The Caldwell Objects

37
Collinder reversed the two dimensions again in
his catalog; his Collinder 416 (NGC 6882)
measures 20' and his Collinder 417 (NGC 6885)
measures roughly 8'. The positions Collinder
gives for the two objects are precisely the same
positions that Trumpler gives for NGC 6882 and
NGC 6885. In other words Collinder's position
for NGC 6885 is the same as Trumpler's position
for NGC 6885, but Collinder's diameter for NGC
6885 equals Trumpler's diameter for NGC 6882.
And Collinder's position for NGC 6882 is the
same as Trumpler's position for NGC 6882 but
Collinder's diameter for NGC 6882 is roughly the
same as Trumpler's diameter for NGC 6885. "My
guess," Archinal says, "is that Collinder simply
copied Trumpler's positions (since they match
exactly) for NGC 6882 and NGC 6885, not
realizing that his identifications were the reverse
of Trumpler's." Wilson, who has also been trying
to solve this puzzle for years, simply tossed her
hands into the air after she searched numerous
catalogs for answers, but found only more
questions. "No one really agrees with each other
on what NGC 6882 or 6885 is!" she says. "Every
reference in [the] amateur literature gives
conflicting information. It is obvious that the two
clusters have been confused with each other since
[the time of William] Herschel."
What a mess! But forget all the confusion.
Let's wipe the slate clean and look at William
Herschel's original discovery notes. (Such a step
is fundamental in solving any identification
problem; one must always refer to the original
sources so far as is possible.) Just where did
William Herschel position NGC 6885 and NGC
6882?
William Herschel made only one observation
of each cluster on different (though subsequent)
nights. First he discovered the object now known
as NGC 6885. On September 9, 1784, he found
NGC 6885 1' east and 27' south
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of 18 Vulpeculae. Interestingly, Herschel's discovery
position of NGC 6885 is not centered on
20 Vulpeculae but on a point about 6.5' westsouthwest
of that star. In fact, Herschel's
discovery position for NGC 6885 matches the
center of NGC 6882 as plotted on the Millennium
Star Atlas. Furthermore, when I took an unbiased
look through my telescope I saw a faint but
dense patch of stars northwest of 20 Vulpeculae
and a brighter but looser scattering of suns to the
southwest of that star; Herschel's position for
NGC 6885 is roughly midway between these two
concentrations. Now at least one piece of the
puzzle appears to make sense. Without question
there is a cluster of stars (or the semblance of
one) at Herschel's position for NGC 6885, and it
is my recommendation that star charts place a
20'-wide NGC 6885 there. Seen another way,
NGC 6882 in the Millennium Star Atlas should
read NGC 6885.
Now let's look at NGC 6882.
On September 10, 1784, William Herschel
discovered NGC 6882 just 1m 12s east and 12'
south of 18 Vulpeculae. Of it he penned " clus- A
ter of coarsely scattered stars" — which is
uncannily similar to his description of NGC
6885. Indeed, some astronomers have argued
that Herschel's observation of NGC 6882 is simply
a repeat viewing of NGC 6885. Supporting
this allegation is the position of NGC 6882 when
plotted on a star chart. William Herschel positioned
NGC 6882 just south of the star 19
Vulpeculae, where no cluster is evident; this may
explain why many of the early star charts do not
show NGC 6882. Archinal found a small oval
asterism at Herschel's position for NGC 6882 on
the National Geographic Society-Palomar
Observatory Sky Survey and once suggested that
this was the missing NGC 6882. In September
1998, Archinal and I used the 31-inch reflector at
the Warren Rupp Observatory, stopped down to
give us the effective aperture
Deep-Sky Companions: The Caldwell Objects