I recently did some tests with the Pentax Astrotracer1 system. To do so, I shot Orion (which was in a SSW direction and around 30° above the horizon on that night) and its surroundings at 180s exposure with a 16mm f/2.0 and a zoom lens at 12mm f/4.0.
The results were surprising:
Both lenses I used during testing showed stars being drawn out to linear streaks on the edges of the image, seeming to radiate almost (but not exactly) from the image center. In the bottom right corner of the first image, some star trails seem to radiate from a different point.
I wonder if that's a lens error showing up, since the constellation of Orion itself was tracked fairly well. If the tracking system assumed a wrong position or sky direction, I would have expected all stars across the field to show similar trails. Also, from using longer lenses, I know that the Astrotracer system itself works well on my camera.
Can anyone help me identify the reason for those star trails?
1 For those unfamiliar with it: The system uses the movable sensor in Pentax cameras to track the apparent motion of stars during long exposures. The control input for it comes from a combined three-axis GPS and magnetic compass unit, so that the camera can calculate the required motion from its position on the earth, the time and the viewing direction and elevation.