On the Prediction of Visibility for Deep-Sky Objects

Knowing in advance the visibility of a given object is important when planning an observation. New amateurs usually search only bright objects since they present smaller difficulties for finding and observing them, except perhaps under severe conditions (e.g., city, full moon). Nevertheless, amateurs gradually extend their observations to more challenging targets, fainter and more elusive. These objects, being more difficult, can offer dramatically different images depending on the telescope aperture and sky quality. Therefore, to predict with certain accuracy the difficulty that an observation will involve may become a puzzling task. Most observers estimates the possibilities of observing a celestial object in a subjective way, depending on experience in handling their telescopes and knowledge on deep-sky objects. It would be however convenient to have a more objective tool to decide whether a given object will be visible or not.