The speed of a film—its ISO rating—indicates how quickly it reacts to light. Faster films allow shorter exposure times, but they also tend to produce grainier images.
Twilight scenes or bright objects such as the Moon are best captured on slow, fine-grained films, which have an ISO rating of 50 to 100.
Most celestial shots require the extra light-gathering ability of at least an ISO 400 film. In recent years, ISO 400, 800, and 1000 print films from Kodak and Fuji have been popular. For ultra-high speed, Konica offers an ISO 3200 print film. For slides, high-speed films such as Kodak's Ektachrome 400 and Ektachrome PI 600 have proved suitable for astrophotography.
For extra speed, most color slide films and some print films can be "pushed," a procedure that extends the film's develop-ment time. Most pliotolabs offer push-processing for a small extra charge. An ISO 800 film pushed "one stop" has the recording power of an ISO 1600 film, though with an increase in graininess.
For very detailed images, some astrophotographers shoot Tech Pan, a fine-grained black-and-white film much sharper than any color film. But for Tech Pan to pick up faint stars and nebulas, it must be hypersensitized—a baking process that increases its speed. Color film can also be hypered, but this is not essential. You can take superb photos with today's ISO 400 and 800 films right out of the box.
While a one-hour photolab can provide fine results, your photographs will be safer with a specialized professional lab. Always shoot a daylight scene at the start of the roll so the lab can work out where the frames are on your film.