Sky and Telescope - November 2017 - 69

he Moon is full about every four weeks; that's a
given. Within about a week either side of full,
the Moon ﬂoods the night sky with reﬂected
sunlight. This can be, among other things,
beautiful, romantic, eerie, tranquil, or even photogenic in its
own right. But it creates real challenges for deep-sky astrophotography. Even when there's no Moon, skies near even
small cities may be plagued by light pollution. High humidity or particles in the air can make matters worse, with light
scattering making the sky even brighter.
So what's a dedicated deep-sky imager to do? Take a deep
breath. Think things through. Then work to make the best of
the situation.
Much like man-made light pollution, the Moon's light
washes out many faint objects, especially dim galaxies. Since
I'm not willing to surrender two weeks of deep-sky imaging
to the Moon every month, and light pollution is all around
me, I've developed a few strategies to keep the shutter open
even under less-than-ideal conditions.

Location, Location, Location
One effective strategy is to choose targets in the darkest part
of the sky. When the Moon is near ﬁrst quarter or last quarter,
I can usually ﬁnd some pretty dark areas of sky in the north,
especially on very transparent nights when there are fewer particles in the air to scatter moonlight. The Moon follows a more
southerly path through the ecliptic during the summer, so it's
farther away from more northern target areas in summer than
in winter (though the nights are so short!). By shooting on
transparent nights and shooting objects only when they're near
the meridian (say, 2 hours on either side), one can minimize
the effects of light pollution and lunar glare.
If you have a portable imaging rig, try to pick a setup
location in the shadow of trees or a building. This will help
to keep direct, intense moonlight from entering your optic.
Choosing a northern target when imaging in the Northern
Hemisphere (or a southern target when imaging from south
of the equator) will also help keep the Moon from shining on
the inside of your optical tube or dew shield.

TOP: F. FA LCHI E T A L. / SCIENCE ADVANCES 2016

Star Clusters and Nebulae
Another way to get the most from bright skies is by choosing
targets that still show well through the glow. Open clusters and
globular clusters are my objects of choice under a bright Moon,
especially if I want to acquire an entire color image data set
in one session using a monochrome camera with color ﬁlters.
Compared to galaxies and nebulae, star clusters are often overlooked by imagers, yet they each have their own appearance
and unique character while adding variety to your imaging
portfolio. They also make great targets for checking the performance of your telescope across the entire ﬁeld of your sensor.
Some choice clusters in the fall and winter months include
NGC 7789, sometimes known as Caroline's Rose Cluster,
located just east of Beta (β) Cassiopeia. It's quite large and colorful, and a ﬁne open cluster to target. Another splashy open

p BRIGHTENING SKIES Top: This map shows the intensity of light pollution in the United States, with white denoting the strongest concentrations. Bottom: Open clusters, including NGC 457 seen at middle, make
great targets even in bright skies. Stacking many short exposures will
keep the background from getting too blown out.

cluster is M35, and it contrasts well when framed in the same
ﬁeld with the more distant cluster NGC 2158.
Many globular clusters are well-placed targets during the
summer and autumn months in the Northern Hemisphere.
Choose a bright one high in the northern sky, far from the
ecliptic, like M5, M13, or M92. Careful processing can bring
out even the faint outer reaches of these objects and reveal
beautifully colored stars.

Other Bright Targets
Small, bright planetary nebulae, as well as some emission
nebulae, can be captured well in a bright sky and found
on most any night of the year. Targets like M42 (the Orion
Nebula), M57 (the Ring Nebula), and M27 (the Dumbbell
Nebula) are bright enough to punch through moderate light
pollution and moonlight, even without using narrowband ﬁlters. And don't be afraid to target nebulae that lie very close
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Sky and Telescope - November 2017

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