Sky & Telescope - October 2023 - 56

OCTOBER 2023 OBSERVING
First Exposure
overlapping frames to create a synthetic
wide-field view while retaining the
exquisite detail that a long-focus instrument
provides. Indeed, some of my best
Moon shots have been captured this way.
SOUTHERN HORIZON The author combined 13 individual frames photographed with an 85mm
lens to create this composite view of the Sagittarius Milky Way. Unlike with typical wide-angle
shots, zooming in on this image reveals considerable detail in individual deep-sky objects. If
desired, the mosaic can be cropped to create a final image suitable for printing or display.
the individual image tiles you'll need
to create your final mosaic. Most of
these programs can display a graphical
overlay to indicate a specific field of
view. Select a rectangular-shaped frame
that matches your particular cameraand-lens
combination. For example, my
Canon EOS 60D with a 50-mm lens has
a field of view measuring 25° × 17°.
Once you're ready to start shooting,
put your camera in Manual mode and
be sure to use the same ISO, aperture,
and exposure setting for each frame.
When shooting in RAW mode, the only
post-processing required will be to set a
consistent white balance across all the
images. This will help everything blend
neatly when you assemble your image.
Getting on Track
On your first attempts with this
method, keep things simple by shooting
with a fixed tripod. The main drawback
to this approach is that your exposure
time will be limited by your lens's focal
length. Over time, the Earth's rotation
causes the stars to drift across the sky
and leave trails on your images. The
effect is more pronounced with longer
lenses. For example, a 16-mm lens (on a
full-frame camera) allows an exposure
of up to 30 seconds long without seri56
OCTOBER 2023 * SKY & TELESCOPE
ous trailing. However, that same camera
equipped with a 50-mm lens will be
limited to just a 10-second exposure
before the star trails start to show. (This
topic was discussed in greater detail in
the February 2023 issue, page 55.)
Fortunately, many lenses in the
50-mm focal-length range have
extremely wide apertures (low f/stop
numbers, such as f/1.8), which allows
them to gather light more quickly than
their slower, wide-angle cousins. This
can partially compensate for the shorter
exposure times you'll need to use. You
can also boost your camera's ISO setting
to keep exposure times suitably short.
For lenses with even longer focal
lengths, you'll need a tracking mount
to achieve untrailed stars. Lightweight
units such as the iOptron SkyTracker
Pro or Sky-Watcher Star Adventurer are
simple, portable options. For larger telephoto
lenses, a more substantial equatorial
mount will be required to carry
the extra weight. Although I've used
this technique exclusively with camera
lenses, there's no reason you can't
extend it to include images captured
with a telescope. For example, if you
have trouble fitting the full extent of the
Andromeda Galaxy (or even the Moon)
in your photo, you can shoot multiple,
Panning for Gold
Once you've imported your images into
your computer, it's a simple matter to
blend the images together with imageediting
software. In Adobe Lightroom
you simply select Photo > Photo Merge
> Panorama Merge. This will launch the
Panorama Merge Preview dialog box.
Three options are presented for the type
of projection used to create the final
panorama: Spherical, Cylindrical, and
Perspective. Experiment to see which
one provides the most aesthetically
pleasing results for your particular set
of shots. Surprisingly, Lightroom and
Photoshop seem to have slightly different
algorithms for blending panoramas,
so if one program fails to work well, try
using the other.
For most of the images in this article,
I've left the ragged, saw-tooth outline
of the individual image tiles visible. If
you prefer, you can crop your resulting
mosaics so they have a more traditional
rectangular shape, which makes
them easier to display on your monitor.
And by carefully planning the initial
arrangement of the tiles, you can create
a final mosaic that supports both horizontal
and vertical crops. This is handy
if you plan to post the image on various
social media platforms, each of which
has its own preferred aspect ratio.
If the prospect of taking dozens of
overlapped images seems a bit daunting,
don't worry - you can start by using
just a couple of frames. Once you're
comfortable with the technique, you
can attempt more elaborate and expansive
shots. Eventually your confidence
will grow, and soon you'll be making
detailed, wide-angle photos that you'll
be proud to post online and show to
your friends and family.
¢ TONY PUERZER is an avid amateur
astrophotographer. He's happy that his
photo mosaics no longer involve scissors,
glue, and posterboard.

Sky & Telescope - October 2023

Table of Contents for the Digital Edition of Sky & Telescope - October 2023