Opus Observes: By the Light of the Moon

Note: Opus is my Celestron C5+, a late 1990s vintage 5-inch f/10 SCT.

11:45 p.m. 6 August 2009 PDT (2009-08-07-0645 UT)

The Full Moon is not usually the best time to do deep-sky observing, but
its effect can be made less significant by concentrating on objects with
some higher peak surface brightnesses, like open clusters, planetary
nebulae, multiple stars. I observed one of each on this night, and each
of them a new target for Opus. The sky, in the darkest part in the
north, was perhaps limiting magnitude 4.3, but in the south, near the
Moon, it was much lower than that.

First up was the open cluster NGC 6774. You can navigate to it in a
number of ways. I found it by starting from second-magnitude Nunki
(sigma Sag, at the top of the handle of the Teapot), going through
fourth-magnitude omicron Sag (which lies near somewhat brighter pi Sag),
and then continuing on an equal distance or so. This should get you to
within a degree or so of the proper spot. In darker skies, it's probably
easier to start from fourth-magnitude rho-1 Sag, but I was finding that
difficult to see in my skies, especially with the nearly Full Moon out
in force. A bit of adjustment in the finderscope was sufficient to put
me on my target.

At 32x (24 mm Pan with an f/6.3 focal reducer) shows a pattern akin to a
pair of ram's horns, about a degree or so across and opening up to the
north. NGC 6774 is located at the vertex of these horns. Under these
skies, it was difficult to see much detail directly, although there
seemed to be a gauze of perhaps 30 stars of around the 11th magnitude,
somewhat elongated north to south, and evidently a little offset to the
west.

My next target was the planetary nebula NGC 6818, nicknamed the Little
Gem. It certainly is small, measuring maybe 15 or so arcseconds across.
I found it similarly to how I found NGC 6774--by starting at
third-magnitude Kaus Borealis (lambda Sag, the tip of the Teapot), going
through third-magnitude pi Sag, and then moving on an equal distance,
plus a little bit north. Again, this got me to within a degree or two of
my destination. In the finder, the Little Gem is nestled within a loose
grouping of several fifth and sixth-magnitude stars.

At 130x (6 mm Radian with the f/6.3 reducer), NGC 6818 is small but
evidently non-stellar. I'm sure it would have been more obviously so if
the skies were darker. With the Orion UltraBlock filter, it became still
more obvious; it took a while to refind focus, though, since the Radian
is long enough to touch upon the bottom end of the diagonal barrel.
(Didn't hit the mirror, though!) The nebula seemed slightly elongated
north to south, as the cluster was. There was no clear tint to the
nebula without the filter; obviously, with the filter, it was blue-green
(but then so was everything else!).

The last target for the night was Struve 2375, a tight-ish double in
Serpens (Cauda). It can be located at the fourth corner of a
parallelogram completed by third-magnitude zeta Aql, delta Aql, and
lambda Aql. This was strangely harder for me to find than the other two
objects, possibly because it was higher in the sky at the time and the
last bit of pointing adjustment required me to look up through the
finder. (Could be I'm getting older...) About a degree and a half to the
west is the open cluster IC 4756, which is marked by a sixth-magnitude
yellow star but consists of a dozen or so ninth-magnitude stars plus a
smattering of tenth-magnitude and dimmer stars.

Once I found it, Struve 2375 was easily split; the seeing was pretty
good, about an arcsecond or thereabouts, and mostly slow. The secondary
was easily seen to be slightly dimmer then the primary, and I estimated
the position angle at 115 to 120 degrees; I later confirmed it at 116
degrees, so I was pretty close on that. (I estimate these by making sure
I look through the eyepiece directly behind it, rather than off to the
side; then the OTA is in the direction of PA 0 degrees, one o'clock is
30 degrees, two o'clock is 60 degrees, and so on. This works like this
only with the diagonal in place.)

Interestingly, both stars in Struve 2375 are themselves doubles. They
are both splittable in principle, but the separations in each case are
very tight, about 0.15 arcseconds, beyond the reach of all but the
largest amateur telescopes in the steadiest of atmospheres. In the 50
or so years since the additional stars were discovered, they have
shared a common motion across the sky, so they are almost certain to be
physically associated.

--
Brian Tung
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