The Erect Image Diagonal; The Eyepiece; Focusing - Celestron NEXSTAR 102GT Instruction Manual

THE ERECT IMAGE DIAGONAL

The diagonal diverts the light at a 45-degree angle from the light path of the telescope.
For astronomical observing, this allows you to observe in positions that are more comfortable than
if you were to look straight through the tube. For daytime viewing, the diagonal uses an erecting prism
that allows you to see land objects right-side-up to attach the diagonal:
1. Turn the thumbscrew on the eyepiece adapter at the end of the focuser barrel until it no longer
extends into (i.e., obstructs) the inner diameter of the focus barrel.
2. Remove the protective dust cap from the focuser barrel.
3. Slide the chrome portion of the diagonal into the eyepiece adapter.
4. Tighten the thumbscrew on the eyepiece adapter to hold the diagonal in place.
5. If you wish to change the orientation of the diagonal,
loosen the thumbscrew on the eyepiece adapter until the
diagonal rotates freely. Rotate the diagonal to the desired
position and tighten the thumbscrew.

THE EYEPIECE

The eyepiece is the optical element that magnifies the
image focused by the telescope. The eyepiece fits either
directly into the focuser or into the diagonal.
To install the eyepiece:
1. Loosen the thumbscrew on the diagonal so it does
not obstruct the inner diameter of the barrel.
2. Remove the protective dust cap from the
diagonal's barrel.
3. Slide the chrome portion of the low power 25 mm eye
piece into the diagonal.
4. Tighten the thumbscrew to hold the eyepiece in place.
5. To remove the eyepiece, loosen the thumbscrew on the diagonal and slide the eyepiece out.
Eyepieces are commonly referred to by focal length and barrel diameter. The focal length of each
eyepiece is printed on the eyepiece barrel. The longer the focal length (i.e., the larger the number) the
lower the eyepiece power or magnification; and the shorter the focal length (i.e., the smaller the num-
ber) the higher the magnification. Generally, you will use low-to-moderate power when viewing. For
more information on how to determine power, see the section on "Calculating Magnification."
Barrel diameter is the diameter of the barrel that slides into the star diagonal or focuser. The NexStar
uses eyepieces with a standard 1-1/4" barrel diameter. The eyepiece adapter includes a fitting that
can be removed to allow 2" visual accessories to be used. Please note that eyepieces used directly in
the focuser without a diagonal may not achieve focus even when the focuser is extended to its maxi-
mum length.

FOCUSING

To focus your telescope, simply turn either of the focus knobs on the sides of the focuser (see figure 2-6).
Turn the focus knob until the image is sharp. Once sharp, turn the knob towards you to focus on an
object that is closer than the one you are currently observing. Turn the knob away from you to focus on
a more distant object than the one you are currently observing.
8
COMPUTERIZED TELESCOPE
StarPointer
Eyepiece
Adapter
Focus Knob
FIGURE 2-6 – VISUAL ACCESSORIES
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Eyepiece
Erect Image
Diagonal – 45˚