WWW.ASTRONOMY.COM 55
are specific to building a wide-field com-
posite. Above you’ll see a typical image
from the Pentax 200mm/U16M setup,
after calibration, stretching, and gradient
removal, representing eight hours of an
HαRGB exposure. It’s reassuring that the
star colors are preserved without exces-
sive blue fringing despite the achromatic
lens system. Also preserved is the detail
in the Hα components of the two main
targets, as well as the intervening faint
Hα signal that bathes this entire region
(and which is often not appreciated when
imaging a more limited field of view).
The image above left shows my ear-
lier, higher resolution images of IC 1396
and Sharpless 2–129 placed into the
Pentax field in Photoshop, after these
were aligned (using the Pentax image as
reference) in RegiStar. Two steps are then
useful in order to achieve seamless
blending of these higher resolution
images against the Pentax backdrop.
First, I assign the two higher resolution
layers to “lighten” mode, adjusting opac-
ity to enhance the details in the target
but also to match the background of the
wide-field Pentax canvas. Second, I cre-
ate a layer mask for this image and use
the brush tool set to black (soft edge) to
selectively hide the edges of the higher
resolution image, adjusting the opacity
of the tool as necessary to achieve the
best result.
Subsequent adjustments are made to
the individual components using clip-
ping masks to achieve the proper color
balance with the wide-field background,
using RGB, curves, and other such tools
as needed.Ta k e i n t h e v i e w
As shown in the accompanying pictures,
this relatively simple technique can
breathe new life into old images. How
many of us realized that the Elephant
Trunk was so close (in angular separa-
tion, at least) to Sh 2–129, or that theSpaghetti Nebula (Simeis 147) was near
the Flaming Star Nebula? And how
many of us painstakingly created mosa-
ics of the North America, Pelican, and
IC 1318 nebulae to capture a wide field
of view, when we could have used the
composite approach described here to
achieve a very similar result?
This approach has reenergized my
interest in wide-field astrophotography
and has enabled me to recycle some of
my older images, painting them onto a
sweeping canvas that gives them a new
perspective. Even if you don’t have a
Pentax setup like mine, try this
approach with whatever wide-field lens
system you have, and you will likely be
pleased with the results.Steve Cannistra is an academic physician
by day, and is often sleep-deprived due to
his passion for astrophotography by night.
More images may be found at his website,
http://www.starrywonders.com.Central Cygnus, including
the North America and
Pelican nebulae (upper left)
and the IC 1318 nebulosities
around Gamma Cygni
(lower right) come alive in
this breathtaking wide-field
shot by the author.