Sky and Telescope - March 2018 - 37

q RICH COLOR When increasing the saturation in your pictures, take
care to monitor each color channel and avoid over-saturating the result,
which presents problems when printing your results. This detailed image
of the Horsehead Nebula, Barnard 33, displays richly saturated color
without going overboard. Here north is to the left.

ECLIPSE: SE AN WA LK ER / S&T; HORSEHE AD: K EN CR AWFOR D

SOLAR ANGLE Composing interesting images of the Sun, Moon,
and planets can be challenging, since all are simply round objects
surrounded by a blank sky. But special events can offer unique opportunities. When photographing last August's total solar eclipse, the
author planned everything long in advance, though in the excitement
of the moment, camera orientation was left to chance. Fortunately,
the error produced a better result than originally planned.

varied hues that occur when ionized elements interact. If we
were to be truly accurate, most tri-color narrowband images
would be dominated by the color that hydrogen-alpha is
assigned to in an image (often green), because it is the most
pervasive element in the universe, compared to sulfur II or
oxygen III. But to best display the colorful interactions of
these elements, imagers equalize the exposures in an image.
Planetary photography also beneﬁts from a pleasing color
balance. For instance, while Mars is a tawny-orange color
(though not saturated), blue and white are often seen in its
thin clouds near the planets limb, and its occasional dust
storms have a distinctive yellow hue. A related aspect of color
is contrast. Astrophotos are tricky to establish proper contrast
in, because most targets are seen against a dark sky. You'd
think that simply making the background black would be the
easiest step, but actually that's not the case. For one thing,
there is no truly "black" sky as seen from Earth's surface.
There are very few places where some artiﬁcial light pollution
isn't visible somewhere in the sky. But our planet's atmosphere itself emits a small amount of light called airglow. This
is commonly seen in nightscape photos taken from dark sites,
though airglow can sometimes be very noticeable even from
rural locations. I've recorded skyglow in backyard nightscape

images taken just a few miles from downtown Manchester,
New Hampshire, the state's largest city.
Given these caveats, deep-sky and nightscape imagers
should strive to establish a dark, neutral background sky, but
it shouldn't be completely black, so that faint nebulosity or
the outer extent of spiral galaxies will still be visible in your
image. Setting the background to pure black during image
processing (known as clipping) often erases these subtle features from an otherwise excellent image.
The only exceptions to this "dark but not black sky" rule
in astronomical photography would be lunar and planetary
images. Shadows in lunar craters really will be black, particularly since the Moon lacks any appreciable atmosphere to
scatter light. Planet images generally appear to be surrounded
by inky black skies because of the high focal lengths required
to magnify the target enough to resolve details; planetary
imagers often shoot at focal ratios of f/20 or more, which
translates into highly magniﬁed views, reducing the background sky brightness.
This brings us to saturation. While we all marvel at the
colorful images of galaxies, nebulae, and star clusters in the
pages of this magazine and elsewhere, the colors of most of
these objects are not the blazing magentas or electric blues
sk yandtelescope.com * M A RCH 2 018

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Sky and Telescope - March 2018

Table of Contents for the Digital Edition of Sky and Telescope - March 2018