Sky & Telescope - November 2023 - 29

T
he planets in our solar system stand out from other
night-sky objects in many important ways. At times
some of them appear brighter than any of the stars,
and they move noticeably. Unlike distant star clusters,
nebulae, and galaxies, which appear as faint, colorless glows
in most telescopes, the major planets reveal fleeting glimpses
of small-scale features and distinct colors in the eyepiece.
Recording such details is perhaps easier than you think.
Dust storms raging across the surface of Mars, storms in
the belts of Jupiter, or tiny gaps in the rings of Saturn can be
photographed by anyone with a mid-size telescope and steady
skies. Here's how you do it.
Which Telescope?
Although the planets are relatively nearby, resolving details
on their disks still requires a lot of magnification. Most any
telescope can produce pleasing planetary images, though
detecting swirling clouds within Jupiter's Great Red Spot
or the Tharsis volcanoes on Mars requires an aperture of at
least 8 inches.
Refractors, reflectors, and catadioptric telescopes are all
available with apertures suitable for planetary imaging, but
each has its own strengths and weaknesses. For example,
an 8-inch apochromatic refractor theoretically provides the
greatest contrast due to its unobstructed aperture, but such
instruments are very expensive, relatively heavy, and feature
tubes longer than many comparable reflectors.
The best-value telescope suitable for planetary imaging is
the Newtonian reflector. But the design's biggest weakness is
that its tube can be prone to flexure, requiring frequent collimation
to ensure consistent optical performance throughout
the night. Additionally, an equatorial mount can place the
eyepiece of Newtonian at an awkward location as the instrument
tracks across the sky.
A good compromise between aperture and portability is
the popular Schmidt-Cassegrain telescope (SCT). This design
utilizes a corrector plate and mirrors to fold a long optical path
into a compact package, resulting in an instrument roughly
one third as long as a Newtonian reflector or refractor with
the same size aperture. Other Cassegrain variants such as the
Classical Cassegrain or Maksutov-Cassegrain are also suitable.
Regardless of which telescope you choose, there are a few
additional accessories you'll need. Resolving the smallest planetary
details requires a long focal ratio, around f/20 or more.
Commercial telescopes are typically offered with focal ratios of
f/4.5 to f/10, so you'll need a quality Barlow lens, eyepiece projection
adapter, or other amplification device (like Tele Vue's
DETAILED DISKS Capturing planetary detail has never been easier.
The author recorded this montage using a 12.5-inch Newtonian reflector
with various high-speed video cameras. He captured the images from
his home in southern New Hampshire - a location often assumed to be
less than ideal for planetary imaging.
uREADY FOR ACTION The author's 12.5-inch Newtonian reflector
awaiting nightfall while aimed at the Moon. Our satellite is a great target
to practice planetary imaging when the planets aren't visible.
sk yand tele scope .o r g * NOVEMBER 2023 29
Powermates) to achieve the desired focal ratio.
Imaging with extremely long focal lengths means an electric
focuser is a must-have accessory. Attempting to manually
focus will simply magnify the vibrations from your hand,
producing an unsteady view that is virtually impossible to
focus. And you'll also be refocusing many times during the
night as the temperature drops and seeing conditions change.
One other useful accessory that has gained popularity in
recent years among planetary imagers is the Atmospheric
Dispersion Corrector (ADC). This device resembles a short
Barlow that contains two weak prisms placed in the optical
path. Earth's atmosphere is loaded with water vapor, which
smears light into its component colors, an effect known as
atmospheric dispersion. The effect appears as red and blue fringing
on the top and bottom of your target planet and becomes
more pronounced the closer the planet is to the horizon. By
adjusting the ADC, you can correct this chromatic smear and
noticeably improve the sharpness of the image. An ADC is a
worthwhile investment, particularly at more northerly locations
where the planets arc across the sky at lower elevations.
ALL IMAGES COURTESY OF THE AUTHOR
http://skyandtelescope.com
Sky & Telescope - November 2023

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