My first Magazine Sky & Telescope - 02.2019 | Page 70

Image Processing
While this is excellent advice, it’s sometimes hard to fi nd a
G2V reference star in an image. The ArcSinH function is a
very powerful tool, and small adjustments with the sliders
can have a strong impact on the image.
When you’re satisfi ed with the stars in your image, click
the Apply button on the bottom right, and then save the
image as a 16-bit TIFF fi le.
Once your narrowband image is stretched to your lik-
ing, you can perform noise reduction by selecting Smooth
Sharpen > Multiresolution Smooth/Sharpen in the pull-down
menu. Moving the sliders to the left will smooth the image,
though I recommend starting with the Finest slider fi rst.
Again, you can apply this to different intensity ranges within
the picture by clicking the Set View Using Image Currently
Displayed button between each correction.
Nebulae Boost
When working on your narrowband image to combine with
your color version, you can ignore the stars and make the
nebula pop as much as you like before noise becomes particu-
larly noticeable. If you shot your Hα images with a one-shot
color camera, you’ll want to convert it to a greyscale image
in order to mix it with the individual red, green, and blue
channels of your color image. To do this, open your Hα image
in ImagesPlus and select Color > Split Luminance from the
pull-down menu.
I prefer to begin to enhance the narrowband image using
the ArcSinH tool, and then apply more adjustments with the
Micro Curves tool (Stretch > Micro Curves). This tool per-
mits you to apply various degrees of enhancement to different
brightness ranges by using the Min and Max check boxes to set
the working range for the tool. Simply click the check box and
then click on an area of your image to set the intensity limit.
To apply multiple iterations of Micro Curves, you need to press
the button labeled Set View Using Image Currently Displayed
after each adjustment (the blue eye button). Otherwise, the
next correction with the tool simply replaces the previous one.
q CHANNEL SPLIT In order to
combine the hydrogen-alpha
image with each color channel
at varying degrees, the color im-
age has to be split into individual
red, green, and blue mono-
chrome images. ImagesPlus
displays these by their respec-
tive color.
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FE B RUA RY 2 019 • SK Y & TELESCOPE
Preparing to Merge
Before you combine the narrowband and color images, you’ll
fi rst have to align the two. This is accomplished in Images-
Plus by opening Image Set Operations > Align Files > Align
fi les - Translate, Scale, Rotate, and then select the two fi les
to align. When the window opens, click on the options for
On Each Image, Translate + Scale + Rotate, and change the
Number of Points to 3. In the Alignment Feature Selection,
fi rst choose the Common Point or Star option, and then click
on a fainter star that appears in both images. Next, select
Common Angle Defi ning Point or Star, and click on a differ-
ent star in each image, followed by Additional Scale Defi ning
Point or Star and select your third star. When all three stars
are chosen, the Align button becomes active. Click it, and in
a few moments, ImagesPlus will save aligned copies of the two
registered pictures.
Now that the two images are aligned, you’ll need to
remove the stars from the Hα
image using the Feature Mask
tool, which appears as a blue
q NARROWBAND BOOST Adding
the Hα to each color channel (with
the most going to the red image)
is performed with the Blend Mode,
Opacity, and Mask tool. Be sure to
change the Blend Mode to Lighten,
and lower the opacity so that the
narrowband image doesn’t over-
saturate areas in the image.