A closer look at your image seems that you may actually have Astigmatism. Here is a brief description of Astigmatism of the web :
"
Astigmatism An optical defect in which star images are elongated into ovals which change from a radial orientation (pointing towards the centre of the field) to a tangential one (at right angles to the centre) as the observer moves the telescope focuser from one side of the best focus to the other. Should not be confused with the star images of an out-of-collimation telescope, which may also be oval but which will not change orientation as the observer passes through best focus." taken from here http://www.aoe.com.au/telescope_dictionary.html .
Nice read.
Theo.
