Sky & Telescope - August 2024 - 57

spot, it's time to begin capturing
the frames you'll later
assemble into a finished photo.
As with any astronomical
image, I recommend shooting
in RAW format for the greatest
amount of processing flexibility.
Also bear in mind that
the more frames you capture,
the more images you'll have to
combine later. That's why you
want each individual exposure
to be as long as practical. How
long can you go?
Unfortunately, you can't
determine the maximum exposure
time from a simple formula
because the upper limit is set
by the brightness of your sky. The best
approach is to take some test exposures
with the lens stopped down a bit, starting
at around f/8. Set your camera's ISO
to 100 or 400 and take exposures of 30,
120, and 180 seconds duration to find
out what looks best. Try to balance the
exposure time and lens aperture setting
so that you get noticeable star trails while
at the same time the camera's histogram
display shows the main hump more or
less centered, or even shifted slightly to
the left. If it's too far to the right, the
brightest stars will be blown out and the
trails will begin to lose their color.
The other consideration is how
brightly illuminated your foreground
is. If it appears overexposed in your test
shots, then you may want to reduce
the exposure time or record a separate,
single foreground frame that you can
blend into your final composition later.
The proper exposure time for your
chosen foreground is a judgment call.
You may want it to be a prominent
part of the composition, or perhaps it
works best as a silhouette. You can even
try " painting " it with light to create a
dramatic effect or help it show up better
in your final image. Light painting is
pretty straightforward. During one or
more of your exposures, simply shine a
light on the parts of the foreground you
want to illuminate. For my Dry Tortugas
image (shown on page 55) I fired a
handheld camera flash repeatedly as I
walked the length of the structure.
pALTERNATIVE SOFTWARE Star Stacker is one of many helper apps available that can quickly
transform a folder of star-trail images into a single image. The app even includes multiple options
for different artistic effects that you can add to your image.
Some Assembly Required
Normally, stacking in astrophotography
means aligning a set of images on the
stars so that their light adds together to
create a single image with an improved
signal-to-noise ratio. In effect, the
finished picture is simply a better
version of a single, individual frame.
However, when it comes to star-trail
photos our goals are different and so is
the approach. If we simply summed the
frames together, the foreground would
get disproportionately bright. If we averaged
them, the stars trails would appear
faint. Instead, we want each pixel in
the assembled photo to be the brightest
value found in the stack of individual
frames. That way, each segment of the
star trail contributes to the final image
and the dark areas stay dark. We can
accomplish this feat with image-processing
software.
My tool of choice is Adobe Photoshop,
though other image-editing software
has similar capabilities. The first step is
to place all the individual frames in a
single folder on your computer to make
the process easier. Next, select File >
Scripts > Load Files Into Stack. This will
place all of your star-trail photos into a
single Photoshop document, with each
individual frame listed as a separate
layer. Then, for each layer, set the blending
mode to Lighten. This is the key. As
you work your way down the stack of
images, you'll see the star trails gradually
lengthen, segment by segment.
Don't try to modify each frame
at this stage - create your stack first
and then Flatten the layers into a
single .psd file. Once you've created
the stacked result, you're ready to start
working toward your final version. You
might want to lighten the picture or
perhaps enhance the saturation slightly
- the choice is yours. As chefs say,
" season to taste. "
If Photoshop isn't something you
want to deal with, a quick app search
will reveal several alternatives. I found
one on the Apple App Store for macOS
called Star Stacker. It adds your individual
shots together and even includes
a number of special effects that you can
apply to your final image. An option for
PC users is Startrails (startrails.de) to
automate the process. There's nothing
here that you can't do manually in Photoshop,
but both are simple to use and
do a great job.
A camera, tripod, lens, cable release,
and some imagination. That's really all
you need to create a captivating startrail
image.
What are you waiting for?
¢Contributing Editor RICHARD S.
WRIGHT, JR. is rarely more content than
when watching the stars trail overhead
while his imagination runs wild.
sk yand tele scope .o r g * AUGUST 2024 57

http://www.startrails.de https://skyandtelescope.org/

Sky & Telescope - August 2024

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