Reciprocity is a loss of film speed for long exposures. In other words, you double the exposure and the film density does not double. Fortunately, with today's films you need not worry about reciprocity very much. Just realize it does affect some film, thus is a factor is picking your favorite. A kit to do it yourself is expensive, but you can buy treated film for less than twice the normal cost and that is the way to go as you start out.
Spectral sensitivity is also a concern.
Many nebulae in the night sky are red from hydrogen emission. However some films are very insensitive at those wavelengths. A red-sensitive film is another factor. For black and white photography, the ultimate film is Kodak Tech Pan. It has just about the finest grain available, has excellent red sensitivity and when hypered is quite sensitive.