The Caldwell Objects

Malcolm J. Thomson of Santa Barbara, California,
describes the planetary as having a curved dark
area between the central star and the following
(eastward) edge. Through a 12-inch reflector,
Christian Luginbuhl and Brian Skiff also viewed
this dark region as well as an uneven edge on the
nebula's western limb.
Images taken through large telescopes and
various filters reveal greater details in the
Blinking Planetary. Especially noteworthy is the
presence of a dense elliptical inner disk and knots
at either end of that ellipse embedded in an outer
halo. Would an Oxygen-III filter help amateurs
bring out further details in NGC 6826 with their
moderate-sized telescopes? I have not seen any
reports of this. In 1995 Austrian astronomer
Gerald Handler (Vienna University) announced
that several relatively cool central stars of
planetary nebulae, including those of NGC 6826
and NGC 6543 (Caldwell 6), show brightness
variations of up to 0.2 magnitude on time scales
of several hours to days. Amateurs with access to
large telescopes and photometers might want to
try their hands at recording these fluctuations.
With Mira, his 16-inch Dobsonian, Jay
McNeil has recently made one of the most
detailed visual studies ever performed on the
Blinking Planetary. He spent a total of about 2½
hours studying and drawing this object on two
separate nights. On both nights the seeing was
better than 1" and the transparency quite good,
and the most pleasant and revealing view came
at 586x. A power of 855x was also used at times to
glimpse even smaller, more intricate details.
According to McNeil, the nebula's brightest part
was an elliptical oval (roughly 30" x 25"), oriented
east-southeast and west-northwest, around the
central star. This oval ring was mottled near the
tips of its major axis and, during moments of
excellent seeing, McNeil detected a sharp
70
15
inner ring about 10" in diameter. The area
between the inner ring and the central star was
darker than any other part of the nebula but not
as dark as the background sky. McNeil also
sighted the southeastern FLIER complex, which
appeared as a very slightly nonstellar nebulous
"knot" less than 3" in extent at the
very edge of the outer envelope's easternmost
point. "With a Lumicon Deep-Sky filter applied at
586x," he says, "this [feature] became nearly as
apparent as the nebula's filtered nucleus at
times." He also sighted the much fainter
northwestern FLIER complex, which looked like
a diffuse knot. At 855x shell structure within the
nebula seemed quite complex, and the view was
reminiscent of NGC 7009, the Saturn Nebula
(Caldwell 55). At times the FLIERs extended very
slightly beyond the nebula's bright outer
envelope "as if the very edges of the major axis
consisted of small 'nipples.'" With extreme
averted vision a low-surface-brightness halo
surrounded the entire structure out to 35". The
individual FLIERs or
Deep-Sky Companions: The Caldwell Objects