Light pollution is a major issue for most of us who star-gaze - see http://www.darksky.org/
While looking for specialised filters to help with this problem I discovered "front filters". These filters fit into the lens mount of the camera body and, counter-intuitively, are behind the lens.
With apologies to those of you who already knew about them, there is a nice illustration of the technology at http://sciencecenter.net/hutech/filters/ffilter.htm
. This also shows how, on unmodified bodies, there is an infra-red filter protecting the camera sensor which means that dust removal is safer.
As I see it the advantages of these filters is that one size fits all lenses potentially saving big bucks and that the filter doesn't rotate even if the front of the lens does while focussing. The downsides are that they can't be used with lenses that protrude into the camera body and, obviously, they do nothing to protect the front of the lens.
Has anyone had experience with these filters?