Sky and Telescope - August 2016 - 68

Imaging Technique

STACKING The ImageIntegration process generates a master
frame from a group of calibrated and aligned images. Hovering
over a selection (in this case the Rejection algorithm) displays
helpful tips about the options available.

data set, depending on how many sub-exposures I'm
combining. The program will open a description of each
method when the cursor is hovered over each option for
a few seconds. Once I've loaded the tool with the images
to be combined, I apply the process by clicking the blue
circle at the bottom left of the window.
In a few moments, the data reduction is complete,
producing a single stacked result. I'll save this ﬁ le and
then repeat the process for each ﬁ lter group.

pixels. If only a few spots need cleaning up, I'll use the
clone stamp tool found at Process > Painting > CloneStamp; if I ﬁnd many, I'll apply the CosmeticCorrection
process again. Next, I'll crop out any non-overlapping
edges in my images using the DynamicCrop process.
This tool crops each image exactly the same, to remove
areas that could interfere with later processing steps.
My next action is to address any light-pollution gradients in the images. Gradients can severely limit the
quality of the ﬁnal picture, particularly in a color image.
Fortunately, there are two excellent tools in PixInsight
to ﬁ x these problems: AutomaticBackgroundExtraction
(ABE) and DynamicBackgroundExtraction (DBE). Both
work well on individual ﬁles or a color image. (See S&T:
Sep. 2014, p. 68, for a detailed article on using these
powerful functions.)
Now I'll combine my individual red, green, and blue
images into a color result using Process > ChannelManagement > ChannelCombination. This tool should automatically assign your ﬁltered images to their respective
channels, but if not, they can be selected manually. Click
the blue "Apply Global" circle at the bottom left of the
window and in a moment your color-combined image
will appear. Be sure to save the result.
Often the color image will appear to have a strong
bias toward one color. This can be corrected using the
ColorCalibration process, but in diﬃcult cases, applying the BackgroundNeutralization process ﬁrst can
help. Both often work well without any changes to their
default settings.
At this point, I'll add any Hα data to the color image
if available. Many experienced imagers struggle with
blending Hα into a color image. Fortunately, PixInsight
has an excellent script for the job. The NBRGBCombination script (under Script > Utilities) is very easy to
use and produces a rich, natural-looking red color in
emission nebulae without aﬀecting star colors. With the

Linear Image Processing
After data reduction is complete, I'll open up each of
my stacked results. These images will appear very dark,
with only a handful of stars visible. This is because most
deep-sky objects are much fainter than starlight, so they
get recorded at the shadow region of the histogram. We
can "stretch" the display in PixInsight using the ScreenTransferFunction process to better display the shadow
regions, which doesn't modify the actual data.
Before performing any additional processing, I'll
inspect the images carefully for any leftover hot and cold
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August 2016 sky & telescope

SCREEN STRETCH Use the ScreenTransferFunction process to display an image while continuing to work with it in its
unstretched (linear) state.

Sky and Telescope - August 2016

Table of Contents for the Digital Edition of Sky and Telescope - August 2016