Sky & Telescope - February 2024 - 63

the mosaic approach, both in terms of acquisition and postprocessing
strategies. Here are some of the most important
things to consider when executing one yourself.
It's All in the Planning
Mosaicking star fields is fairly straightforward: Consider the
size of your target, then determine how many fields your
particular scope and camera combination requires to cover
it. But you can't simply shoot adjacent fields and assume
they will snap together like pieces of a puzzle. Instead, you
need to precisely aim for the fields to overlap enough that
you'll have identifiable alignment stars common to each of
the overlapping segments in order to accurately and seamlessly
stitch them together.
While you can plan and execute an image mosaic
manually, you'll have better luck if you use control
software to point your telescope to record the necessary
frames. Most imaging-control software includes mosaic
scripting. Some popular examples are Sequence Generator
Pro (sequencegeneratorpro.com), TheSkyX Imaging Edition
(bisque.com), and Voyager (software.starkeeper.it). I find
that the free software N.I.N.A., short for Nighttime Imaging
'N' Astronomy (nighttime-imaging.eu), works very well
for automating all my image-acquisition tasks, including
capturing mosaics (S&T: Sept. 2023, p. 28).
Let's consider a two-panel image for your first mosaic project.
Once you've learned the ropes, you can increase the size
of the mosaic almost without limit.
Perhaps the most important consideration when planning
a mosaic is determining how much overlap between
panels is sufficient to produce a seamless result. You need
to consider having enough stars in the overlapping area to
use as alignment points, as well as adequate space to ensure
gradients or other inconsistencies between the overlapping
frames won't create a visible seam in the final product. A
20% overlap should be adequate for most purposes, though
you may need to increase this if your telescope produces
noticeable field distortions, as is common particularly in
wide-field instruments.
The Framing Assistant in N.I.N.A. makes mosaic planning
easy. For it to work correctly, you'll need to input information
about the imaging system's focal length and sensor size. Start
by searching for your target in the Sky Atlas located in the
main menu along the left side of the screen. Then click on
Send to Framing Assistant in the top right of the window. In
a few moments, the Framing Assistant opens with an image
of your target taken from one of the all-sky surveys that the
software utilizes for this purpose. Verify that the information
about your camera and telescope focal length is correct.
Next, increase the number of vertical or horizontal panels
you'd like to shoot using the Horizontal panels and Vertical
panels sliders until the simulated frames cover the entire
target. You can adjust the Panel overlap setting if needed,
though I find the default 20% works well with my equipment.
When you're satisfied with the framing and panel overlap,
click Add target to sequence and choose the options Add target
to sequence > Sequencer > Deep Sky Object Instruction Set.
The program then opens the Advanced Sequence tab. Here
you'll need to decide whether to shoot data for both panels
each night or concentrate on one field at a time. There are
good arguments for both approaches, but I find I get more
consistent results by recording data for both panels each night
until they are all complete. This ensures that the exposures
across the two fields are subject to the same transparency and
seeing conditions, making them slightly easier to assemble.
A related question for monochrome-camera users making
color composites is whether to shoot through all the necessary
filters during each session, or just one or two per night.
I recommend the latter so I can tailor the filter choice to
the observing conditions. For example, I tend to image with
color filters on the best nights, using the luminance filter in
particular on the very best nights. Narrowband images are
a bit more forgiving of seeing and sky brightness and so are
desirable on most nights, including when the Moon is up.
1
2
A GOOD PLAN The Framing Assistant in N.I.N.A. makes it easy to
plan your mosaic. After entering the specifics about your camera and
telescope, set the number of horizontal and vertical panels in the Targets
section. Then simply drag and rotate the field-of-view simulation to
compose your shot.
sk yand tele scope .o r g * FEBRUARY 2024 63
http://www.sequencegeneratorpro.com http://www.bisque.com http://software.starkeeper.it http://www.nighttime-imaging.eu http://skyandtelescope.org
Sky & Telescope - February 2024

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