This sounds complicated, but
fortunately, most astro-imaging
software includes tools to do this. I
prefer ImagesPlus (mlunsold.com), which
makes this sort of work very easy to do.
Here’s how I incorporate the technique
into my image-processing workflow.
Start with the stars
Begin by collecting your colour data, as
well as the narrowband or UHC-filtered
images. It doesn’t matter if your camera
is a DSLR or monochrome deep sky
camera, as the process for combining
the narrowband and colour images is
the same, though let’s assume you are
using a colour camera. Process both
sets of images to bring out the most
detail you can without enhancing noise.
When processing the colour photo,
don’t worry too much about enhancing
the nebula. Concentrate instead on
bringing out the stars in the picture.
The goal is to make stars appear as
natural as possible without imparting a
bloated or washed-out look to them, nor
a colour bias; you should see a good deal
of blue, yellow and orange stars in the
stretched result.
I prefer to use the ArcSinH tool to
stretch my images, found in the pull-
down menu (Stretch > ArcSinH). When
this tool opens, start by changing the
Scale Function from None to nth Root
X^n in the top-right of the window.
In the middle section, move the BkGd
Weight and Power sliders to bring out
the stars in the image. Try to avoid the
temptation to enhance the nebula at
this point, as that will occur later with
the addition of the narrowband data.
Use the red, green and blue sliders
in the Decompress Colour section at
the bottom of the ArcSinH window to
help preserve the star colours during the
stretching process. Some imagers prefer
to use a sun-like star (known as a G2V
star) as a colour reference and adjust
the colours until that star appears
neutral white without becoming saturated. While this is
excellent advice, it’s sometimes hard to find 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.
WCOLOUR STRETCH
Begin by stretching the
stacked colour image with an
emphasis on preserving the
colours and profiles of the
stars using the ArcSinH tool.WALIGNMENT After
stretching the individual
colour and narrowband
images, both pictures
should be aligned. Be sure
to click Translate + Scale
+ Rotate and choose 3
alignment points.SSTAR MASK After alignment, the stars in the narrowband image
need to be masked out so that they don’t affect their appearance
in the final colour image. Some bright stars, particularly ones
embedded within bright nebulae, can be removed from the mask
before applying the tool, so the result shouldn’t contain any odd
holes that could create artifacts in the final image.http://www.skyandtelescope.com.au 61