Astro filter
http://www.lumicon.com/sto…/…/15-LUMICON-Nebula-Filters.aspx
http://www.astrodon.com/narrowband.html
http://www.astronomik.com/en/datasheets.html
http://icas.to/space/index-e.htm
http://www.astronexus.com/node/4
http://www.prairieastronomyclub.org/…/some-available-light…/
http://www.koenvangorp.be/deepsky/filters.html
http://www.astrosurf.com/buil/filters/curves.htm