 NGC
7139 in a very overlooked planetary nebula in Cepheus.
Although it has been detected in a 4-inch telescope, most observers
will require at least a 6-inch and dark skies. And while this
planetary isn't faint enough to be considered a challenge object,
the surface brightness isn't all that high (probably somewhat
fainter than the 22.2 mag/arc-secē
quoted above).
In
smaller apertures (8 inches or less) this will probably be an
averted vision object, appearing as a faint round hazy patch of sky,
just a bit brighter than the surroundings. A UHC or OIII
filter will help bring it out, particularly at low magnification
(less than 100x) or with less than dark skies.
In my
18-inch f/4.5 it appeared as a "faint round smudge" at 94x
without a filter. I was able to recognize the nebula without
having to identify the field stars but it is faint enough that it
could have been easily overlooked. A better view came at 270x,
where it was readily seen, but frustratingly faint as far as seeing
any detail. At this magnification it appeared as a soft round
glow with hints of some sort of central structure. No central
star was observed. An OIII filter helped, even at such high
magnification. Averted vision brought out tantalizing hints of
internal structure that I simply could not pin down.
Having
never seen an image of this planetary I wrote in my log, "I bet
the image is cool." Looking at the image above, I was
probably seeing hints of the faint stars that lie within the nebula
rather than real detail. Regardless, it was an interesting
view. Like most planetary nebulae this one has a personality
all its own. Be sure to try a variety of magnifications!
The field in an 6-inch f/8
at 60x. North is down and east is to the right. |
