How to interpret aberrations ?
Aberrations come in many forms and flavors. The aberrator can generate most of them and the links on the left will show you more .
Defocus: Focussing is needed to get your scope’s focal point and your eyepieces focal-point at the right place. During startesting more light is directed from the inner airy-disc to the diffraction rings as you go from in-focus to either side of focus.
Coma: 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. Misalignment aberrations often can be corrected by proper collimation (see other sites).
Astigmatism: A well known aberration in human eyes but also in telescopes. Again misaligment can be part of the problem, but can be solved. Often asymetrical in the out-of focus images.
Spherical: Comes in two flavours, lower and higher spherical aberrations. Lower spherical is a very well known feature of telescopes. Both lower and higher lead to a significant difference in the out of focus images. One side of the focus is brighter than the other.
Tube Currents: This aberration has nothing to do with the optical quality of mirrors/lenses in a telescope. But it is a telescope induced error that is mainly the effect of the design of a telescope. Especially closed telescopes with secondary obstructions (SCT,MCT) are known to show tube-currents during cooldown. When tube-currents are visible no star-testing should be made.
Turned Down Edge: This is a type of error that is known only from reflectors. In this case the edge of the mirror is having a different curve than the other parts of the mirror. Some people simply cover the outer edge of such mirrors to erase the effects. They reduce the aperture that way but increase the image quality.
Turbulence: The most common aberration astronomers will see. Too often vision will be degraded due to turbulence. The turbulence can be nearby your observation point due to thermal effects in the surroundings (houses). However the higher atmospheric layers also are turbulent. Clouds (even thin ones) and other elements disturbe your views.
Roughness: Due to minor deformations in the mirror made during production a mirror can suffer from roughness. The effects are much alike turbulence except that they are static and will not disappear.