Astro-Imaging
Electronic astronomy is now becoming more affordable. Astronomical CCD cameras are getting better and cheaper every year.
Using a tripod mounted camera with a standard 50mm lens, exposures of about 30 or 40 seconds duration are possible before the Earth's rotation becomes apparent. The celestial sphere will be dragged across the film plane causing stars to appear as short arcs centred on the pole position (currently near Polaris). Longer focal length lenses worsen the problem. Exposures have to be made shorter to prevent star trails from becoming visible.
Attaching a camera to an equatorial mounting allows exposure times to be increased, since the camera can track the celestial sphere as the Earth rotates and cancel out star trails. As well as normal telephoto lenses, telescopes can also be used for photography. Both require precise star tracking since motors and gears are not normally made to the necessary accuracy. A separate guide telescope can be used to monitor a suitable star during the exposure. This is the technique that I have used for comet photography.
Taking photographs through a telescope is the most difficult skill to master. So far I have only managed to take short exposures of the Sun and Moon through my telescope. Exposures that are longer than a fraction of a second require an accurate equatorial mounting and a smooth tracking system.
