THE VIEWFINDER
As with most astronomical telescopes, both ETX model
telescopes present a fairly narrow field of view to the observer.
As a result it is sometimes difficult to locate and center objects
in the telescope’s field of view. The viewfinder, by contrast, is a
low-power, wide-field sighting scope with crosshairs that let you
easily center objects in the eyepiece of the main telescope’s
field of view.Atelescope’s viewfinder is a tremendous aid in the
location of faint astronomical objects before observation is
made through the telescope’s eyepiece.
ETX-90EC 8 x 21mm Erect-Image Viewfinder:
Standard equipment with the ETX-90EC, the 8 x 21mm Erect-
Image Viewfinder has 8-power magnification and an aperture
of 21mm. This viewfinder presents a correctly oriented image
both up-and-down and left-to-right.
ETX-125EC 8 x 25mm Right-Angle Viewfinder:
Standard equipment with the ETX-125EC, the 8 x 25mm Right-
Angle Viewfinder has 8-power magnification and an aperture of
25mm. This viewfinder presents a correctly oriented image up-
and-down, but is reversed left-for-right.
Focusing the Viewfinder
Both ETX model viewfinders are factory pre-focused to objects
located at infinity. Individual eye variations may require that the
viewfinder be re-focused. To check the focus look through the
viewfinder. Point the viewfinder at a distant terrestrial (land)
object such as a telephone pole or light pole. If the viewfinder
image is not in sharp focus, follow the procedure for your type
of telescope.
ETX-90EC
1. If the viewfinder is mounted in the viewfinder bracket (5,Fig.
7), remove it by slightly unthreading the six alignment
screws (4 and 6, Fig. 7) until it can slip easily out of the
bracket.
2. Loosen the knurled lock-ring (2, Fig. 7) located near the
viewfinder’s front lens cell (1, Fig. 7). Unthread this ring
(counterclockwise, as seen from the eyepiece-end of the
viewfinder) by several turns.
3. Look through the viewfinder and focus on a distant object.
Slowly rotate the front lens cell in one direction or the other,
until the object appears sharp. One or two rotations of the
lens may have a significant effect on image focus.
4. Lock the focus in place by threading the knurled lock-ring
clockwise up against the viewfinder’s lens cell.
5. Place the viewfinder into the viewfinder bracket on the main
telescope. Gently tighten the six alignment screws. Proceed
with Aligning the Viewfinder.
ETX-125EC
Turn the focus ring (2, Fig. 7) at the base of the viewfinder
eyepiece (3, Fig. 7) in either direction until a sharp focus is
reached.
Aligning the Viewfinder
In order for the viewfinder to be useful, it must first be aligned
with the main telescope, so that both the viewfinder and the
main telescope are pointing at precisely the same location. To
align the viewfinder follow this procedure:
1. The viewfinder bracket (5, Fig. 7) includes six alignment
screws (4 and 6, Fig. 7). Turn the three rear alignment
screws (6, Fig. 7) so that the viewfinder tube is roughly
centered within the viewfinder bracket.
N OT E : Do not ove rtighten the alignment screw s. W h e n
tightening one screw it may be necessary to loosen one or both
of the two other alignment screws.
2. Using the SP26mm eyepiece, point the main telescope at
some easy-to-find, well-defined land object, such as the top
of a telephone pole. Center the object precisely in the
e y e p i e c e ’s field of view, then tighten the vertical and
horizontal locks (6 and 10, Fig. 1) so that the object does not
move (1, Fig. 8).
3. While looking through the viewfinder, turn one or more of the
three front viewfinder alignment screws (4, Fig. 7), until the
crosshairs of the viewfinder point at precisely the same
position as the view through the eyepiece of the main
telescope (2, Fig. 8).
Re-check that the viewfinder’s crosshairs and the main
telescope are now pointing at precisely the same object. The
viewfinder is now aligned to the main telescope. Unless the
alignment screws are disturbed or the viewfinder jarred, the
viewfinder should remain aligned indefinitely.
Using the Viewfinder
To locate any object, terrestrial or astronomical, first center the
object in the crosshairs of the viewfinder; the object should also
then be centered in the field of view of the main telescope.
O b s e r ving T i p: If higher observing magnifications are
intended, first locate, center, and focus the object using a low-
power eyepiece (e.g., SP 26mm eyepiece). Remove the low-
power eyepiece and replace it with a higher-power eyepiece;
the object should still be centered in the field of view. Objects
are much easier to locate and center at lower powers; higher
powers are employed simply by changing eyepieces.
CHAPTER
1
Fig.8: Aligning the Viewfinder.
1
2
(before
alignment)
(after
alignment)
View through the
telescope eyepiece
View through the
viewfinder
3
1
4
Fig.7: The Viewfinder. (1) Front lens cell; (2) Knurled lock-
ring (ETX-90EC) or focus ring (ETX-125EC); (3) Viewfinder
eyepiece; (4) Front alignment screws; (5) Viewfinder bracket;
(6) Rear alignment screws.
5
2
ETX-90EC
2
6
ETX-125EC
3
