SH2-71 is a curious planetary nebula. Its bi-polar structure isn't initially apparent. But looking closely at the heavily stretched and starless Ha data, two extended bits of nebula can be seen.
These areas can be seen better in this image by the 2.5 meter Isaac Newton telescope:
https://academic.oup.com/mnras/article/489/2/2195/5551494
The article says this was shot with a filter that passes both Ha and NII and they don't know which bandwidth is responsible. I'm seeing it, albeit faintly, in my 5nm Ha filter.
That same article also discusses the possibility that this is a three star system. It seems there only one confirmed triple system planetary nebula: NGC 246. This discussion of the challenges the system presents certainly brought to mind the basic premise of the Three Body Problem trilogy!
I didn't see this very faint part of the nebula until I was nearly done processing several months after finishing the data capture. My hesitation in processing was that I found the nearby Ha emission cloud really interesting but wasn't sure if it would work in the composition. Lately I've been doing a lot of RGB-SHO compositions so I thought I'd see how that worked. The RGB itself looks remarkably like the HOO version. The SII is very weak. I started with continuum subtracting the SHO data but it didn't seem to produce any meaningfully different structures. Then I tried subtracting the Ha from the Red and the nebula just disappeared, leaving an even background.
With so little difference between the NB and BB data I decided to try combining them. Red is R+H+(Sii-Sii background), Green is G+O*.2, B is B+O. This delivered the colors you see. I did lose some of the nebula's structure with this mix so I added back some of the Ha to the luminance. Stars are pure RGB.
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