Sky & Telescope - November 2023 - 32

Solar System Imaging
Another important step is to give the telescope ample time
to reach thermal equilibrium. If your telescope is warmer
than the surrounding air, it will radiate heat, which will blur
the image. Give your telescope about an hour to cool before
starting to image. The process can be sped up with the use of
fans or more exotic active-cooling devices.
The biggest limiting factor in the quality of your images
will be the atmospheric seeing conditions above you. It's best
to monitor weather conditions using websites that cater to
astronomers, such as astrospheric.com or meteoblue.com to
see if it's worth setting up your telescope at all.
With these preliminaries out of the way, there's one more
critical step you need to perform before hitting the record button:
Focus the camera. Achieving perfect focus is pretty easy
under steady seeing conditions. More likely, you'll end up racking
the focuser back and forth trying to find a good approximation
of focus on a less-than-optimal night. While some
control programs include a focusing aid, I find it more reliable
to focus by eye using a high-contrast area on my target such
as a moon's shadow on the cloudtops of Jupiter, or the Cassini
division in Saturn's rings. It may take several minutes of trial
and error to achieve sharp focus, but take as long as you need. I
often refocus frequently throughout the night as the temperature
and seeing conditions change.
At last, you're ready to record! As you image throughout
the night, keep in mind that your target is a rotating planet.
That means there's an upper limit on how
long you can image before that rotation
begins to smear the fine details you're trying
to capture. For example, Jupiter rotates
quickly, so I limit my videos of it to 1
minute or less when the seeing conditions
are excellent. Saturn is noticeably smaller
and fainter, and I can go about 90 seconds
or so before its rotation begins to affect
uCOMPLETE CONTROL Specialized cameracontrol
software such as FireCapture (firecapture.
de), seen at right, allows more options than most
software that comes with a camera. Both FireCapture
and SharpCap (sharpcap.co.uk) offer additional
control of electric focusers and filter wheels.
image quality. With Mars, the limit is closer to 3 minutes.
Neither Venus nor Mercury rotates fast, so you can record
longer videos of each, though you'll still want to keep an eye
on your file sizes.
Planetary Processing
Once you've recorded and copied your data off the control
computer, it's time to stack and process your data. There are
several programs that can accomplish this, including AutoStakkert!
3 (autostakkert.com), AstroSurface (astrosurface.com),
and RegiStax 6 (astronomie.be/registax). Each one will distill
your video or image files into an image suitable for additional
processing steps, such as sharpening. Mac users also have several
options, including Lynkeos (lynkeos.sourceforge.io) and
Planetary System Stacker (https://is.gd/PlanetarySS).
While AutoStakkert! 3 includes a rudimentary sharpening
feature, its writer suggests that other planetary-imaging
software offers better options. I prefer to stack my video files
in AutoStakkert! 3 and then bring the stacked image into
WaveSharp (https://is.gd/WaveSharp) to utilize its powerful
wavelets sharpening tool. (WaveSharp is the successor to
RegiStax 6 currently in development, though it only includes
sharpening tools and does not stack images.)
Both RegiStax 6 and Autostakkert! 3 offer two ways to stack
your images. There's single-point alignment, which is most useful
for planets when they're only a few arcseconds across, such
as Uranus and Neptune, or Mercury, Venus,
and Mars when they're far from Earth.
The second stacking option is multi-point
alignment, which lets you select dozens of
points to align and stack on. This works
best on close-ups of lunar craters, on Jupiter
and Saturn, and on Mars, Mercury, and
Venus when their disks are large.
All stacking programs function in a
similar fashion: They first evaluate and
order the frames, then you choose how
many to include in that stack. Generally,
the more frames you stack, the better the
signal-to-noise ratio is in the resulting
image, allowing you to aggressively sharpen
it. How many frames you stack will vary
depending on the seeing conditions. When
the seeing is steady, you might be able to use
half the frames you've acquired, but when
conditions are poor perhaps only 20% of
your frames will make the cut. Adding more
frames to a stack taken under poor seeing
will only produce a blurry result. If you're
unsure of how many to stack, AutoStakkert!
3 permits you to create up to 8 custom
stacks at a time so you don't have to go back
and redo the lengthy process.
With your data all stacked, it's time
to sharpen the results. I prefer using the
32 NOVEMBER 2023 * SKY & TELESCOPE
http://www.astrospheric.com http://www.meteoblue.com http://www.autostakkert.com http://www.astrosurface.com http://www.astronomie.be/registax http://lynkeos.sourceforge.io https://is.gd/PlanetarySS https://is.gd/WaveSharp http://sharpcap.co.uk
Sky & Telescope - November 2023

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