Sky and Telescope - November 2017 - 71

p GALACTIC ENHANCEMENT Galaxies are perhaps the hardest target to shoot under bright skies. However, some nearby specimens, such as M31
seen here, contain many small H II regions that can be captured well through a Hα filter even in a light-polluted sky or when the Moon is nearly full.

Salman's gallery found online at sharplesscatalog.com. It
lists objects by constellation and provides information on
each object's size and brightness, with notes to suggest what
ﬁlters might work best.

Short Exposures
When shooting under light pollution or moonlight, you'll
need to record shorter exposures (and many of them) than
you can in darker conditions. This is because the bright sky
saturates your camera's detector quickly, washing out your
target before you can capture enough photons in a single
exposure. Stacking many short images also does a better job
of removing artifacts and other unwanted signal in individual frames, including airplane and satellite trails.
In bright conditions, I often shoot exposures of about 1/3 to
½ of what I usually use in more ideal conditions. For example,
I usually take 10-minute exposures with red, green, and blue
ﬁlters on a dark night. With the Moon up, I typically shoot
5-minute exposures. These shorter exposures have a lower
background brightness with more dynamic range between the
darkest and brightest pixels in the image compared to a long
exposure under the same conditions.

Banishing Gradients
Once you have your images recorded, one of the ﬁrst processing hurdles you'll encounter is eliminating brightness
gradients. Images recorded under brightly lit skies will inevitably capture unwanted brightness gradients along with the
precious photons that we're really after. Fortunately, most
image-processing software these days includes tools that can

reduce the impact of unwanted light gradients in images. I
use PixInsight's DynamicBackgroundExtraction and AutomaticBackgroundExtraction tools to address these problems,
which is one of the ﬁrst steps in my deep-sky image processing workﬂow (S&T: August 2016, p. 66). Another option is the
plug-in tool for Adobe Photoshop called GradientXterminator
(https://is.gd/zryU32), which is an effective and easy-to-use
solution after converting your image to TIF format. There are
also many online tutorials describing gradient removal methods for other image-processing software.

Strategies for Success
Most amateur astrophotographers know that there's a sort
of "Drake equation" for the probability of getting in a good
night of imaging. And the news isn't good. For most of us,
astrophotography is limited due to clouds, wind, work, family
commitments, and pesky equipment malfunctions. But with
a few carefully chosen strategies, you can stop the Moon from
robbing half of your imaging time every month and mitigate
the impact of light pollution on your results. Use the brightest
nights - whether from moonlight or light pollution - to make
sure your equipment is in top shape for those clear, transparent, and moonless nights that we all crave. Target objects that
outshine your local light pollution, or simply take test shots
to help you frame new targets. Taking these steps can greatly
expand the number of nights you can be out collecting photons, rather than letting your equipment sit idle.
¢ RON BRECHER shoots deep-sky targets from his backyard
observatory in Guelph, Ontario. Visit his website at astrodoc.ca.
s k y a n d t e l e s c o p e . c o m * N O V E M B E R 2 0 17

http://www.sharplesscatalog.com https://www.is.gd/zryU32 http://www.astrodoc.ca http://www.skyandtelescope.com

Sky and Telescope - November 2017

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