Sky and Telescope - April 2017 - 37

illumination (as well as by moonlight). This allows you to
reveal ionized gases that permeate the Milky Way and other
galaxies from even badly light-polluted areas.
It's no wonder that narrowband imaging has grown so
popular, and there are tools to help get the most out of this
technique. Here's how you can assemble your own in the
popular software PixInsight (pixinsight.com).
While several narrowband ﬁlters exist, those isolating the
wavelengths of hydrogen alpha (Hα), doubly ionized oxygen
(O III), and singly ionized sulfur (S II) are most commonly
used by amateur imagers. We'll use frames taken through
these ﬁlters to create a three-color image by assigning each
channel in a descending-wavelength order - an approach
popularized by scientists using the Hubble Space Telescope
and known today as the "Hubble palette."

Scripting Color
As with most deep-sky CCD image processing, you should
calibrate, align, and combine all your individual exposures
into single master ﬁles for each ﬁlter used. Once completed,
you can then begin to assemble them into a color image using
PixInsight's powerful Script functions. My preferred technique
begins by opening the master Hα, S II, and O III images, then
opening the SHO-AIP script found in the script pull-down
menu (SCRIPT > Multichannel Synthesis > SHO-AIP). The letters SHO stand for sulfur, hydrogen, and oxygen, while AIP is
the group that developed the script. This script uses the Hubble
palette mentioned earlier as its default palette, in which S II is
assigned to the red channel, Hα to the green channel, and O
III to the blue channel.
Once the script is opened, assign your master ﬁles in the
Pictures Selection section accordingly. The next stop is the
Layer to Mixing SHONRVB section, an acronym for Sulfur,
Hydrogen, Oxygen, Nitrogen, Rouge, Vert, Bleu. As the SHO
palette is selected by default, note that 100% of the S II
master is mapped to "Layer Red". Likewise, Hα is assigned to
"Layer Green", and O III chosen for "Layer Blue". But you can
easily create alternative narrowband palettes by moving the
slider controls to change both the channel assignments and
contribution amounts of the master ﬁles to your liking.
At this point, direct your attention to the check boxes at
the top right of the script dialog. The most critical setting is
to check the Auto STF box. Those having some familiarity
with PixInsight know that when viewing linear data, the STF
(Screen Transfer Function) AutoStretch must be turned on
to see an unstretched, dark image. The other three options
might require some experimentation, but for best results try
checking Mixing with Rescall (meaning rescale), and Backg
Auto Egualise (Background, Equalize), leaving the Optimise
STF box unchecked.
Now you're ready to create a color image from the three
grayscale masters. This is done in the "Mixing L SHONRVB"
section - click on the line to expand its dialog. Leave all
the default settings in this section, and click the Mixing
SHONRVB button to produce the color image. Should you

p COLOR SCRIPTING PixInsight's SHO-AIP script lets you blend
monochromatic, narrowband master files into a color image.
t RAPID RESULT
This view of IC
5070, the Pelican
Nebula, exhibits
a strong green
bias and magenta
stars - a typical
result of using the
Hubble palette
combination before adjustment.

wish to try a different mix but keep the original result, click
the New Picture button at the top of the window before
clicking the Mixing SHONRVB button a second time.
While the resulting image is a good start, you might
notice two artifacts of the SHO palette. First, it appears
predominantly green. This is because Hα is almost always
the strongest wavelength generated by emission nebulae.
Since the SHO palette assigns Hα to the green channel, green
overwhelms the relatively weaker contributions of the red and
s k y a n d t e l e s c o p e .c o m

* APRIL 2017

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Sky and Telescope - April 2017

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