Sky & Telescope - February 2019 - 66

IMAGE PROCESSING

by Timothy Jensen

DEEP COLOR Adding narrowband image
data, particularly data recorded through
a hydrogen-alpha (Hα) ﬁlter can add faint
details and rich color 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 color camera to record color and
Hα that were combined in ImagesPlus.

Giving Nebulae a
A

mong the most photographed objects in the night sky
are emission nebulae. These expansive ﬁelds of eyecatching red and magenta nebulosity, with occasional
ﬂourishes of orange, make for beautiful celestial portraits.
However, getting those rich colors isn't always easy, particularly if you live and shoot from light-polluted locations.
While one-shot color 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 specialized 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 wave-

FE B RUA RY 2 019 * SK Y & TELESCOPE

lengths except those surrounding ionized hydrogen emitting
light at 656.3 nanometers. 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 only passes 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 color 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 color
image obtained using a one-shot color camera. The trick is to
blend the Hα image into a color photograph to bring out the
nebula while at the same time maintaining a natural color
balance. One common practice is to replace the luminance
channel with a Hα image in a color photo. However, this
can lead to an odd color bias that will require much work to
correct. A better solution is to blend the Hα data into the red
channel of an RGB color image. Care must be taken, since the
Hα data can overpower the broadband red image. Addition-

ALL IMAGES COURTESY OF THE AU THOR

BOOST

Here's a novel technique to
bring out emission nebulosity
in your astrophotos.

Sky & Telescope - February 2019

Table of Contents for the Digital Edition of Sky & Telescope - February 2019