For faint deep-sky targets such as nebulas and galaxies, faster telescopes—ones with f-ratios of f/4 to f/7—are a definite advantage because they allow shorter exposure times. Alternatively, you can speed up a slower telescope with an accessory called a telecompressor.
Even at fast f-ratios and with ISO 800 film, however, you will need exposures of 15 to 60 minutes. It would be wonderful if we could simply attach a camera at prime focus, open the shutter, and walk away. Try it at these focal lengths and the result will be horribly trailed stars, despite the most precisely aligned mount.
Errors in the drive gears are the chief culprit, but flexing mounts and tubes, atmospheric refraction, and wind all contribute to wiggly stars. Some method of guiding the telescope is called tor.
Schmidt-Cassegrain owners usually turn to off-axis guiders. These devices contain a small prism that picks off a star image just outside the film frame, allowing the photographer to monitor the guide star's position while photographing through the same telescope. Owners of refractors and Newtonian reflectors often opt for a separate piggybacked guidescope—perhaps a small 2.4 or 3 inch (60 or 75 mm) refractor.
The least expensive but most difficult method of guiding is to do it by hand, tweaking speed controls to keep a star centered on the illuminated cross hairs of the guiding eyepiece. Today there is an alternative. Automatic guiders using CCD chips detect any wandering of the star's image and send pulses to the telescope's motors, which move the scope so the star returns to its centered position. The constant, unfailing corrections result in a photograph more perfectly guided than any human is capable of, especially while falling asleep at the eyepiece at 3 am.