I don't believe these are diffraction spikes caused by obstructions (normally secondary supports etc.) but are probably artefacts produced, when imaging at fast focal ratio, with CCDs fabricated with micro-lenses over the pixels. This was a hot topic in the SBIG forums several years when the ST-10XE (non-micro-lensed) was superseded by the ST-10XME (micro-lensed). Many people who rushed to replace their ST-10XE with an ST-10XME, because of its significantly better QE due to the micro-lenses, could not understand why they were suddenly seeing "diffraction spikes", often only vertically, in images made with fast refractors. It was eventually shown that the effect was only observed at fast f-ratios. The horizontal artefacts on the bright stars are probably caused by the pinching effect described by Ollie. HTH Derrick