I think I have come to the end of using this lens. I got very good results finally with the lens at f4.5 with very good stars across the field.
I put the lens back on to the Canon 550D, as a trial to see what sort of results I got there, and much to my surprise the excessive off axis aberrations were back - with the lens at f4.5. So what I achieved with the ST8300 did not replicate on the DSLR.
Anyway - one lesson - I think buying anything less than a full frame DSLR for astrophotography is a waste of money and time.
cheers
Martin
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