The Moon with Earthshine taken at the prime focus of a telescope. Taken with a Canon 20Da DSLR camera through a 130mm aperture refractor at f/8 with 1040mm of focal length and a 2 second exposure at ISO 800. The telescope was on an equatorial mount tracking the Moon at the lunar rate.

Prime focus usually refers to the camera being placed at the focal plane of a telescope without any additional optical elements, such as a tele-extender or tele-compressor, to change the focal length of the optical system. Really this is a loose definition, as technically a camera with any kind of camera lens is at "prime focus" of that lens. Generally, prime focus means the camera is attached to a telescope with more focal length than a normal camera lens.

Because prime focus usually means quite a bit of focal length, everything becomes more critical, such as focus and tracking. More focal length means more magnification, and that means that errors are magnified also.

Prime focus shots through a telescope can include relatively short exposures, such as this 2 second exposure of the crescent Moon with Earthshine, taken with a 5 inch f/8 refractor. They can also include exposures whose total time can run up to several hours (usually broken down into a series of much shorter exposures).

This image was focused through the camera's viewfinder with the Canon Right-Angle Finder at 2.5x magnification.

As the focal length and exposure times get longer, the degree of difficulty increases.

The Moon and Sun also move across the sky at slightly different rates than the stars. Many mountings have special settings for these different rates. If your mount does, be sure to use the lunar rate when shooting the Moon; the solar rate when shooting the Sun, and the sidereal rate when shooting the stars.