i am a fan of black and white images, particularly of this region so i really like this shot. and this is a great focal length for the region too - lots of promise for really attractive details (especially the complex area under the horse's chin). i agree that sharpening artefacts look like they're just starting to show at the top of the head.. but this could perhaps be masked out rather than reducing sharpening across the whole image.
on the topic of longer subs, i'm prepared to be a little contentious and say that i think you might not notice much difference going to 20 min subs, but then i'm biased by too much experience with DSLRs with low read noise. i know that with DSLRs, the read noise and therefore sub-exposure length becomes irrelevant even with greater than 1-2 minute subs. but don't take my word for it.. have a look at the middle row of images in this test:
http://www.philhart.com/canon_test
now i know that CCDs have higher read noise than DSLRs, but it's surely lower with modern cameras than it used to be. and the signal is low with HA so that also increases the desired sub-exposure length.
so in theory, yes longer subs will be better, but in practice i think the difference is so low that i'd go with paul on this one.. you'd only need to lose one 20 minute sub due to problems and you'd have lost more than you'd gained. ie a little increase in total exposure time is worth more than the increase in sub length?
having said all that, i'd love to see an objective test that does show a difference between 10/20 minutes subs with a modern ccd.
phil