CFHT

The Canada France Hawaii Telescope is Canada's premier optical and infrared telescope. When it first opened in 1979, its 3.6 metre diameter mirror made it the sixth largest telescope in the world. Today, many telescopes have much larger mirrors, but CFHT remains important to international astronomy because of its large field of view. Modern telescopes were designed for tiny CCD chips - they can see very faint things, but only over a small patch of the sky. CFHT was designed for large photographic plates - by replacing those plates with a mosaic of CCD chips, astronomers are now able to take detailed pictures over a large area of the sky.

CFHT is located at the summit of Mauna Kea, a dormant volcano on the Big Island of Hawaii. Mauna Kea rises 4200 metres above sea level - at this height, the atmosphere is dry and calm, the sky is dark, and the weather is generally clear. These conditions make Mauna Kea the finest site for astronomical observations in the Northern Hemisphere. Many other of the world's best telescopes are located on Mauna Kea, including Subaru, Keck I and II, Gemini North and JCMT. CFHT is lucky to enjoy one of the one of the best locations on the summit.

CFHT is jointly owned and operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique of France , and the University of Hawaii. Astronomers from Canada, France and Hawaii use the telescope free of charge. Twice a year, researchers submit proposals that outline what they want to do with the telescope. A committee of scientists chooses the best programs and divides up the telescope time. Astronomers will sometimes travel to CFHT to use the telescope themselves - other times, the data will be taken for them, and they'll receive the results by mail or over the Internet.

MegaCam

MegaCam is a new astronomical camera that has just been built for CFHT. It was designed for large observing projects - surveys that cover a wide area of the sky, look at light at a variety of wavelengths, and find very faint objects. In short, MegaCam is ideally suited for the Canada France Hawaii Legacy Survey! MegaCam and its supporting instruments were built jointly by CFHT in Hawaii, the Departement d'Astrophysique, de Physique des Particules, de Physique Nucleaire et d'Instrumentation Associee and the Observatoire de Paris, section de Meudon in France, and the Dominion Astrophysical Observatory in Canada. MegaCam is the world's largest modern astronomical camera - it can take a picture one square degree in size. That's about the same area covered by four full moons!

Instead of using film like an ordinary camera, MegaCam takes its giant pictures using CCD chips. You can think of the surface of a CCD chip as a series of buckets. Light coming from the night sky falls into those buckets - the more light that comes from a particular part of the sky, the more full the buckets on that part of the chip will be. Once the picture has been taken, a computer reads out the amount of light in each bucket and converts it to a digital number. When you display the picture on a computer, the brightness at each point corresponds to how much light fell in the bucket at that point. CCD chips are used in most astronomical cameras today - they are much more sensitive to light than ordinary photographic film.

Instead of one giant CCD chip, MegaCam uses a mosaic of forty chips. Each chip measures about three by six cm and has nine million pixels on it - the "buckets" in the analogy above. This means MegaCam pictures will be very detailed. They'll also take up a lot of computer memory - each picture will need over 700 megabytes of memory for storage. That's more data than will fit on a CD-ROM, just from one picture! To deal with all the images MegaCam will produce, a data centre called Terapix has been set up. It will write programs to process MegaCam images and create tools for astronomers to use when working with the large data sets.