Thanks for the comments.
My processing regime was based on Paul Haese's
article, using very similar settings.
Started with 1783 frames, processed in Registax5, final stack of about 900 frames, saved as TIFF. I have only adjusted wavelets 2 & 3 , approx. 13 & 22. No other adjustments in Regaistax.
The ring becomes visible even then.
I then used AstraImage 3, Lucy Richardson Deconvolution PSF size 0.7, iterations 3. Gamma adjusted down a fraction,minimal unsharp mask.
Photoshop CS5, canvas resize, evel adjust - middle slider moved to 0.95, samart sharpen - radius 9.8, 19% & saved for the web.
Is it possible that the ring is caused by misalignment of some frames?
Question regarding exposure:
Using IC capture I try to fill the histogram as much as possible. Usually set frame rate to 30, gamma to 100 & adjust gain to get nice histogram. In this case gain was around 875 from memory. Is this the best approach?
I've tried using high gain setting ( around 1000) as some people suggest, but end up with very grainy images.
Thank you again for great tips.