Monday, May 23, 2011
NGC 6781 (top) is a planetary nebula (PN) in the constellation of Aquila. The main ring measures approximately 2 arcminutes in diameter and is about 1 kpc distant. The true 3D geometry of a PN cannot usually be grasped from images alone and studies have indeed shown that NGC 6781 is a toroidal shell (bottom, Schwarz & Monteiro 2006) typical of so-called bipolar nebulae. If you look carefully you can make out numerous filaments of dust which are responsible for the patchy reddening observed in this nebula. NGC 6781 was observed on 10 May 2011 with RSS in Fabry-Perot mode which scanned through the emission lines Halpha 6563 (green), [NII] 6583 (including HeI 6678, red) and [SII] 6730 (blue). The ionising central star is best shown at bluer wavelengths (see here).
Credit: Observers were Ted Williams, David Buckley, Brent Miszalski (processing and write-up), John Chisholm and Nicola Loaring.