Sky and Telescope - July 2016 - 70

Image Stacking
SAVING THE STACK
Save your result as a
16-bit TIFF ﬁle with no
compression, and be
sure to select "Embed
adjustments in the
saved image but do
not apply them" to get
a raw linear ﬁle that
can be adjusted in an
another image-processing program such as
Adobe Photoshop.

apply them" option. I prefer to save my photos as 16-bit
TIFF Images with no compression. Use this option even
if you've stacked JPEG images to avoid introducing compression artifacts in your ﬁnal image.
Now you can open the stacked image in your favorite
image-processing program such as Adobe Photoshop and
work on it. Note that the image will appear dark at ﬁrst
because it has yet to be stretched to reveal faint details.

Advanced Stacking Functions
DeepSkyStacker also has many advanced settings useful
for more complex stacking routines.
The program can group and manage diﬀerent sets
of images or exposures simultaneously. For example, it
can calibrate and combine photos of the same object shot
on diﬀerent nights, when you might have used diﬀerent
exposures and have diﬀerent sets of darks and ﬂats.
After an initial set of images is loaded into the main
ﬁle group tab at the bottom, you can simply load additional ﬁles by clicking on the corresponding tab before
you add those additional ﬁ les.

PUSH PROCESSING The low-noise images produced in DeepSkyStacker allow you to really bring out faint spiral arms in galaxies, such
as this colorful shot of M33 in Triangulum. This image is the result of 27
8-minute exposures captured with a modiﬁed Canon 600D at ISO 800
and an Astro-Physics 130EDFGT refractor.

July 2016 sky & telescope

Another advanced feature in DeepSkyStacker is its
comet stacking tools (found at the right side of the
screen). The program allows you to stack a series of
comet photos either by isolating the comet and removing
the trailed stars, or by aligning the stars and removing
the comet. Access either method by clicking the Edit
Comet Mode icon and selecting the comet's location in
each of your images.
Now you'll need to click the Stack checked pictures
command and select the Comet tab. Here you'll click
the Stars + Comet Stacking option and then hit the OK
button. In a few minutes, you should have a sharp comet
image against a ﬁeld of well-tracked stars. The success
of this method will depend on how fast the comet is
moving against the star background and the length of
time between exposures. See the program's help ﬁle for
additional information u.

Troubleshooting
While I've had great success with DeepSkyStacker,
occasionally it needs some adjustments to get the best
results. For some newcomers, the program appears to
function, but only part of an image is displayed when it
completes a stack. You might need to download the latest beta version of DeepSkyStacker, particularly if you're
using a recently introduced camera. The beta version is
actually the most recent, fully functioning program with
the latest RAW decoding library.
If your ﬁnal stack didn't properly register and stack
your images, go back to the "Register checked pictures"
command and then check the "Automatic detection of
hot pixels" box in the Actions tab. Also, try reducing the
"Star detection threshold" in the Advanced tab.
Stacked images can sometimes display dark and light
streaks, often caused by dark frames recorded at diﬀerent temperatures from your light frames. These artifacts
can be reduced in the Stacking Parameters window.
Here you'll select the Dark tab and check both the "Hot
Pixels detection and removal" and "Dark Optimization"
boxes. Additionally, you may need to experiment with
the "Dark Multiplication Factor" settings.
Most often, your stacked result is not as colorful as
you might prefer. Don't worry - it's normal for stacked
linear images to appear dark, with low contrast and little
color saturation. You'll adjust all of these issues in subsequent image processing.
Overall, DeepSkyStacker is easy to use with some practice, and it has the ability to streamline the tedious stacking process that is necessary with most deep-sky astrophotography. Stacking is the best way to greatly increase
your image quality by improving the signal-to-noise ratio,
which then allows you many more processing options. ✦
Jerry Lodriguss's new book A Beginner's Guide to Astronomical Image Processing will be out later this year.

Sky and Telescope - July 2016

Table of Contents for the Digital Edition of Sky and Telescope - July 2016