Regardless of their maximum aperture, most lenses perform better stopped down a bit. Experiment to see where your lenses perform best. Film You have an incredible selection of film compared to just a decade ago. Today there are several films that really are perfect for astronomical applications. Film sensitivity is high and grain and resolution is very fine.
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.