I didn't know the outer shell structure in the NW before. But when I remember right I still didn't look at A 21 with my 27", I have to revisit it.
With my old 16" I tried to see the main (double) shell structure as a challenge which is mainly shining in Ha. But with [OIII] it was visible, also like the faint interior of the PN. Howard has sketched the double shell also like Rainer Mannoff with 16" under Moroccan skies. My sketch with 16" is down below.
And Alvin is right, the PN is visible with very small aperture. Together with Martin Schoenball we both observed this PN with a very simple 70mm Reflector which has a spherical mirror. With [OIII] it was visible without any problems and I think it could be visible even with 50mm-60mm or a 50mm binocular with filters.
Reply With Quote