Why do people doing CCD imaging often stack, say, five 1-minute exposures instead of taking just one 5-minute exposure?
Modern digital cameras capture faint astronomical objects with much shorter exposures than their film-based counterparts did, but it still takes an exposure of many minutes to produce a good picture. So-called image stacking is the easiest way to achieve the equivalent of a long exposure, for two reasons. The first is that many image-processing programs have reduced the stacking process to a few mouse clicks. And the second is that most telescope drives can manage 1- or 2-minute exposures unattended, without the effort and auxiliary equipment needed for guiding a long exposure. As a bonus, it’s far less frustrating when you lose a minute-long exposure to a gust of wind or an airplane flying through the field than it is to lose many minutes of work.
Technically speaking, one long exposure with a digital camera has a higher signal-to-noise (S/N) ratio and creates a better picture than an equivalent exposure duration obtained by stacking frames. But this is only marginally so for astrophotographers working under suburban conditions, where skyglow is the dominating source of noise after an exposure time of just a minute or two. It is only under very dark skies, or when using very narrowband filters, that there is a significant S/N advantage to shooting a single long exposure.
— Dennis di Cicco