There are currently so many options and variants of trackers on the market, from different brands with different features, that anyone wanting to approach the world of wide- and medium-field astrophotography is bound to feel overwhelmed and have many doubts about which instrument might be right for them.
Let's start by defining an astro-tracker as an instrument that allows us to track the apparent movement of the celestial sphere (in reality, it is the earth that moves, not the sky) so as to avoid motion blur or the star trail effect (which is a technique in its own right, already covered in another article). By doing so, it allows us to lengthen exposure times, thus obtaining more detail of the night sky and a better signal-to-noise ratio (which we will explore in another article) compared to a static shot on a classic tripod.
A distinction can be made regarding the load capacity of the instrument. There are smaller, portable astro-trackers useful for small setups, such as a mirrorless camera with a wide-angle lens, or even a smartphone. Others, larger and heavier, allow for cameras or dedicated astronomical CMOS sensors with telephoto lenses or small telescopes. Furthermore, they allow for the installation of a bar with a counterweight to mount even medium-sized telescopes. Therefore, based on the weight and size of our equipment, we can make an initial selection.
Another distinguishing feature is the presence or absence of a motor or even an automatic GoTo pointing system, which, as the name implies, allows us to select a celestial object from the relative catalog and, with a simple click, the astro-tracker or mount will point exactly at that object, placing it in the center of the frame or eyepiece—a great convenience! Some trackers are purely mechanical, lacking any electronics.
These are usually instruments dedicated to small setups that allow for relatively short exposure times. Those equipped with motors and control boards managed by desktop or mobile apps, on the other hand, are the ones that, depending on their price range, can even allow us to take deep-sky photos, with exposures lasting several minutes.