My first Magazine Sky & Telescope - 02.2019 | Page 69

ally, ionized hydrogen doesn’t just emit
light in the red. It also produces emission
at 486 nm (the blue/green hydrogen-
beta line), and another, weaker emission
at 434 nm (blue). So if we take our H α
data and mix a little into the green and
blue channels as well as the red, we can
achieve a much more natural color bal-
ance with a lot of detail in the nebula.
This sounds complicated, but for-
tunately, most astro-imaging software
includes tools to do this. I prefer Images-
Plus ( mlunsold.com ), which makes this
sort of work very easy to do. Here’s how I
incorporate the technique into my image-
processing workfl ow.
t COLOR STRETCH Begin
by stretching the stacked col-
or image with an emphasis
on preserving the colors and
profi les of the stars using the
ArcSinH tool.
Start with the Stars
Begin by collecting your color data, as
well as the narrowband or UHC-fi ltered
images. It doesn’t matter if your camera
is a DSLR or monochrome deep-sky cam-
era, as the process for combining the nar-
rowband and color images is the same,
though let’s assume you are using a color
camera. Process both sets of images to
bring out the most detail you can without
enhancing noise. When processing the
color photo, don’t worry too much about
enhancing the nebula. Concentrate
instead on bringing out the stars in the
picture. The goal is to make stars appear
as natural as possible without imparting
a bloated or washed-out look to them,
nor a color bias; you should see a good
deal of blue, yellow, and orange stars in
the stretched result.
I prefer to use the ArcSinH tool to
stretch my images, found in the pull-
down menu (Stretch > ArcSinH). When
this tool opens, start by changing the
Scale Function from None to nth Root
X^n in the top-right of the window.
In the middle section, move the BkGd
Weight and Power sliders to bring out
the stars in the image. Try to avoid the
temptation to enhance the nebula at this
point, as that will occur later with the
addition of the narrowband data.
Use the red, green, and blue sliders in
the Decompress Color section at the bottom of the ArcSinH
window to help preserve the star colors during the stretching
process. Some imagers prefer to use a sun-like star (known
as a G2V star) as a color reference and adjust the colors until
that star appears neutral white without becoming saturated.
t ALIGNMENT After stretch-
ing the individual color and
narrowband images, both
pictures should be aligned.
Be sure to click Translate +
Scale + Rotate and choose 3
alignment points.
p STAR MASK After alignment, the stars in the narrowband image
need to be masked out so that they don’t affect their appearance in
the fi nal color image. Some bright stars, particularly ones embedded
within bright nebulae, can be removed from the mask before applying
the tool, so the result shouldn’t contain any odd holes that could cre-
ate artifacts in the fi nal image.
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