Ken Tapping, January 19, 2011
If you have a digital camera, you'll know that an important feature is how many “megapixels” it uses to form its images. Each pixel represents a tiny sensor mounted on a surface in the camera onto which the lens projects the image. Each sensor tells a computer chip the average brightness and colour of the light falling on it. The result is a dot in the image with the size of the patch covered by the sensor, having a uniform brightness and colour. We refer to these dots as picture elements or “pixels.” Obviously, the more sensors and the smaller they are, the smaller and more numerous the pixels are too. The image is made up of more dots and looks sharper. Our eyes work exactly the same way. In our case, there are sensors called rods and cones on the back of the eye onto which the lens at the front of the eye projects the image.
Imagine having eyes with only one small pixel. You would be able to sense only the brightness and colour of one small dot. It would be frustrating and very tedious, but you could still get an image of the world around you by scanning your eye to and fro and up and down, so that your single pixel sweeps over all points in the image.
For most of the history of radio astronomy we have had to work with single-pixel imagers. In the case of radio telescopes, we use concave mirrors (also known as dishes) to collect the radio waves and form a radio image because it is much easier to make and support large concave mirrors than large convex lenses. With the mirror, the image forms in front rather than behind, as is the case with lenses.
Radio telescopes consist of large dish antennas, usually with a single sensor at the focus to pick up the radio energy detected. We make images by scanning the antenna up and down and to and fro over the piece of sky we are imaging. Because the radio emissions from cosmic sources are very weak, we have to spend some time, ranging from seconds to hours, on each point in the map to detect enough energy to measure. The result is that some imaging projects turned out to be so slow that, considering the other demands for telescope time, they could not be done.
Canada and other countries around the world are collaborating to develop the biggest and most sensitive radio telescope ever – “The Square Kilometre Array,” which will comprise thousands of small dish antennas. With the large investment needed to make such an instrument, it is fortunate that technologies are being developed to put more pixels at the focus of each antenna, immensely speeding up the imaging capability of the telescope. Instead of one sensor, we can now have an array of sensors at the focus of the telescope. One such device for use on the SKA is under development at our observatory in Penticton. If you drive by, you will see a 10-metre dish quite close to the road, with a rather large box mounted at its focus. The box contains an array of sensors and their control electronics. For years we have called radio telescopes “radio eyes.” That has not strictly been true, but now it is becoming so.
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