Monster PNe Sharpless 2-174 and Sharpless 2-216

[Uwe Glahn]

Last new moon I could visit an high alpine location with nearly perfect sky conditions. Because of the bad seeing I tried some lager PNe.

First candidate was Sharpless 2-174 (PNG 120.3+18.3), in my knowledge the 16'th largest PNe. With 27" + 113x + [OIII] is was visible as a brighter and large diffuse nebula with direct vision. The conical form shows a hard and good defined NW edge and a diffuse S end.

Sh2-174.jpg
inverted version

Next one was Sharpless 2-216 (PNG 158.6+0.7). With an diameter of ~1,7° the second largest PNe in the sky. The first attempt with the 27" failed because of the small maximum field of only 0,7°. Also the brighter edge was not visible. Than I tried it with my small grab and go 4" f/4 Newton + [OIII]. The field with 21x was nearly 4,7° (21mm Ethos, thanks Marc, 4,8mm AP) - a perfect starting point. To my surprise the PN was immediately to see without any problems. The brighter E edge was visible with direct vision. With averted vision the PN becomes a round glow, a really spectacular view that I never expected.

Sh2-216.jpg
inverted version

[reiner]

Hi Uwe,

my results for Sharpless 216 were similar as yours. I could not detect it with my 22" as the field is too small. I could see it with my 14" some time ago. And, similar to your observation, I could see it with my 80mm finder and UHC, though it was not easy.

Sharpless 174 appeared to me with OIII filter elongated, but very diffuse. I could not see a sharp NW edge as in your sketch, it was too faint.

This PN has a funny behavior. If you switch from OIII to H beta, the faint patch moves. Sharpless 174 is one of the large PN with strong interaction with the interstellar matter (ISM), which distorts the PN shell. The OIII emitting region is that around the CS, while the trailing shell is mostly HII and hence visible with H beta (though I could not see any structure in it), which is the reason for the "jumping" PN. With UHC, both parts are visible and the PN appears fairly round. This observation was together with Matthias Kronberger on my 22".

Here is an animated gif of this "jumping" PN, based on OIII and HII frames by Stephane Zoll



Here is a paper about this runaway PN
http://adsabs.harvard.edu/full/1994AJ....108..978T