have an astronomical source and light from it comes into the telescope’s mirror and gets focused down and imaged onto the CCD chip. What you have on the chip is an exact image of the sky. So when we say we’re making an image of a gamma-ray burst, and we show you this fuzzy dot in the sky, that’s an actual image of the sky that was made with one of these solid-state detectors.

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This material is based upon work supported by NASA under Grant Nos. NNX09AD33G and NNX10AE80G issued through the SMD ROSES 2009 Program.

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