 Abell
31 is a large faint planetary nebula that is best viewed at low
magnification. An OIII filter is required to see it in all but
the largest scopes. Many observers report seeing a large,
round glow. Look for a prominent parallelogram of four
10th magnitude stars. The brightest portion of the nebula
envelopes the south-eastern star, extending toward the star to the
southwest.
In my 18-inch at 94x I
at first only had a very vague impression that anything was there,
even with the OIII in place. Scanning the field helped, as the
glow often jumped out at me as it came into view, only to become
vague yet again. The overall impression was of an egg-shaped
glow elongated in the east-west direction. In time I began to
suspect two "streaks" that formed a narrow elongated
horseshoe shape. This horseshoe looped around the
south-eastern star. The H-Beta filter also brought out the
nebula, although not quite as well as the OIII, and the
"horseshoe" wasn't nearly as obvious. I did suspect
the vaguest impression of a glow without a filter, but I could not
be certain.
The morphology of this
planetary nebula is very interesting. It is apparently an
ancient planetary that is interacting with the surrounding
interstellar medium. Note how the color DSS image (right)
shows both red and blue portions to this nebula. The blue
portion sits close to the progenitor star and shines strongly in
OIII. The southern (top) portion is bounded by a wide
"bow shock" where the nebular material is apparently
interacting with the surrounding medium. 
The field in an 8-inch at 35x.
North is down and east is to the right. |
