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   astrodon on November 15, 2015 at 10:09 am

   Great question but not easy to answer. The answer starts with doing a web search on you target to find out what it’s emitting. OIII3 is our preferred filter for any object with oxygen emission. So, it comes down to wheterh you use a 3 or 5 nm H-a. IF the object has considerable NII, then you may want to use our 5 nm H-a that will capture both H-a and NII. If it doesn’t then you can get better contrast and fainter objects with the 3 nm H-a. Some of my planetary nebula have little H-a and mostly NII. In that case I’d either use our 5 nm H-a or our NII 3 nm filter. I have a 10-slot filter wheel with 3 and 5 nm H-a and NII. Don

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