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ENGLISH | 5
If you have an astronomical refracting telescope and you
were to insert an eyepiece directly into the telescope’s
focuser, you would see an image that was upside down
and mirror reversed. Because observing straight through
the telescope can be difficult to use when looking directly
overhead, astronomers use a mirror diagonal to make the
eyepiece more accessible. The mirror in the diagonal also
flips the image so that it is correctly oriented up and down.
However, it will still appear backwards left to right. This is
perfectly normal.
Optical systems such as spotting scopes and binoculars
give a correctly oriented image, but they were designed
for terrestrial observing. They use complex sets of prisms
that flip the image so that it appears correctly oriented.
Astronomers avoid using additional glass elements in the
light path such as mirrors or prisms because each time light
strikes an optical surface some of the light is absorbed by
the glass and is lost to the observer’s eye. The prisms in
spotting scopes and binoculars will reflect the light path 4
to 5 times before sending it to the eyepiece, losing a little
bit more light each time. When looking at faint targets,
such as galaxies or nebulae, the astronomer wants his or
her eye to capture every photon of light possible to see the
faintest detail. This is not a problem for daytime terrestrial
telescopes where daylight is plentiful—and nobody wants to
see wildlife upside-down or backwards! If you really want to
use your scope for terrestrial viewing with a fully corrected
image, there are optional prism assemblies available, but
check with the manufacturer as not all refractors can reach
focus with all erecting prisms.
Newtonian reflector telescopes were also designed for
astronomical use and images of terrestrial objects will
appear upside-down. Because the design uses two
mirrors, the images you see are correct left to right and
up and down, but the views are rotated, depending on the
angle of the focuser and the ground as well as how you
hold your head to the eyepiece. Using a diagonal like the
refractor would actually cause more problems by mirror
reversing the image and not really correcting the image at
all. Unfortunately for Newtonian telescopes, there is no
easy way to correct this.
Astronomers don’t mind an upside-down view since
there is really no up or down in space. It is all a matter of
perspective. Because the Earth we live on is roughly shaped
like a sphere, a person in Australia looking at the Moon with
the naked eye would see it upside-down compared to a
person simultaneously observing it from Alaska. They are
looking at the same object, but from different perspectives.
IMAGE ORIENTATION
Image orientation as seen with the
unaided eye & using erecting devices
on refractors & Newtonians.
Reversed from left to right, as viewed
using a Star Diagonal on a refractor.
Inverted image, normal with Newtonians
& as viewed with eyepiece
directly in a refractor.
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