Sky & Telescope - July 2020 - 39

can reload the image and adjust the Parameters slider on the
bottom right toward "preserve more nebulae." If that doesn't
work, then you can restore it by holding the control key (Ctrl)
and left-clicking on the comet. Once you've determined the
proper setting, perform the same action on all your individual
images. I prefer to do this manually, rather than using the
batch setting, because the program may still remove the
comet on a few exposures. When completed, save the image
using File > Save As from the pull-down menu. Straton will
automatically add "withoutStars" to the end of the ﬁle name.
You can now stack your sets of starless, comet-registered
images with any astronomical image-processing program. I
create a luminance image and an RGB image, open both in
MaxIm DL 6, and process with the Digital Development tool
to perform a non-linear stretch. I save the results as 16-bit
TIF ﬁles, which I'll combine later in Photoshop.
Next, you have to process the star-ﬁeld data. This time you
need to remove the comet as it moves relative to the stars in
each exposure. For this I prefer a free program called Fitswork
(https://is.gd/ﬁtswork). Begin by opening one of the images
aligned on the ﬁeld stars (atlas01 stars.ﬁt). Select Processing
> Background Flatten > Variable Flatten from the pull-down
menu at the top of the screen.
A new window opens where you can protect the stars and
deep-sky objects from the tool's action using the Flatten
Radius and Mask Bright Areas sliders. You should usually only
need to increase the Mask Bright Areas slider, which highlights everything in the image that will be masked as yellow.
Right-click the mouse to remove any mask that appears over
the comet nucleus. When you're satisﬁed with the mask preview, click the Calculate button, followed by the OK button,
and then save the result. Repeat this action for each image
and then combine the results into a single FIT ﬁle (or luminance and RGB images when shooting LRGB sets). Stretch the
results to create scaled TIF ﬁles just like the ones you made for
the starless comet ﬁles earlier.
The next step is to create the ﬁnal LRGB composite in
Adobe Photoshop. Open all four TIF ﬁles and start by selecting
the luminance star-ﬁeld image. Usually there's a tiny spot of
the comet's nucleus remaining in the photo, which you can
use as the registration point for the other layers. Convert the
color space from Grayscale to RGB by choosing Image > Mode
> RGB Color from the pull-down menu. This is necessary so
that the color layers added later will, in fact, appear in color.
Select and copy the starless comet luminance image using
Edit > Copy from the pull-down menu. Next, click on the
luminance star-ﬁeld image and select Edit > Paste, which will
place the starless comet image on top of the star ﬁeld. Open
the Layers window (Window > Layers) and change the layer
blending mode from Normal to Difference and use the Move
tool to nudge the starless comet image until the nucleus lines
up with the tiny spot in the star-ﬁeld image. When the layer is
in position, change the layer blending mode from Difference
to Lighten. The comet then appears in all its glory in the star
ﬁeld. Lastly, use Layer > Flatten Image to combine these layers.

p ADDING COLOR STARS Copy and paste the RGB star-field image
and align it to the luminance image below by changing the color starfield layer blending mode to Difference, then nudge it into place with
the Move tool. When perfectly aligned the stars turn black, and you
can change the layer blending mode to Normal.

p COLOR COMET Adding the starless comet image to the working
stack is done in a similar way - paste it on top of the color star-field
layer and change the blending mode to Difference. Be sure to hide
the color star-field layer by clicking its "eye" icon in the layer window
so that you are aligning the color comet layer to the luminance layer
at the bottom of the stack.

p SHUFFLING LAYERS The final step is to change the order of the
layers so that the luminance is above the color layer, then changing
its blending mode from Normal to Luminosity.
s k y a n d t e l e s c o p e . o r g * J U LY 2 0 2 0

https://www.is.gd/fitswork http://www.stars.fi http://www.skyandtelescope.org

Sky & Telescope - July 2020

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