Sky & Telescope - February 2023 - 66

Imaging Technique
for each segement. My preferred software is Adobe Camera
Raw (ACR), but Adobe Lightroom has identical functions.
For simple panoramas I'll first try the Merge to Panorama
command. It can work great, with one click. Or it can fail
frustratingly, often on scenes similar to ones it previously
worked well on. Scenes over water or with dark end segments
often can foil the software. If it fails, my next recourse is to
send the processed files into Photoshop and try its Photomerge
command. This sometimes works for image sets that ACR or
Lightroom balk at.
The Adobe competitors Affinity Photo, Capture One, and
ON1 Photo RAW also have good panorama stitching functions.
But I've found that any sets that give Adobe's software
trouble usually also fail in the competitor's programs.
Advanced Stitching
For problem pans, and usually for 360° scenes and multitier
panos, I resort to PTGui (which stands for Panorama
Tools Graphical User Interface), a $160 program for Linux,
MacOS, and Windows (ptgui.com). While it can process raw
files from most cameras, it will not read any of the settings
applied to the frames in other programs. So I export processed
raw files as 16-bit TIFFs for each segment, then import
the TIFFs into PTGui.
Hitting PTGui's Align Images magically results in a seamlessly
stitched and blended panorama . . . usually. Sometimes
it also fails. In such cases the software at least indicates
which segments have issues and offers the option to manually
create alignment " control points. " This involves examining
adjacent frames and clicking on identical stars or landscape
features in each frame. The process can be tedious. And you
have to know the sky well enough to identify the same stars
on multiple frames.
I always export the stitched segments to a layered Photoshop
file (PSD). This allows me to manually edit the
automatically generated masks to fix minor issues such as
horizons with " fault lines " resulting from PTGui aligning on
the moving stars, rather than on the stationary ground. Be
warned: Even single-layer panos can be tens of thousands of
pixels wide and many gigabytes in size. This will likely tax the
ability of your aging computer to process and save them.
A key feature of PTGui is that for 360° scenes you can
rotate and move the entire scene around to improve the final
composition. Other programs, if they work at all on such
scenes, can't do that. What they give you is what you get.
Although all stitching programs can be balky at times, the
results can be wonderful, and much easier to create than in
the old analog days when we had to cut and paste (literally!)
prints together to make panoramas. Try some simple panos
to start. I think you'll soon be hooked on the wide-fields and
pixel-peeping resolution that panoramas can provide.
¢ ALAN DYER is coauthor with Terence Dickinson of The
Backyard Astronomer's Guide. A gallery of his panorama images
can be viewed at https://is.gd/nightpanos.
66 FEBRUARY 2023 * SKY & TELESCOPE
uPTGUI PROJECTIONS The dedicated stitching program PTGui
can handle complex panos, like this one consisting of 40 segments
(four strips of 10 images each), merging them using a choice of map
projections, such as Equirectangular (bottom) or Circular fisheye
(right). The latter style of 360° panorama is possible only with sets of
images that cover the entire sky from horizon to zenith.
http://www.ptgui.com https://www.is.gd/nightpanos
Sky & Telescope - February 2023

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