Collimation Terms

updated: 2024-04-17

On‐Axis	Stars at or near the center of an image that are created by light parallel to the optical axis of the scope.
Off‐Axis	Stars around the perimeter of an image that are created by light entering the scope at an angle.
On‐Axis coma	A star image aberration seen in on‐axis stars.
Off‐Axis Astigmatism	A star image aberration existing when stars images are not symmetrical about the center of the image. Inside‐Focus.
Out‐of‐focus image	where the scope focal plane is past the camera
Outside‐Focus	Out‐of‐focus image where the scope focal plane is before the camera
Round Star	Star shape is round
Oblong Star	Star shape is oval
Pointy Star	Star shape is oval with its major (long) axis pointing towards the center of the image (upper‐right in this example)
Flat Star	Star shape is oval with its minor (short) axis pointing towards the center of the image (lower‐left in this example)
Balanced Image	An (outside‐of‐focus) image that is rotationally symmetrical