having spent the weekend surrounded by people imaging the horsehead, i should retract my comment about the sharpening artefacts.. the brightening around the edge of the horsehead is clearly a real feature. not the first time i've mistaken features for artefacts.. i was once cloning out bok globules from a nebula before i realised my mistake
anybody listening should also probably take my comments about sub-exposures with a grain of salt too. if you're chasing really faint stuff then photon statistics start to count against you with shorter subs.
i'll go and crawl back under my rock and hand back over to the experts..