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MORE PIXELS

If you have a digital camera, you’ll know that an important number 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.

If you have a digital camera, you’ll know that an important number 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 dish antennas, as large as possible, 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 were 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. If you drive by the observatory, 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.

Jupiter is now getting low in the southwestern sky in the evenings, but still easy to spot. Uranus still lies very close to it, but you will need a telescope to see it. Saturn rises around midnight, and Venus around 5 a.m.The Moon will be full on the 19th.

Ken Tapping is an astronomer with the National Research Council’s Herzberg Institute of Astrophysics, and is based at the Dominion Radio Astrophysical Observatory.