Sky & Telescope - February 2019 - 69

"+" icon found along the left side of the screen. You'll need to
experiment with the Star, Threshold, and Masked Area sliders
to get the right mix of star removal without adding artifacts
to the image. For some very bright stars, it may be best to
leave them in the image. Do this in the Special Star/Area
Processing section at the bottom of the window. Check the
select box and the Include button, and then click on the stars
you want to keep in the image, and increase the selected area
using the Radius slider. If you ﬁnd that removing a star leaves
too big a hole, you can try increasing the Radius slider setting
for that star. Once you have an image you are happy with, hit
Apply and save the result as a new TIFF ﬁle.

Adding Hydrogen-Alpha
Now that both images are ready, open the color shot and
split it into its individual channels by choosing Colors >
Split Colors from the pull-down menu, and choose RGB. In
a moment, you'll be presented with the three color channels,
and each will be a color corresponding to its particular channel (red will appear red). Convert these images to greyscale
using the Colors > Interpret - Mix Colors command with the
Grey button selected.
To blend in the Hα image, choose Special Functions >
Combine Images Using > Blend Mode, Opacity, and Masks
tool from the pull-down menu. When selected, the Combine
Images window opens, as well as a smaller Combine Images
Setup window; input the name of your combined red and
narrowband image in the Combination File Name area (for
example, red+Hα) then
q FINAL COMBINATION When
the narrowband image is combined with each color channel,
they can be re-combined using
the Combine LRGB, LCMY, HSL
tool. Additional color balance
adjustments can be made using
the sliders for each channel
before clicking Apply.

p FINAL COMBINATION Many targets within the Milky Way can benefit
from adding narrowband data to a normal color image. This picture of
the Rosette Nebula (NGC 2244 in Monoceros) takes on new life with the
addition of several hours of Hα data.

click OK. In a moment, all of the open images that you have
will appear in the Combine Images window.
You only need to combine the starless Hα and the red
channel images at the moment, so remove the blue and green
images from the list by clicking on their ﬁle names, and press
Delete Image at the bottom left. Make sure the Hα image is on
the top of the stack with the red channel image below it using
the up or down arrows. Now, with the Hα image selected,
set the Blend Mode to Lighten and lower the Opacity slider
to about 0.70. Click the Combination button in the Display:
section to preview how your combined red and Hα will appear.
You can adjust the Opacity setting until you get the effect you
like. Press ﬂatten and then save the image. Your enhanced red
channel is ready. Repeat this process on the green and blue
images, with an opacity setting of around 0.3 for the green
and 0.25 for the blue.
Once the three enhanced color channels are ready, recombine them into a color image using Color > Combine LRGB,
LCMY, HSL. In this window, assign the appropriate image to
its color channel by clicking on the desired color button and
then the corresponding image. Now hit the Apply button, and
your enhanced color image will appear. Use the Crop tool to
trim off any non-overlapping areas around the edge of the
frame, and be sure to save your result.
This processing technique can help you create stunning,
highly detailed images of nebulae even under less-than ideal
skies. And don't be afraid to experiment with other narrowband wavelengths. Some nebulae have large contributions
from other ionized elements, particularly doubly ionized
oxygen (O III), which is found in the blue-green region of the
spectrum at both 495.9 and 500.7 nanometers. These narrowband enhancements can often turn an ordinary image
into an eye-popping portrait.

¢ TIMOTHY JENSEN images the night sky from his backyard
observatory in rural North Carolina.
sk yandtele scope.com * FE B RUA RY 2 019

http://www.skyandtelescope.com

Sky & Telescope - February 2019

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