Aberrations come in many forms and flavors.
The aberrator can generate most of them and the links on the left will show you more . Focussing is needed to get your scope s focal point and your eyepieces focal-point at the right place. Due to misalignments Coma effects can be prominent in a telescope. Stars appear like comets having tails on one side. If you do not center the star used in startesting coma effects might be induced from your eyepiece.