This section is relevant to unguided widefield images captured with a film or digital camera. It’s quite obvious that in order to capture fainter objects you have to increase the length of the exposure and how long the shutter is held open. However, if a camera shutter is left open, pointing at the sky, the apparent motion of the stars around Polaris causes the stars to “trail.” If you want to have pin-point sharp images of the stars then you have to limit the exposure time so that the motion of the star does not cause the image of the star to move over the CCD or negative. There are two factors that have to be taken into account.
First, the focal length of the lens – higher magnification increases the motion of image across the CCD or film.
Second, the apparent motion of the stars increases as you observe stars that are further away from the celestial pole i.e. increasing declination.
As a rule of thumb an exposure time of 20 seconds at the celestial equator (90 degrees to Polaris) is a reasonable exposure time for a 50mm lens. If you really zoom in, the stars will be oval rather than sharp points. At half the exposure time, the stars will be more point like but fainter objects may not be captured. Using 20 seconds, for a 50mm lens at celestial equator as a reference, it is possible to mathematically calculate the values for various focal lengths at specific declinations.
Regarding usage of the tables, 90 degrees would represent a star at the North or South pole and 0 degrees is the celestical equator. To understand the maths behind the tables I would recommend “Astrophotography for the Amateur” by Michael Covington.
