Sky & Telescope - September 2016 - 70

Image Stacking

BOILING ATMOSPHERE Left: This single frame of a white-light solar recording shows "seeing cells" distorting some areas of
the image much more than others. AutoStakkert! extracts only the sharpest areas in each frame. Right: A high-contrast area is
chosen as an anchor point to roughly align the video.

the software to select the best areas of each frame. The
Global setting is useful when the background is changing
in brightness, which can happen during daytime imaging
with changing transparency. The Global setting ensures
that each alignment point uses the exact same set of
frames for stacking, thereby avoiding brightness inconsistencies in your ﬁnal stacked image.
After selecting the Quality Estimator, it is time to click
the 2) Analyse button, which will start the quality estimation of all frames and, in the case of a surface recording, will also perform the surface stabilization. Image
stabilization is a crucial stage in processing the recording,
so when it's complete, you should check how well this was
executed automatically. Go to the frame view window and
drag the Frames slider around to ensure that each frame
is coarsely aligned and only small movements are still
visible. If there are large movements visible after surface
stabilization, try placing the alignment anchor on a diﬀerent feature and rerun the analysis stage.
When image analysis is completed successfully, the
label of the Frames slider turns green and the frame
view window shows all the frames sorted by their average quality, from best to worst. The main window will
also show a Quality Graph, indicating the frame quality over time (as a gray line) and the distribution of the
sorted frame qualities (green line), scaled from their
best (top left) to worst (bottom right) values. By clicking
on the Frames label, you can switch between browsing
through the quality sorted or unsorted frames.
The Quality Graph will give you an indication of how
many frames you might want to stack; typically anything
below the horizontal 50% quality line (halfway up the
image) can be ignored. By pressing Ctrl and right clicking in the quality graph at the point where the majority
of frames are below the 50% line, you can automatically
set the corresponding percentage of frames you want
to stack. However, this quality graph only gives a rough
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September 2016 sky & telescope

indication, and it's advised to check the frames by eye as
well, as this gives a better indication of what your frames
really look like and how many of them you might want to
stack for your recording.

Alignment Points
Now that the quality evaluation is complete, it's almost
time to stack your videos. AutoStakkert! allows you to
make multiple image stacks from the same video simultaneously, using both ﬁ xed numbers of frames and/or
frame percentage numbers (e.g., you can ask it to always
stack 50% of all the frames). This is particularly handy
when you are not sure how many frames you should
stack. This percentage number strongly depends on the
imaging conditions (most importantly, the seeing). A
good rule of thumb in planetary imaging is to use about
50% of the frames; use more when the seeing is very
good and fewer when the seeing is poor. When you have
a very bright subject, such as the Moon or Sun, fewer
frames are often preferred.
To correct for ﬁne-scale movements caused by seeing
distortions, the best results are produced by using multiple alignment points (APs). These APs deﬁne regions of
the images the program will analyze and track to compensate for seeing distortions. Each of these APs uses a local
quality estimation to build up a unique subset of the best
frames for that area, allowing the software to build a map
of the image as it would appear without gross distortions.
The APs are typically placed with quite a bit of overlap, allowing the program to discard the bad ones. When
the contrast is inadequate for a particular AP to track,
the resulting blurry stack from that particular AP is
ignored and replaced by the data from an adjacent AP.
Determining where and how many APs should be
used isn't easy. You can get very good results with the
Place AP grid button, which places APs automatically in
a grid-like structure. However, manual placement of APs

Sky & Telescope - September 2016

Table of Contents for the Digital Edition of Sky & Telescope - September 2016