Sky and Telescope - November 2017 - 70

Astrophoto Strategy

to a bright star, such as IC 59 and IC 63, which are tucked up
very close to Gamma (γ) Cassiopeia.
Comets are another class of targets to watch for in lessthan-ideal conditions. If you get the chance to catch a nice
bright comet against a ﬁeld of background stars or a star cluster, take advantage of it. Comets come and go very quickly
with very little warning, and they may be out of sight or in
a less interesting ﬁeld before conditions become ideal for a
better shot. Many comets are at their best in twilight, when
bright skies are unavoidable.
Galaxies may be the toughest objects to capture under
bright skies. The majority are small, dim, and fuzzy. It's best if
you can wait for (or go to) a darker sky to shoot them, but if
that's not an option for you, then try to shoot plenty of images
and process them carefully to tease out the faintest outer
details. As in all astrophotography, excellent focus is critical.
In bright skies, it's even more so, because good focus gives the
maximum possible contrast both in stars and extended objects.

The Narrowband Solution

p FULL OF STARS Top: Globular clusters, such as M10 seen here, also
make fine targets under most any skies. Middle: Planetary nebulae can
easily punch through moderate light pollution. The full Moon was up
while recording most of the narrowband data used in this composite of
M27. Bottom: Shooting nebulae through narrowband emission-line filters
(including Hα, O III, S II, and N II) greatly reduces the impact of light pollution and moonlight in deep-sky images. This color composite of NGC
6960, the western side of the Veil Nebula, reveals plenty of reddish Hα
and teal-colored O III.

N O V E M B E R 2 0 17 * S K Y & T E L E S C O P E

A popular deep-sky imaging solution to both moonlight
and light pollution is to shoot with a monochrome camera through narrowband ﬁlters. These ﬁlters block all light
except for a narrow range of wavelengths - roughly 3 to 10
nanometers wide - where speciﬁc elements emit visible light.
These strips are known as emission lines. Narrowband ﬁlters
(particularly hydrogen-alpha, or Hα) have the added beneﬁt
of blocking most sources of strong light pollution as well as
moonlight, permitting users to image on practically any clear
night of the year. Narrowband ﬁlters are most effective when
shooting emission nebulae, including most of the bright nebulae on the Messier list, as well as planetary nebulae. They're
ineffective on reﬂection nebulae, though. These ﬁlters can
also be used with one-shot color cameras, including DSLRs.
I tend to shoot a mix of both natural color (RGB) and narrowband data to take advantage of most any clear skies I get.
My color pictures are often made up of data recorded through
ﬁve ﬁlters - typically red, green, blue, Hα, and doubly ionized
oxygen (O III). I usually wait for a transparent and moonless
night to shoot my broadband red, green, and blue images, and
often shoot Hα and O III data when the Moon is up.
Notice I don't use a clear (luminance) ﬁlter. Rather, I
synthesize a luminance channel by combining all my other
ﬁltered shots into a monochrome result that can be used as a
luminance image. I've found that the luminance ﬁlter is more
severely affected by light pollution and moonlight than the
individual color ﬁlters, so I tend to get a better result this way.
This approach works particularly well for planetary nebulae and emission nebulae, since much of their light is emitted
in the same narrow ranges of wavelengths passed by narrowband ﬁlters. In addition to the bright targets mentioned
earlier, you can go after some fainter quarry off the beaten
path using narrowband ﬁlters. The Sharpless catalog contains
many emission nebulae perfect for scopes of all sizes. A good
resource for choosing targets from this catalogue is Dean

Sky and Telescope - November 2017

Table of Contents for the Digital Edition of Sky and Telescope - November 2017