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Greg it was Mike S. that convinced me to stop down. I am glad I took his advice. It was me who was pig headed by insisiting on using the lens wide open. The very slight aberrations become quite noticeable with long exposure astro work. The lens is not baffled as well as high end OTA's.
When attempting to optimise a system we scientists call this being stuck in a local minima when the real one is over the next tiny maxima.
Bert
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