Imaging is a fussy beast and any tilt in the imaging train can give undesirable results in our images.
With the advent of large CCD chips, collimating of the chip to the front of the camera is critical.
This method involves reflecting laser light from the CCD surface and observing how the reflections rotate on a screen when the camera is revolved.
The CCD is properly aligned when the bright reflection from the CCD cover slip is stationary on the screen, despite rotating the camera through 360 degrees.
Watch the video and check you are using the right spot to collimate
Here I show how to get it right