There are two types of star tracker out there, one of which has counterweight (or sometimes called balance weight) attached on the other side of the tracker rotational axis, while the other doesn't. MSM Star Tracker is of the latter. Let's call this type unbalanced trackers for short.
Compared with balanced trackers, there are some disadvantages that come with unbalanced ones:
- When the center of mass of the camera passes the topmost point during tracking, a brief disturbance to angular tracking speed is more likely to happen with unbalanced trackers than with balanced ones;
- The torque produced by the unbalanced weight of the camera may give ball heads, mounts or whatever connected to the tracker a higher tendency to get loose;
- Battery may drain faster than the balanced trackers, as the unbalanced torque from camera exists as a load to the tracker motor.
Fortunately, with a little tweak in the camera setup all of them would be eliminated and what's left are the advantages that we buy unbalanced trackers for - compactness, superior portability, covering over 95% landscape astrophotography scenarios.
The tweak is so easy that there is only one simple rule to follow - the 15° rule. Assume polar alignment is properly done hereafter. Now think of a plane which is vertical to the ground and contains the tracker's rotational axis. Let's call it center plane. If we are facing right against to the side of tracker with the rotating plate, the center plane appears to be the red line in the picture below.
All you need to do before pressing camera shutter is make sure the center of gravity of camera (plus lens) falls into the 15° zone shown between the red and yellow plane in the picture below. Note that if we are facing the tracker side with the plate, the yellow plane sits on the right side of the red plane. This is the 15° rule.
Here is an example (see the picture below) where the center of gravity of camera falls into the 15° zone. You can save this image to your phone album in case you need it out in the field.