The Caldwell Objects

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Blinking Planetary
NGC 6826
Type: Planetary Nebula
Con: Cygnus
RA: 19 H 44 M 48.2 S
Dec: +50° 31' 30"
Mag: 8.8; 8.5 (O'Meara)
Dim: 27" X 24"
Dist: ~2,200 light-years
Disc: William Herschel, 1793
W. HERSCHEL: [Observed 6 September 1793] A bright point, a little
extended, like two points close to one another; as bright as a
star of the 8.9 magnitude, surrounded by a very bright milky
nebulosity suddenly terminated, having the appearance of a
planetary nebula with a lucid centre; the border however is not
very well defined. It is perfectly round, and I suppose about
half a minute in diameter. It is of a middle species, between the
planetary nebulae and nebulous stars, and is a beautiful
phenomenon. (Η IV-73)
GC/NGC: Planetary, bright, pretty large, round, a star of
magnitude 11 in the middle.
WHILE HE WAS WORKING AT THE HARVARD-
Smithsonian Center for Astrophysics in Cambridge,
Massachusetts, in the late 1970s, Peter
Collins took me into the modest dome of the 9inch
Clark refractor nestled on the building's roof
and pointed the telescope to the northern wing of
Cygnus, the Swan. "Have you ever seen the
Blinking Planetary?" he asked. The answer was
"no," but his words set my mind racing. "How
can a planetary blink?" I wondered. The object he
showed me, of course, was NGC 6826. The reason
that planetaries of this type blink is simply a
matter of where the object's telescopically
harvested light hits the observer's eye.
68
Most planetary nebulae have a dense inner shell
surrounded by a fainter outer halo. When you
look straight at a planetary, the fainter outer
envelope disappears because its light falls on the
eye's fovea centralis. The fovea is populated
largely by cone cells. Cone cells are responsible
for our visual acuity, but they provide little light
sensitivity. To see dim objects we rely upon the
rod cells lining the surrounding retina. There are
rod cells near the fovea centralis, and they will
pick up relatively bright objects like stars and the
condensed regions of planetary nebulae. But dim
objects will not register with them. However,
when you use indirect, or averted, vision, light
from a planetary's dim
Deep-Sky Companions: The Caldwell Objects