The Caldwell Objects

known as NGC 2244; and NGC 2239 should be
struck from all catalogs and atlases.
To find the Rosette, use the same stellar
guideposts that lead to Hubble's Variable Nebula
(Caldwell 46). Follow the line of naked-eye stars
that flows southwest of 3rd-magnitude Xi (ξ)
Geminorum: 15, 13, and 8 Monocerotis. The
Rosette is a fuzzy patch of light 2° east-northeast
of 8 Monocerotis. I can see the cluster easily with
the naked eye from Volcano, Hawaii, and I can
see the nebulosity that way too, with averted
vision; confirm your own naked-eye sightings
with 7x35 binoculars. My naked-eye magnitude
estimate of 4.4 for NGC 2244 includes the star 12
Monocerotis, while Archinal's estimate
(magnitude 4.8) does not. Since there is no way to
visually subtract 12 Monocerotis from the nakedeye
view, my estimate better reflects what is
actually seen.
I find it fascinating that no one noticed or
suspected nebulosity around this naked-eye
cluster until the late 1800s. That I can see the
glow in 7x35 binoculars is even more astounding,
since the nebulosity was first picked up with
a 48-inch reflector, and only a tiny piece of it at
that. In the 4-inch at 23x there is no mistaking the
Rosette's existence, though the nebula looks like
a pale specter of glowing gas riddled with dark
lanes. One could argue that knowing the nebula
exists helps one see it. But consider this: when
Walter Scott Houston persuaded Canadian
amateurs Fred Lossing and Rolf Meier to look for
the Rosette with a 16-inch reflector, the team
reported that they could only barely see the
brighter parts. By the late 1970s, however,
observers were seeing it consistently in 12-inch
and larger telescopes, and Houston later saw it
with a 5-inch "apogee scope." I can only conclude
that the nebula is so close to the threshold of
visibility that even the slightest light pollution or
atmospheric contamination will obliterate it. I
once thought
The Caldwell Objects
49 & 50
that the sea-level atmosphere was dense enough
to render the nebula invisible, but this is not true.
While observing the 1999 Leonid meteor shower
from the southeastern coast of Hawaii Volcanoes
National Park, I took a break to look at the
Rosette Nebula with my naked eye and had
absolutely no trouble seeing it. Light pollution,
rather than altitude, seems to be the biggest
obstacle to seeing the nebula.
Of course, any arguments about the nebula's
visibility were squelched during the nebula-filter
revolution of the 1980s. An ultra-high-contrast
(UHC) filter, for example, is designed to transmit
the emission lines of doubly ionized oxygen at
wavelengths near 5000 angstroms. Such a filter
pulls the nebula out from the sky background
and makes it visible to the otherwise naked eye
— even from some light-polluted regions. Under
truly dark skies like Hawaii's, however, there is
absolutely no need for such a filter. The trick is to
use averted vision or to block the cluster with a
foreground object, like the roof of a house, and
look for the nebulous glow; this latter trick is
particularly effective at revealing the nebula's
northwestern segment, which I estimated to
shine at around 5th magnitude. Alternating
between averted and direct vision will cause the
nebula to swell and contract, respectively.
Altitude does help, of course, and I am
amazed at how much detail the 4-inch reveals at
4,200 feet. At 23x, with a nearly 3° field of view,
the nebula's northwestern section is most
prominent and looks smoky gray. If I concentrate
on that region alone, black knots of obscuring
dust suddenly pop into view, and the longer I
look, the more prominent they become. It's an
amazing, almost mesmerizing, effect — seeing
dark material slowly appear within a smooth
glow, then overtake it, as if it had eaten its way
out. I have to pull away from the eyepiece to
break the spell, because the
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