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Originally Posted by acropolite
I would guess that the vanes being not aligned with the light path as BD suggests would be the cause of the double spike.
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If the veins are angled to the light path they will only produce a wider spike - not a double spike. I also noticed the curve in the vanes in Dave's photo but assumed it could have been due to the camera lense. On closer inspection I think your right Laurie - certainly something to look at. The clear double spikes on the left of the star in Dave's original image don't exactly match the oposing spikes on the right - in that the ones on the right seem to converge together. Perhaps this is strong support for the curved vane that Laurie interpreted. There is still something not aligned to produce those dominant double spikes on the left though - I don't think Registax could missalign them like that in a stack.
Remember that with a four vane spider there are actually 8 diffraction spikes that eminate from the airy disc - it is just that they pair up giving the illusion of only 4 diffraction spikes. A 3-vane spider will have the impression of 6 diffraction spikes eminating from the airy disc.