Sharpless 2-216 is the closest known planetary nebula and also one of the oldest. Its apparent size of 1.6 degrees is over five times bigger than the main shell of the Helix Nebula. Only its eastern rim is visible (barely) on the POSS. Sharpless 2-216 is probably unobservable, but you never know with guys around like Jay McNeil and Rich Jakiel.
Jim, Jim, Jim, Jim, Jim!!! Oh, this is a good one! I have to hand it
to ya'... Sharpless 2-216 (aka GN 04.41.3, Simeis 288, Marsalkova 44, LBN
742, YM 22) is definitely first class "retinal torture". And there
aren't many PNe in the sky which fall directly into this category via me
;-) I have tried for this object with Mira, my 16"; however, there really
are no "brighter" knots in Sh 2-216 as in Sh 2-240. This thing is very
old, diffuse, and glowing with a fairly even surface brightness not much
brighter than any background sky that I know of. If anyone even suspects
this one, I definitely want to know about it....
The recent posts concerning the planetary nebula Sharpless 2-216 prompted me to put it on my observing list before departing to my dark sky observing site in rural Florida this weekend. I had little hopes of seeing anything in the position of this extensive nebulosity but, as I should have learned long ago, the night sky can surprise a jaded observer like myself ...
Using my 18" f/5 reflector and a 35mm Panoptic eyepiece, I swept the field of Sh2-216 and located the area of the bright eastern rim (based on the inspection of a RealSky image and Parker's Emission-Line Survey of the Milky Way. With no filtration the telescope revealed nothing unusual - only a fine Milky Way star field . I suspected that an [OIII] filter would be of use as the Parker book includes an image of this object in [OIII] light that shows a relatively bright glow.
The addition of the [OIII] filter darkened the field from a luminous gray to black and I quickly spotted a dimly glowing vaguely defined filament about 45 arc minutes long and perhaps 20 minutes wide running north to south. Rich Jakiel, my observing buddy that made the trip with me, thought the filament to be only 10 to 15 arc minutes wide. We carefully swept over the filament to the surrounding jet black sky and upon returning to the area of the filament would note a persisting glow in the same spot. I was still skeptical - it wasn't supposed to be this easy ! Removing the filter, I thought I might be seeing a Milky Way patch that would appear nebulous through the [OIII] filter - but in 'straight' light there was no clustering of stars to deceive us. I slowly accepted that the glow was the 'impossible' planetary and it was not the exceeding difficult (or invisible) object I had suspected it would be.
I returned to the area again the next evening just to confirm my observations and try other filters on the planetary. Although this evening wasn't quite as transparent as the previous one, I again spotted the filament and I put my doubts to bed - Sh2-216 is in the realm of the visual amateur astronomer with a moderately large Dob and good skies.
For the curious , my observing site in Florida is only a few feet above sea level and suffers very little light pollution . The naked eye limiting magnitude is close to the 7th magnitude on good nights although the variable seeing conditions were frequently poor on these evenings. The position of the filament 'center' in 2000.0 coordinates is 4h 45m 35s, +46 48m 45s. Happy hunting!--Dave Riddle