We can learn a lot about the heavens simply by looking at them with our eyes. But our eyes are poor light-gatherers. We therefore enlist the help of optical instruments, particularly telescopes. Telescopes have bigger apertures (lens-openings) than our eyes and can therefore gather much more light. Astronomers not only look through telescopes, they also use them as giant cameras to record images on photographic film. Using film provides a means of making permanent records of observations. Also it enables astronomers to capture images from stars that are much too faint to be visible to the naked eye. This is because photographic film is able to store light. In this way we can detect the light from objects that lie near the very edge of the Universe, some 15,000 million light-years away. There are two basic kinds of light telescopes, refractors and reflectors. Refractors gather and focus starlight by means of lenses; reflectors by means of mirrors. Reflectors can be made much bigger than refractors because the mirrors can be supported from behind. Lenses have to be supported around the edge, which causes distortion in large lenses. Whereas the biggest refractor mirror is only 102 cm (at Yerkes Observatory, USA), the biggest reflector has a mirror 6 metres across (Zelenchukskaya, Russia). Other famous reflectors include the 5-metre, or 200-inch Hale telescope at Mt Palomar, and the 2.5-metre, or 100-inch Hooker telescope at Mt Wilson, both in California; and the 4-metre Mayall telescope at Kitt Peak observatory in Arizona. The Hubble Space Telescope now in orbit around the Earth has a 2.4-metre diameter mirror.
