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Hi Tony,
I have never seen anything like this before. I know that this probably doesn't help much, but I've done just about everything wrong whilst taking images and I've never seen this before.
The brighter stars look a bit like the image I use for collimation, i.e. just out of focus, though I don't think I have ever gotten such a bright central spot.
What sort of camera are you using? Have you got another, even a guide camera to take an image?
First things first you have to isolate the cause, is it optical or is it an artefact of the imaging system? Chuck an eyepiece in, put the biggest magnification eyepiece in, with a barlow if you've got one and just defocus the image. Do the stars look like you've imaged? If so it's a simple focus problem. If not, then maybe it's an out of focus reflection off some part of the imaging system.
Don't trust focus masks for wide field imaging, there great for high magnification work, but for wide field, isolate a star, put the camera in focus mode and move your focuser back and forth until the star image is the smallest possible, or if you've got software that does FWHM calculations, use that.
When the weather clears next in Melbourne (I'm about to spend four weeks OS, that should help), report back on the results.
Cheers
Stuart
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