There is some great information about filters and nebulae at:
http://pages.sbcglobal.net/raycash/filters.htm
Filter Performance Comparisons for Some Common Nebulae
by David Knisely Prairie Astronomy Club
The following is a summary report of visual observations of emission nebulae comparing the performance of various filters intended for such objects. The instrument used is a 10" f/5.6 Newtonian, working at 59x, 70x, and 141x, as well as a few unaided-eye observations using the filters hand-held and looking up at the sky (for Rosette, North America, California, and Barnard's Loop). The filters used were Lumicon's DEEP-SKY, UHC, OIII, and H-BETA, and were usually all mounted in a modified Lumicon multi-filter adapter. This allowed rapid comparisons between filters, thus avoiding some of the judgement problems caused by the time needed to change filters or reports from inaccurate single-observation anecdotal accounts. Observing was done from a dark-sky site (visual naked-eye limit 6.5 to 7.0). For detailed descriptions of the objects, see any of the various observing handbooks. Two methods were used for rating filter performance. In the first method, each filter was given a 0-5 point "Score" performance ranking behind it for each object observed; Example: OIII (4) means the OIII gave a large improvement in the view over non-filter use and contributes 4 points to its overall score total. Items such as overall surface brightness, area of nebulosity observed, and contrast of detail were used to judge how well a filter improved the view. However, since this judgement contains some of the personal preferences of the observer, the exact results may be somewhat subjective in the long run. Different observers might have slightly different ratings of various filters on various objects, so small differences in judgments are to be expected. Still, the scoring does on average give a reasonable idea of overall filter performance.
