Sky & Telescope - March 2021 - 61

the barred spiral galaxy M108, the Owl lies 2.3° southeast
of Merak, or Beta (β) Ursae Majoris, just outside the bottom of the Big Dipper's bowl. It appears as a large, diffuse
disk that's relatively bright. In smaller scopes, the " eyes " may
not be readily apparent, but they're very prominent in larger
apertures. At an estimated distance of more than 2,000 lightyears, it would have a diameter of 1.7 light-years.
Nebula ﬁlters enhance the visibility of most planetary
nebulae, and this is true for M97: It's much easier to detect
with a ﬁlter. Some planetaries require the use of a ﬁlter to
detect them at all, while even the brightest and easiest usually
show some additional structure when a ﬁlter is inserted in
the light path (see sidebar below). The table on page 64 lists
the ﬁlters I used for the observations discussed here - the
entries in brackets indicate that the listed ﬁlters aren't necessary for enjoying the view but might improve it.
Located just 8° from Polaris, IC 3568 in Camelopardalis is available to northern observers all year but is ideally
placed during spring. This is a relatively young planetary at a
distance of around 9,000 light-years, and its apparent size of
10″ yields a diameter of about 0.4 light-years. Several American astronomers have contributed to our understanding and
enjoyment of this planetary nebula. The ﬁrst, Robert Aitken,
discovered this planetary on August 31, 1900, while observing Comet Borrelly-Brooks (1900 b) from Lick Observatory.
The planetary has the appearance of a uniform, gray or bluegreen disk, with a visible central star. Employing a nebula
ﬁlter will enhance its visibility. Lawrence Aller described it
as the " theoretician's planetary nebula, " acknowledging its
presumably simple spherical morphology. The Hubble Space
Telescope image of IC 3568, however, shows an intricate
structure, which inspired James Kaler to coin its nickname:
The Lemon Slice Nebula.

S ONE NAME, TWO OBJECTS Earlier observers associated the planetary nebula with the open cluster - in fact, both objects bore the same
designation, NGC 2818, until the cluster gained a new listing, Collinder
206. In fact, the notion that the planetary nebula's progenitor star was a
cluster member prevailed until recent studies showed the two objects to
be at different distances, with the planetary in the foreground. Mid-northern observers should be able to access this nebula in southern Pyxis.

The catalog designation NGC 2818 refers both to a
planetary nebula in Pyxis and the open cluster (also listed as
Collinder 206) that lies adjacent to it. Scottish astronomer
James Dunlop, observing in New South Wales, Australia,
discovered them both on May 28, 1826. Apparently, Dunlop
assumed that the nebula was a condensation of nebulous
material associated with the cluster. John Herschel identiﬁed
it as a planetary nebula when he observed it on April 7, 1837.
NGC 2818 was widely assumed to be associated with the cluster - in fact, the progenitor star was assumed to be a cluster

DIE T M A R BÖCK ER / ER NST VON VOIGT / CA PELL A OBSERVATORY

Which Filter?
Two types of filters are generally useful for observing planetary nebulae.
They can be grouped into deep-sky
ﬁlters, like the O III and hydrogen-beta
filters, and light-pollution ﬁlters such
as the Ultra-High Contrast (UHC)
filters. They differ in how much of the
visible spectrum that they pass. O III
filters usually have a bandpass about
10-15 nm wide and transmit the bluegreen light emitted by doubly ionized
oxygen at 496 nm and 501 nm. This
O III emission is the principal radiation
emitted by planetary nebulae and an
important criterion for classifying an
object as such. A hydrogen-beta filter
blocks most light except a thin band

centered on 486 nm, the wavelength
emitted by hydrogen atoms in a
nebula. The wider bandpass of the
popular UHC filters is usually about
20-30 nm wide, which includes both
the O III and hydrogen-beta lines. The
effectiveness of these filters varies;
some objects " respond " to one filter
more than another. Very few planetary nebulae benefit from use of the
hydrogen-beta filter, whereas an O III
or UHC filter will improve the contrast
significantly in a majority of targets.
For some objects, the improvement
will be subtle, but in others, a filter will
mean the difference between detection or invisibility.

Filters are usually employed by screwing the filter into the eyepiece barrel.
Happily, commercially available filters
and eyepieces from most manufacturers have compatible thread sizes. A
second strategy is to manually insert
the filter into the light path by holding it between the eyepiece and the
observer's eye. This technique has the
advantage of allowing a rapid comparison of the filtered and unfiltered
view, which in effect causes emission
nebulae to " blink " (appear to brighten
due to the contrast gain of the filter).
The disadvantage is that " blinking " a
filter like this effectively is a bit of an
acquired skill.

sk yandtelescope.org * MARCH 2 021

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Sky & Telescope - March 2021

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