Focusing And Aligning The Viewfinder - Meade ETX-90AT Astro Instruction Manual

Fig. 20a: ETX-90 Viewfinder:
(1) Front lens cell;
(2) Knurled lock-ring;
(3) Viewfinder eyepiece;
(4) Alignment screws;
(5) Viewfinder bracket;
Fig. 20b: ETX-105/-125
Viewfinder:
(6) Focus ring;
(7) Viewfinder eyepiece
14
Focusing the Viewfinder
The ETX model viewfinders are pre-focused at the factory. Individual eye variations
may require that the viewfinder be re-focused. To check the focus, point the
viewfinder at a distant terrestrial (land) object such as a telephone pole or light pole
and look through the viewfinder. If the viewfinder image is not in sharp focus, follow
the procedure below that applies to your telescope model.
ETX-90AT
1. Remove the viewfinder by slightly unthreading the six alignment screws (4, Fig.
20a) and slipping it out of the bracket.
2. Loosen the knurled lock-ring (2, Fig. 20a) located near the viewfinder's front lens
cell (1, Fig. 20). 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. Replace the viewfinder into the viewfinder bracket. Gently tighten the six alignment
screws. Proceed with Aligning the Viewfinder.
ETX-105AT or ETX-125AT
Turn the focus ring (6, Fig. 20b) at the base of the viewfinder eyepiece (7, Fig. 20b)
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:
1. Turn the three rear alignment screws of the viewfinder bracket (4, Fig. 20a) so that
the viewfinder tube is roughly centered within the viewfinder bracket.
Tip: Do not overtighten the alignment screws. When tightening one screw it may
be necessary to loosen one or both of the two other alignment screws.
2. Point the main telescope at some tall, well-defined land object, such as the top of
a telephone pole. Center the object, as precisely as possible, in the SP 26mm
eyepiece's field of view. Tighten the vertical and horizontal locks (6 and 9, Fig. 1)
so that the tube cannot move and the object remains centered (1, Fig. 21).
3. While looking through the viewfinder, turn one or more of the three front
viewfinder alignment screws (4, Fig. 20a), until the crosshairs of the view-finder
point at precisely the
same position as the
view through the
eyepiece of the main
telescope (2, Fig. 21).
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.
(before
alignment)
View through the
viewfinder
(after
alignment)
Fig. 21: Aligning the Viewfinder.
1
View through the
telescope eyepiece
2