Yep, the diagonal of that small sensor is about 16mm.
In the first image, your disk's diameter is larger than the diagonal of the sensor! In the third image, your disk is smaller (i.e., better focus) but still probably about a third of the sensor diagonal, say 5mm.
Your OTA is f/7. That means that for each 7mm the focus is off by, that disk would change size by 1mm. To see a disk of 5mm diameter, your drawtube is off by something about 7*5mm = 35mm.
Check your notes to see if the third image has a longer drawtube than the first image. If so, add between 35mm to 70mm of additional space between the camera and the drawtube and try again. If the third image has a shorter drawtube than the first image, then you need to remove somewhere between 35mm to 70 mm of spacing.
You can try to achieve focus in daytime by changing spacers. If you can get a tree that is 100 meters away in focus, then for infinity focus at night, you will need to shorten the drawtube by 3.6mm for a objective with a focal length of 600mm -- based on thin lens formula from high school physics. Just apply the thin lens formula for trees that are at different distances.
Chen