Thanks all
LOL, Miaplacidus, as my job as a cleaner I do need to find (and remove) dust, so , yep like finding it in the night sky too
Thanks Bert for the suggestion. The lens has a very nice iris with a large number of leaves, though, as you say I would be better putting an aperature stop in front of the lens so as to reduce light scattering. Its a fantastic idia and I will do it next time.
Scott