NGC 7293 'Helix Nebula'
Object type: Planetary Nebula
Size: diameter ~ 12.8'
Visual Brightness: ~ 6.5
Constellation: Aquarius
Date: 2007-07-15/18
Location: Postalm/Austria (1300m)
Telescope: TMB 80/480 (3.1" Apo f=480mm) + 0.8 TV reducer/flattener
Camera: Starlight XPress SXV-H9
Astronomik H-alpha (13nm) and OIII filter
Image type: HasGOIII
Binning: all 1x1
Exposure time: Ha
12x15m = 3h00'
OIII: 7x15m = 1h45'
Exposure time total: 4h45'
Discovered by Karl Ludwig Harding before 1824.
The Helix Nebula is one of the closest of all planetary nebulae: Lying at a distance of perhaps 450 light years, it is the only planetary nebula for which a parallax could be obtained by ground-based observations. Nevertheless, its distance is quite uncertain: The first determination by A. Van Maanen yielded about 85 light-years, Becvar (1961) has 590, L. Kohoutek (1962) 280, I.S. Shlovskii (1956) and P.A. Ianna & H.A. McAlister (1974) 160, the Sky Catalogue 2000.0 gives about 300 ly, and C.R. O'Dell (1963) obtains 450 light-years.
It is also one of the apparently largest planetaries known: Its apparent size covers an area of 16 arc minutes diameter, more than half of that of the full moon; it halo extends even further to 28 arc minutes or almost the moon's apparent diameter (These dimensions were taken from Stephen J. Hynes who quotes AAT and ESO photos). Although the nebula is quite bright, its light is spread over this large area so that it is not an easy object for visual observing; the Herschels have apparently never cataloged or observed it.
A 3D model of the nebula may be found here: http://hubblesite.org/newscenter/archive/releases/2004/32/image/a/format/web_print/
More information may be found at:
http://seds.org/messier/xtra/ngc/n7293.html