62 AUSTRALIAN SKY & TELESCOPE February | March 2019
When you’re satisfied with the stars in your image, click
the Apply button on the bottom right, and then save the
image as a 16-bit TIFF file.Nebulae boost
When working on your narrowband image to combine with
your colour version, you can ignore the stars and make
the nebula pop as much as you like before noise becomes
particularly noticeable. If you shot your Hα images with
a one-shot colour 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 colour image. To do this,
open your Hα image in ImagesPlus and select Colour > 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 permits
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.
Once your narrowband image is stretched to your liking,
you can perform noise reduction by selecting SmoothSharpen > 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 first.
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.Preparing to merge
Before you combine the narrowband and colour images, you’ll
first have to align the two. This is accomplished in ImagesPlus
by opening Image Set Operations > Align Files > Align files -
Translate, Scale, Rotate, and then select the two files 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, first choose
the Common Point or Star option, and then click on a fainter
star that appears in both images. Next, select Common
Angle Defining Point or Star, and click on a different star
in each image, followed by Additional Scale Defining 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
‘+’ icon found along the left
side of the screen. You’ll needTCHANNEL SPLIT In order
to combine the hydrogen-
alpha image with each colour
channel at varying degrees,
the colour image has to be
split into individual red, green
and blue monochrome images.
ImagesPlus displays these by
their respective colour.TNARROWBAND BOOST Adding
the Hα to each colour 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
oversaturate areas in the image.IMAGE PROCESSING