NGC 246 in Cetus is a reasonably large planetary nebula, about 4 min arc across. Magnitude about 8.5. Slightly brighter in OIII (blue) than in H-alpha (yellow). SII was insignificant, and we have not included any.
We've managed to resolve the mag 12 white dwarf separately from its somewhat brighter binary companion.
For the answer in the back of the book, see the stunning, jaw dropping,
Gemini South shot.
According to the Gemini shot docco, the edge at the top of the frame is leading, creating a stronger shock as it hits interstellar medium, and that is why it is noticeably brighter than the bottom edge.
H-alpha 8hrs, OIII 8hrs, both in 1 hour unbinned subs. Field 11 min arc, 0.55 sec arc/pixel. Seeing was just under 2 sec arc.
Cheers,
Mike and Trish
Edit: Trish did a reprocess with H-alpha = red, OIII = turquoise, and slightly less noise reduction. I think it's better.
Mike