Hi Bill, there are a few producers of clip in filters that insert between the sensor and bayonet mount, like my favourite
http://www.astronomik.com/en/clip-fi...ml?___store=en
But they're only for canons and some do Nikon. You may need to jump ship as canon seems far better supported generally. Bit if you are already thinking narrowband, I'd recommend a cheap mono to try out or a one shot colour ccd or CMOS that will be more sensitive