Orion 9995 instruction manual Appendix a Enhancing the Pointing Accuracy, Cone Error Calibration

Note: The following appendices apply to the Sirius EQ-G with GoTo hand controller only.

Appendix A: Enhancing the Pointing Accuracy

The Sirius EQ-G produces pointing accuracy adequate for most applications. If higher accuracy is required, “cone error” calibration may be necessary.

Cone Error Calibration

“Cone error” is found in all German equatorial mounted telescopes. Cone error results from the optical axis of the tele- scope not being aligned to the R.A. axis of the mount. This affects the pointing accuracy of the Sirius EQ-G. Three-star alignment automatically compensates for some of the cone error, but pointing accuracy will be optimized by mechanically minimizing the cone error. The following calibration procedure should be performed before the initial use of the telescope and periodically thereafter to ensure peak accuracy.

Testing for Cone Error

This test is performed at night using two bright stars located on opposite hemispheres of the night sky. Confirm the telescope is properly polar-aligned using the polar axis finder scope. Perform the one-star alignment using an eastern star as the alignment star (see “One-Star Alignment”). After completing the star alignment, choose a bright star in the western sky from the Sirius EQ-G object database and press ENTER to slew the telescope to the star. If the optical axis is perfectly aligned to the R.A. axis, the telescope will accurately put the star in the center of the eyepiece. This shows that there is no significant cone error in your telescope setup and you will not need to perform the calibration. It is acceptable if the star is slightly off- center as long as it appears in the eyepiece field of view and reasonable close to the center. Many factors determine the pointing accuracy of the Sirius EQ-G. Incorrect star alignment, loose R.A. or Dec. lock-knobs, or cone error. If the Sirius EQ-G puts the star outside the eyepiece field of view, you need to determine which of these factors is causing the pointing inac- curacy. To determine if the inaccuracy is caused by cone error, simply move the telescope about the R.A. axis by pressing the Left or Right direction button. If the star can be moved into the eyepiece field of view without adjusting the Dec. axis, it is likely that cone error exists in your telescope setup.

Cone Error Calibration Procedure

1.Insert an illuminated reticle eyepiece (not supplied) into the focuser (or diagonal) of the telescope. Confirm the tele- scope is properly set up and balanced, and the finderscope is aligned with the optical tube of the telescope. The mount also needs to be accurately polar aligned.

Note: Steps 2 to 4 are to identify R.A. and DEC movements in the reticle eyepiece. If you are already familiar with the move- ments, proceed to step 5.

2.Find a bright star and position the telescope so the star is centered in the eyepiece field of view.

3.Look into the eyepiece. Move the telescope about the R.A. axis using the R.A. direction buttons on the hand controller while carefully observing the movement of the star.

4.Keep moving the telescope about the R.A. axis back and forth to keep the star within the eyepiece field of view. Rotate the eyepiece in the focuser (or diagonal) until the movement of the star becomes parallel to one of the illu- minated crosshairs (Figure 15.2). This crosshair will repre- sent R.A. movement in the course of this procedure, and the perpendicular crosshair will represent Dec. movement. Tighten the set screws to secure the eyepiece in position. Make sure the eyepiece will remain stationary while the telescope is moved.

Figure 15.2.

Aligning the R.A. motion of the star with the crosshairs.

5.Point the telescope north so the tube is parallel with the R.A. axis of the mount.

6.Loosen the R.A. lock knob and rotate the telescope about the R.A. axis until the counterweight shaft is parallel to the ground (Figure 17a).

7.Using the Dec. direction button on the hand controller, adjust the telescope in Dec. so Polaris lies on the R.A. crosshairs of the illuminated reticle eyepiece (Figure 16).

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Figure 16. Adjust the telescope in Dec. (with the hand controller) to place the star on the R.A. crosshair.

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