60 AUSTRALIAN SKY & TELESCOPE February | March 2019
AMONG THE MOST PHOTOGRAPHED OBJECTS in the
night sky are emission nebulae. These expansive fields of
eye-catching red and magenta nebulosity, with occasional
flourishes of orange, make for beautiful celestial portraits.
However, getting those rich colours isn’t always easy,
particularly if you live and shoot from light-polluted locations.
While one-shot colour cameras like DSLRs or mirrorless
cameras can capture the entire spectrum of visible light, often
their utility is compromised by light pollution and skyglow.
One way around this limitation is to image through
narrowband or other specialised filters to increase the contrast
in the images. Ultra High Contrast (UHC) filters sold by
various manufacturers are a type of combined narrow-
bandpass filter. They work by passing specific wavelengths of
light, commonly found in nebulae, while blocking unwanted
wavelengths from most sources of skyglow. Oceanside Photo
and Telescope’s new Triad filter works in a similar fashion.
Individual narrowband filters allow only a fraction of the
visible spectrum to be seen by the camera. Most commonly
used is the hydrogen-alpha (Hα) filter that blocks all
wavelengths except those surrounding ionised hydrogen
emitting light at 656.3 nanometres. Since hydrogen emission
is typically the most common light from emission nebulae,this is an excellent filter for highlighting details in faint
nebulosity. It’s most effective when paired with monochrome
cameras, since it passes only a very small portion (often
between 3 and 7 nm) of the deep-red region of the spectrum,
so images recorded through this filter with a colour camera
(such as a DSLR) appear as a washed-out red.
However, it is still possible to use these monochrome images
captured with Hα filters to enhance an ordinary colour image
obtained using a one-shot colour camera. The trick is to blend
the Hα image into a colour photograph to bring out the nebula
while at the same time maintaining a natural colour balance.
One common practice is to replace the luminance channel
with a Hα image in a colour photo. However, this can lead to
an odd colour bias that will require much work to correct. A
better solution is to blend the Hα data into the red channel of
an RGB colour image. Care must be taken, since the Hα data
can overpower the broadband red image. Additionally, ionised
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
Hα data and mix a little into the green and blue channels as
well as the red, we can achieve a much more natural colour
balance with a lot of detail in the nebula.Here’s a novel technique to bring out emission nebulosity in your astrophotos.
DEEP COLOUR Adding narrowband image
data, particularly data recorded through a
hydrogen-alpha (Hα) filter can add faint details
and rich colour to images taken under less-
than-ideal skies. The author took this example
of the emission nebulosity IC 434 surrounding
the Horsehead Nebula using a one-shot colour
camera to record colour and Hα that were
combined in ImagesPlus.Giving nebulae a
ALL IMAGES COURTESY OF THE AUTHORIMAGE PROCESSING by Timothy Jensen