The Caldwell Objects

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agrees with the value published by Christian
Luginbuhl and Brian Skiff.
NGC 2392 takes magnification well, so use as
much as your night-sky conditions will allow.
Don't be afraid to crank up the power to 100x per
inch of telescope aperture. (I did not do this with
my 4-inch for the sake of consistency with the
other observations reported in this book.)
Through the 4-inch at 130x the nebula displays a
crisp nucleus surrounded by a sharp inner shell
and a fainter outer extension. Careful study of the
inner envelope at this magnification shows two
discrete regions of intensity to the northwest and
north and a third knot or enhancement toward
the southeast. These features tend to warp the
shell's circular appearance into an ellipse. Pay
extra attention to the northern enhancements, for
I find they have a beaded or feathery texture.
Through the 4-inch the outer halo is rather
uniform, and
158
none of the radial patterns seen in observatory
photographs can be detected at 130x.
Owners of larger telescopes should be prepared
to spend hours with this incredible planetary. At
the 1998 Texas Star Party I studied NGC 2392
with McNeil and his 16-inch f/5.1 Dobsonian. We
used astonishingly high powers, up to l,200x, and
were rewarded with some striking details. The
central star appeared as a solitary sun floating in
a bay of darkness fringed by a crisp inner ring.
Three distinct beads and two arcs of light
punctuated the inner ring, though there was no
hint of duplicity. The outer halo appeared as a
pale green sea that brightened suddenly into a
mottled garland at the nebula's outer edge. Try as
we might, we could not see the Eskimo's face,
and I do not know of any visual observer who
has.
Before moving on to the next Caldwell
object, return to low power and move your
telescope a little more than 2° northeast of the
Eskimo, where you will encounter one of the
sky's truly neglected deep-sky objects. NGC 2420
is a magnitude-8.3 open cluster whose southern
edge sits right on the ecliptic. It is hardly ever
mentioned in the amateur literature. In the 4-inch
at 23x it's a lonely sight — perhaps two dozen
11th- to 12th-magnitude suns arranged like fine
mesh around a dim 10'-wide cometlike glow.
Increasing the magnification only enhances the
effect. The cluster looks like a chipped and dusty
diamond that has lost its luster.
Deep-Sky Companions: The Caldwell Objects