Tuesday, 17 May 2011 | Piotr
This is quite handy filter – for visual it cut's out the light pollution noticeably. The visibility of the nebula is improved, but much less than with O-III filter (on O-III glowing nebulas). If you start looking off-axis then you may see that stars can look like split red-green sometimes.
For imaging nebulas it is also quite good as it pass O-III/H-beta and H-alpha regions. I used it as a nebula luminance filter and shot R,G,B with this filter stacked to get quasi-narrowband nebula look (higher contrast of M27 vs the background sky).