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Actual image orientation as seen
with the unaided eye
Inverted image, normal with
Newtonians and as viewed with
eyepiece directly in other scopes
Reversed from left to right, as
viewed using a Star Diagonal on a
refractor or Schmidt-Cassegrain
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The image orientation changes depending on how the eyepiece is inserted into the telescope. When using the star
diagonal with refractors and Schmidt-Cassegrains, the image is right-side-up, but reversed from left-to-right (i.e.,
mirror image). If inserting the eyepiece directly into the focuser of a refractor or the visual back of the Schmidt-
Cassegrain (i.e., without the star diagonal), the image is upside-down and reversed from left-to-right (i.e., inverted).
Newtonian reflectors produce a right-side-up image but the image will appear rotated based on the location of the
eyepiece holder in relation to the ground. Newtonian reflectors are best for astronomical use where right-side-up
does not matter.
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To focus your refractor or Newtonian telescope, simply turn the focus knob located directly below the eyepiece
holder. Turning the knob clockwise allows you to focus on an object that is farther than the one you are currently
observing. Turning the knob counterclockwise from you allows you to focus on an object closer than the one you
are currently observing.
The Schmidt-Cassegrain focusing mechanism controls the primary mirror which is
mounted on a ring that slides back and forth on the primary baffle tube. The
focusing knob, which moves the primary mirror, is on the rear cell of the telescope
just below the star diagonal and eyepiece. Turn the focusing knob until the image
is sharp. If the knob will not turn, it has reached the end of its travel on the
focusing mechanism. Turn the knob in the opposite direction until the image is
sharp. Once an image is in focus, turn the knob clockwise to focus on a closer
object and counterclockwise for a more distant object. A single turn of the
focusing knob moves the primary mirror only slightly. Therefore, it will take many
turns (about 30) to go from close (near) focus to infinity.
For astronomical viewing, out of focus star images are very diffuse, making them
difficult to see. If you turn the focus knob too quickly, you can go right through
focus without seeing the image. To avoid this problem, your first astronomical
target should be a bright object (like the Moon or a planet) so that the image is
visible even when out of focus. Critical focusing is best accomplished when the
focusing knob is turned in such a manner that the mirror moves against the pull of
gravity. In doing so, any mirror shift is minimized. For astronomical observing,
both visually and photographically, this is done by turning the focus knob
counterclockwise.
Note: If you wear corrective lenses (specifically glasses), you may want to remove them when observing with an
eyepiece attached to the telescope. However, when using a camera you should always wear corrective lenses to
ensure the sharpest possible focus. If you have astigmatism, corrective lenses must be worn at all times.
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ure 3-4
Figure 3-5
The emblem on the end of the
focus knob shows the correct
rotational direction for
focusing your telescope
.