Interpreting what you see in Star Testing
A perfect system shows a series of rings of equal brightness.
If extra-focal image appears to be wobbly, or if it has an irregular sort of blobby turbulent appearance, it is probably tube currents in the telescope -- you need to wait until it settles down to equilibrium temperature
If it looks a bit like you are watching the river-bed through a flowing river, it is atmospheric turbulence, i.e. bad seeing. The difference between what you see with tube currents and with atmospheric turbulance is that the "ripples" from the atmosphere tend to move in the same direction. You will probably need to wait another night when the atmosphere is less turbulent.
Coma will exist in any non-spherical (e.g. paraboloidal) primary mirror if it is not properly collimated. The intra- and extra-focal images show the rings as brighter on one side than the other.
Astigmatism shows as elliptical rings -- the orientation of the major axis of the ellipse moves through 90 deg between intra- and extra-focal images. Their actual orientation in the eyepiece may vary from that shown in the drawing.
An under-corrected mirror has a bright outer ring inside focus and a bright inner ring outside focus.
An overcorrected mirror shows the reverse of the above -- there is a gradation of brightness through the rings.
If you have an extra- focal image which has a bright outer ring (as does an overcorrected mirror) significantly brighter than the next one (i.e. not a gradation of brightness through the rings), you probably have a turned edge. The intra-focal image will not show a change in brightness and the outer ring appears fuzzy.
Referenced from http://www.astunit.com/tutorials/startest.htm