In short, wow. I use the 13nm H-alpha filter in-between a DSLR and MPCC. Because it is T-threaded, I can add and remove filters without disturbing the orientation of the camera, and without adding a lot of extra backfocus to the imaging train (which would throw off the spacing for the MPCC. This filter allows me to take subframes that are essentially as long as I like, and really does the job on nebulae. For my purposes, the filter is sensibly perfect.