Orion PRO 120 EQ instruction manual Balancing the Telescope, Installing the Finder Scope

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a b c d

Figure 4a - 4d. Proper operation of the equatorial mount requires that the telescope tube be balanced on both the R.A. and Dec. axes.

(a)With the R.A. lock lever released, slide the counterweight along the counterweight shaft until it just counterbalances the tube. (b) When you let go with both hands, the tube should not drift up or down. (c) With the Dec. lock lever released, loosen the tube ring lock clamps a few turns and slide the telescope forward or back in the tube rings. (d) When the tube is balanced about the Dec. axis, it will not move when you let go.

the mounting plate and rethread into the tube rings. Note that the side of the mounting plate with the central “groove” will be facing up. Use the small wrench to secure the tube rings to the mounting plate.

10.Loosen the black mounting plate securing knob as well as the metal safety thumbscrew on the top of the equatorial mount. Place the mounting plate, with tube rings attached, in the dovetail slot on top of the equatorial mount. Position the mounting plate so that it is centered on the dovetail slot. Re-tighten the mounting plate securing knob until the mounting plate is secure. Then, tighten the safety thumb- screw. The safety thumbscrew will ensure that the mount- ing plate (and telescope tube) will not fall off the EQ mount if the mounting plate securing knob should come loose.

11.Open the tube rings and lay the telescope optical tube in the rings at about the midpoint of the tube’s length. Rotate the tube so that the focuser is at a convenient position for viewing. Close the tube rings and tighten them.

Installing the Finder Scope

To place the finder scope (Figure 3a) in the finder scope brack- et, unthread the two black nylon screws until the screw ends are flush with the inside diameter of the bracket. Place the O-ring that comes on the base of the bracket over the body of the finder scope until it seats into the slot on the middle of the finder scope. Slide the eyepiece end (narrow end) of the finder scope into the end of the bracket’s cylinder opposite the adjustment screws while pulling the chrome, spring-loaded tensioner on the bracket with your fingers (Figure 3b). Push the finder scope through the bracket until the O-ring seats just inside the front opening of the bracket cylinder. Release the tensioner and tighten the two black nylon screws a couple of turns each to secure the finder scope in place. Insert the base of the finder scope bracket into the dovetail holder on the top of the focuser. Lock the bracket into position by tightening the knurled thumbscrew on the dovetail holder.

Inserting the Diagonal and Eyepiece

Loosen the thumbscrew on the 1.25" adapter on the focuser and remove the small dust cap. Insert the chrome barrel of the star diagonal into the focuser and secure with the thumb- screw. Loosen the thumbscrews on the diagonal and insert

the 25mm eyepiece into the focuser and secure it with the thumbscrews.

Your telescope is now completely assembled and should appear as shown in Figure 1.

4. Balancing the Telescope

To ensure smooth movement of the telescope on both axes of the equatorial mount, it is imperative that the optical tube is properly balanced. We will first balance the telescope with respect to the right ascension (R.A.) axis, then the declination (Dec.) axis.

1.Keeping one hand on the telescope optical tube, loosen the R.A. lock lever. Make sure the Dec. lock lever is locked, for now. The telescope should now be able to rotate freely about the right ascension axis. Rotate it until the counter- weight shaft is parallel to the ground (i.e., horizontal).

2.Now loosen both counterweight lock knobs and slide the weights along the shaft until they exactly counterbalance the telescope (Figure 4a). That’s the point at which the shaft remains horizontal even when you let go with both hands (Figure 4b).

3.Retighten the counterweight lock knobs. The telescope is now balanced on the right ascension axis.

4.To balance the telescope on the declination axis, first tight- en the R.A. lock lever, with the counterweight shaft still in the horizontal position.

5.With one hand on the telescope optical tube, loosen the Dec. lock lever. The telescope should now be able to rotate freely about the Dec. axis. Loosen the tube ring clamps a few turns, until you can slide the telescope tube forward and back inside the rings. (this can be aided by using a slight twisting motion on the optical tube while you push or pull on it) (Figure 4c).

6.Position the telescope in the mounting rings so it remains horizontal when you carefully let go with both hands. This is the balance point for the optical tube with respect to the Dec. axis (Figure 4d). If the optical tube does move, then you must adjust the position of the optical tube in the tube rings until it does balance in the declination axis.

7.Retighten the tube ring clamps.

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