http://www.skyandtelescope.com.au 63to 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
Nowthatbothimagesareready,openthecolourshotand
splititintoitsindividualchannelsbychoosingColours>
SplitColoursfromthepull-downmenu,andchooseRGB.
Inamoment,you’llbepresentedwiththethreecolour
channels, and each will be a colour corresponding to its
particularchannel(eg.redwillappearred).Convertthese
images to greyscale using the Colours > Interpret - Mix
ColourscommandwiththeGreybuttonselected.
ToblendintheHαimage, choose Special Functions >
CombineImagesUsing>BlendMode,Opacity,andMasks
toolfromthepull-downmenu.Whenselected,theCombine
Imageswindowopens,aswellasasmallerCombineImages
Setup window; input the name of your combined red and
narrowbandimageintheCombinationFileNamearea(for
example, red+Hα) then
TFINAL COMBINATION
When the narrowband image
is combined with each colour
channel, they can be re-combined
using the Combine LRGB, LCMY,
HSL tool. Additional colour
balance adjustments can be made
using the sliders for each channel
before clicking Apply.
SFINAL COMBINATION Many targets within the Milky Way can
benefit from adding narrowband data to a normal colour 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 colour channels are ready,
recombine them into a colour image using Colour > Combine
LRGB, LCMY, HSL. In this window, assign the appropriate
image to its colour channel by clicking on the desired colour
button and then the corresponding image. Now hit the Apply
button, and your enhanced colour 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 ionised elements, particularly
doubly ionised oxygen (O III), which is found in the blue-
green region of the spectrum at both 495.9 and 500.7
nanometres. These narrowband enhancements can often
turn an ordinary image into an eye-popping portrait. TIMOTHY JENSEN images the night sky from his rural
backyard observatory.