Sky & Telescope - February 2021 - 22

Colorful Planetaries

(Continued from page 19)
some make great targets for imagers with smaller instruments. Here are some of my favorites.

Skulls, Owls, and Squids

FEBRUARY 2 021 * SK Y & TELESCOPE

S COLD TARGET The small but bright planetary NGC 2392 is a great
short-exposure target for long-focal-length telescopes equipped with
small detectors. South is up in this image.

S SPIRALING NEBULA The Helix Nebula is a large, eye-shaped
planetary that displays a wide color palette and fine detail in most any
imaging setup.

NGC 2392: A DA M BLOCK / M OUN T LEM M ON
SK YCEN TER / UNIV ERSIT Y OF A RIZON A

Let's start our planetary nebula hunt in central Ursa Major.
Messier 97 (the Owl Nebula) is a top-tier imaging target for
amateurs. Its 2.8′ diameter may be on the small side, but its
proximity to spiral galaxy M108 (about 48′ northwest) makes
for a great composition if you include both targets by using a
small, wide-ﬁeld instrument with at least a 1°-wide ﬁeld of
view. The Owl displays a reddish shell with a bluish or greenish inner region and two dark voids making up the eyes that
give the Owl its distinctive nickname. Extremely deep narrowband exposures show a very faint outer halo that expands
the nebula's size beyond the stated diameter - a common
refrain for many of these targets.
Hopping over to Perseus, Messier 76, commonly referred
to as the Little Dumbbell, has the distinction of being one
of the faintest targets in the Messier catalog. Nevertheless,
like many PNs, its light is concentrated in a relatively small
area and is a great target for imagers with long-focal-length
optics. But imagers with wide-ﬁeld instruments shouldn't
pass over this tiny target, because it resides in a ﬁeld ﬁlled
with faint hydrogen-alpha (Hα) emission nebulosity. While
visual measurements put the nebula at slightly more than 1
arcminute across, deep photographs show it to be more than
twice that size. M76 sports a mix of red and teal hues. Highresolution images give this complex nebula the appearance of
a baby's teething ring.
Heading south to Cetus, you'll ﬁnd NGC 246 - a challenging, faint target. Spanning 4.1′, NGC 246 is a larger PN,
requiring lots of exposure to display the intricate, mottled
details in the nebula's center and around its edges.
A small, bright PN that's well-placed for late winter/early
spring imaging is NGC 2392 in Gemini. Northern Hemisphere amateurs can relate to its appearance - NGC 2392
reminds me of a cold observer wearing a fur-lined parka
staring back at me. As one of the smallest planetaries on my
list, you'll need a long-focal-length scope to resolve this PN's
complex inner arcs surrounded by a striated circular halo.
Fortunately, NGC 2392 has a relatively high surface brightness and shows up in short exposures.
If you want a break from imaging galaxies early in the
year, try photographing one of my personal favorite planetaries: Abell 31, or Sharpless 2-290. Located in Cancer, it has
one of the largest apparent sizes of any planetary, spanning
about half the Moon's diameter. That means that images
taken with even modest instruments can reveal a wealth of
detail. Abell 31 is quite faint, so you'll need a lot of exposure
time on this object to get a high-quality result. Frames shot
with a Hα ﬁlter display a distinct, sharp, shock front, where
the nebula is crashing into the interstellar medium on the
southern edge, while the northern part appears diffuse. O III
emissions dominate the inner, bluish area, and large instru-

Sky & Telescope - February 2021

Table of Contents for the Digital Edition of Sky & Telescope - February 2021