Hello all - thanks for the link on how to correct tilt....I clearly have it.
I have now operated the lens at f2, f2.5, and currently f3.5. I believe there is a significant improvement particularly in the lower left/right. The existence of tilt in the upper half of the image is clearly discernible now, so I will attempt a fix on that.
If I can null that out, it will be interesting to dial back the lens to f2 to see how things may have improved there.
I will keep you posted, but here is the result so far (full size FIT - 16Mb)- so, this is the 50mm L lens, stopped down to f3.5, 30 minutes exposure, ST8300, Baader 7nm Ha filter.
http://www.martinpughastrophotograph...d%20ST8300.fit
thanks for all the pointers.
Martin