Sky & Telescope - June 2022 - 67

S II
O III
Hα
ISOLATED WAVELENGTHS
Images recorded through narrowband
filters that pass only
specific wavelengths of light found
in nebulae are combined in a variety
of false-color palettes. In this
image of Sh2-132 in Cepheus, data
captured through S II, Hα, and O III
images (above) are blended into a
false-color result seen at far left.
" enhanced color " results - these pictures blend narrowband
data with the color channels of the corresponding wavelength
in the visible spectrum. For example, Hα is mixed with the
red channel of an RGB image to emphasize emission nebulosity.
Likewise, O III is added to both the blue and green
channels of a natural-color image to enhance the elements'
contributions, which is particularly helpful when targeting
planetary nebulae and supernova remnants. This technique
retains a natural color balance while enhancing the visibility
of dim nebulae.
Imaging exclusively with narrowband filters offers the widest
range of options when creating a color image. A naturallooking
color palette can't be generated from narrowband
data alone because the filters only pass a tiny fraction of the
visible spectrum. This affects star colors in particular. Additionally,
the three primary narrowband wavelengths don't fall
neatly into corresponding regions of the visible spectrum to
make a natural-color image. Doubly ionized oxygen (O III)
emits light at two wavelengths near the crossover between
blue and green light (500.7 and 495.9 nm), while Hα, N II,
and S II appear in the red part of the spectrum within a span
of about 15 nanometers at 656.28, 658.4, and 671.6 nm,
respectively. For these reasons, photos made solely from narrowband
data are referred to as " false-color " images.
Professional astronomers working with the Hubble Space
Telescope and other observatories established the narrowband
palettes we typically use today. The most common narrowband
color combination is referred to as the Hubble Palette
since it was popularized in many of the Space Telescope's
most iconic images beginning in the early 1990's. Hubble
astronomers use a descending-wavelength palette by assigning
S II, Hα, and O III to the red, green, and blue channels,
respectively. At about the same time, astronomers using the
Canada-France-Hawaii Telescope (CFHT) chose to combine
narrowband data by assigning Hα to the red channel, O III
to green, and S II to blue. Each of these approaches presents a
very different look, as you can see on page 40.
Another popular narrowband technique is to shoot
through only Hα and O III filters, and combine the results by
treating Hα as red and assigning O III to both green and blue.
This is particularly useful when the target object contains
little or no S II component.
Still other astrophotographers aim to get the best of both
worlds by shooting their targets with both narrowband and
broadband filters. This allows the nebulae to be processed one
way, while achieving natural hues in the field stars.
You don't need to limit yourself to a single result either. I
often make multiple versions of an image with the same narrowband
dataset.
Processing Tips
We perceive most of the detail in the lightness component
(brightness, contrast, graininess, smoothness) of a color image,
while the chrominance component provides color information.
I try to account for this fact when I process my color pictures
by separating the workflow into steps that affect lightness separately
from those that focus on color. With this foundation,
I use a different approach for broadband images compared to
narrowband, regardless of the type of camera used.
When I process natural-color astrophotos, my approach is
to complete almost all color corrections before applying any
non-linear stretching or detail enhancement. I aim to achieve
a neutral background and stars that display natural-looking
hues (red, gold, blue, and white). One important step toward
achieving this result is to remove any uneven field illumination
(such as vignetting and light-pollution gradients) after
combining the images into a color composite.
For narrowband images, I also remove gradients fairly
early in the workflow. However, I defer all other color adjustments
until I've fully processed the lightness component of
the image, including optimizing the sharpness, smoothness,
brightness, and contrast. After that, I adjust the color by
aiming for a pleasing combination of tones that reveal the
chemistry and structure of my subject.
sk yand tele scope .o r g * JUNE 2022 67
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Sky & Telescope - June 2022

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