Sky & Telescope - August 2022 - 68

S&T Test Report
border between the sky and foreground.
This option works well if you don't have
a complex foreground with tall trees
or buildings of varying height. You can
click and hold your mouse to adjust the
angle of this boundary to match your
composition. The next option, Gradient,
allows you to set an area between the
foreground and sky that will include a
transition zone between the two areas.
This is also useful if you're shooting over
a field with a low horizon, for example.
For images with a complex foreground,
the best choice is the Irregular
mask. Clicking this option immediately
brings up the directions to " Fill the sky
with brush, " meaning you paint a mask
that covers the sky area in your photo.
You drag the brush tool around the sky
area you'd like aligned while carefully
avoiding having the mask cover any
of the foreground you want to remain
stationary. The brush size is adjustable
with your mouse wheel. You don't want
to paint your mask too close to the foreground
objects; if the mask covers any
of the foreground, the stacked result
will display an unnatural glow around
the foreground edges.
After painting the sky mask, there are
a few more options to consider before
stacking can commence. The next two
selections are Auto brightness and High
dynamic range. The first is intended to
avoid blowing out the highlights when
stacking many frames. It often produces
good results. However, I found the tool
to sometimes be overly aggressive, particularly
when I only had a few images
to combine. The High dynamic range setting
is intended to keep the brightness
of the final result within the range that
can be represented in a 16-bit file. It's
best used if you intend to do additional
processing to the final stacked image in
another program.
If you didn't include dark frames
Sequator isn't always successful when
processing RAW camera files. The image at
right includes 12 RAW frames with the Auto
brightness and High dynamic range options
activated, producing a noisy, over-processed
result. The photo at far right stacks the same
12 frames that were first processed in Adobe
Camera Raw and saved as 16-bit TIF files
before being imported into Sequator.
68 AUGUST 2022 * SKY & TELESCOPE
frames and the star positions change
greatly throughout the series. This
option will analyze the stars near the
horizon and edges of the frame and
try to make them appear round rather
than elongated. There are two options
here: Lens and Complex. Selecting
Lens will correct minor distortions
due to lens design and some distortion
when shooting through telescopes
(more on this later). Complex is best
suited for photos recorded through
extremely wide-angle lenses, though
the software's author recommends only
stacking up to 5 minutes total exposure
when shot this way.
At this point you can apply the
The stacked result above includes sixty
6-second exposures to achieve the noise characteristics
similar to a 10-minute exposure.
(Noise images) when adding all your
files, select Reduce dynamic noises. This
does a very good job eliminating hot
pixels, which grow into red-, green-, and
blue-colored lines in the composited
result. It's also useful when imaging
on warm nights and your dark frames
aren't the same temperature as your
light frames.
Reduce distor[tion] effects is particularly
useful when you stack a lot of
Reduce light pollution command, which
does an impressive job of detecting and
suppressing image gradients. There are
two options in this tool: Deep sky or
Uneven. The Deep sky light-pollution
filter corrects simple linear gradients
and works best with telephoto images
or photos taken with a telescope. Most
nightscape photos taken under lessthan-pristine
skies will benefit from the
Uneven option, and the aggressiveness
slider below it allows you to adjust how
strong this filter is applied. Additionally,
there's an Intelligently-aggressive check
box, which, when selected, analyzes the
Sky & Telescope - August 2022

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