SkyandTelescope.com September 2014 69Understanding the Problem
Gradients are often due to unwanted light sources such as
light pollution, but there are many natural light sources
that will introduce gradients to your deep-sky images,
including the zodiacal light, aurorae, and even natural
airglow. Each of these light sources add signal to your
images — an unwanted signal.
Because gradients are unwanted signal in our photos,
the most accurate way of removing them is to mathemati-
cally subtract them from the image. Ideally, this subtrac-
tion should be the fi rst thing you do to your calibrated
images before any stretching or sharpening. At this stage
your image is linear (unstretched), which is the best time
to achieve the most accurate gradient removal.
To get the best image under less-than-ideal conditions,
you need to subtract the gradients and nothing else, but
you can’t produce an image that contains only gradients.
You can, however, get close enough to carry on with your
science or artistic rendition. Currently there are two
methods for getting pretty close to producing a faithful
image of gradients. The fi rst is to shoot “superfl ats” dur-
ing your imaging session. Superfl ats are fl at-fi eld calibra-
tion images recorded near the time and area where you
captured your target data. For the majority of amateurs,
this is impractical, and takes away valuable imaging time.
Furthermore, superfl ats do not take into account chang-
ing conditions such as encroaching twilight.
The other method is to create an artifi cial representa-
tion (model) of the gradients in your image. So how can
you create an artifi cial gradient? The most plausible solu-
tion is to measure small areas (samples) from strategic
locations in the original image and produce a simulation
of the gradient. Fortunately, there are tools that make this
process much easier than it might seem.
All other conditions being equal, the larger your cam-
era’s, telescope’s, or lens’s fi eld of view, the more obvious
the gradients will be, but even images only 1°° wide or less
can and do show gradients.PixInsight
Although there are a few programs today that off er point-
measured gradient removal, I prefer the tools available in
PixInsight 1.81 from Pleiades Astrophoto (http://pixinsight.
com). I fi nd that this program off ers much more control
and produces better results than the others available
today. Here’s how I achieve my best results.
I usually begin with a calibrated and combined image
with any non-overlapping edges cropped off. My fi rst stepis to apply an automatic ScreenTransferFunction (STF)
by selecting Image > STF AutoStretch from the pulldown
menu to increase the image contrast, revealing any gradi-
ents in the image.
Once identifi ed, PixInsight has two tools that can
suppress gradients: AutomaticBackgroundExtractor and
DynamicBackgroundExtraction (ABE and DBE for short).
ABE, as its name implies, is mostly automatic and doesn’t
enable you to interact with the image. Launch the ABE
module by going to the PROCESS > BackgroundModel-
ization > AutomaticBackgroundExtractor. You can then
adjust a few parameters that will launch the sample-gen-
eration and background-modeling engines. Set the Target
Image Correction to Subtraction and then click the blue
square at the bottom-left to produce a background model
and corrected image. Unfortunately, this tool doesn’t
always discriminate between the subject and background,
often resulting in inaccurate gradient corrections.Dynamic Background Extraction
My preferred method uses the DBE tool (PROCESS >
BackgroundModelization > DynamicBackgroundExtrac-
tion), which I fi nd to be the most fl exible and user-friendly
gradient removal method available to amateurs today.
The best way to know whether you’re removing the
gradient and nothing else after applying ABE or DBE on
an image is not by looking at the corrected image, but at
the background model image that the ABE or DBE tools
created and later subtracted.
Regardless of how wonderful your image might look
after your fi rst application of ABE or DBE, you should
carefully check that the process only removed the gradi-
ent signal — nothing more, nothing less. A gradientFacing page: Practically everyone shooting deep-sky astropho-
tos will at some time encounter brightness gradients across an
image. Notice the gradient in this photo of the Virgo Cluster,
which appears bright at the top-left and dark at the bottom-
right. Author Rogelio Bernal Andreo explains how to remove this
unwanted signal from your own images using PixInsight.PixInsight’s AutomaticBackgroundExtractor (ABE) tool enables users to
quickly generate a gradient map of an image and subtract it. Although
this works well in some instances, it often indiscriminately maps galax-
ies and bright stars as brightness gradients that should be suppressed.
This gradient map of the sky background was taken from the image on
the facing page using the ABE tool. Note its lumpy, uneven appearance.PixInsight.indd 69 6/23/14 12:17 PM