Orion ATLAS EQ-G User Manual download pdf (Page 21)

Note: The following appendices apply to the Atlas EQ-G

with GoTo hand controller only.

Appendix A: Enhancing the

Pointing Accuracy

The Atlas EQ-G produces pointing accuracy and tracking

accuracy adequate for most applications. If higher precision is

required, “cone” error calibration may be necessary.

Cone Error Calibration

“Cone” error is a common inaccuracy found on all German

equatorial mount designs. Cone error results from the optical

axis of the telescope not being aligned to the R.A. axis of the

mount. This affects the pointing accuracy of the Atlas 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 calibra-

tion procedure should be performed before the initial use of

the telescope and periodically thereafter to ensure peak accu-

racy.

Testing for Cone Error

This test is performed at night using two bright stars located

on opposite hemispheres of the night sky. Confirm that 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 com-

pleting the star alignment, choose a bright star in the western

sky from the Atlas 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

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 Atlas EQ-G. Incorrect star alignment,

loose R.A. or Dec. lock-knobs, or cone error. If the Atlas 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.

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.

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 16). 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.

5. Point the telescope North and set the latitude scale to

your local latitude using the altitude adjustment L-bolts.

Alternatively, place Polaris in the polar axis finder scope if

your polar axis finder scope is accurately aligned with 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 (as shown in Figure 18a).

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 17).

8. Without moving the R.A. axis, adjust the azimuth control

knobs (Figure 2) to orient Polaris in the center of the eye-

Figure 16. Aligning

the R.A. motion of the

star with the crosshairs.

Figure 17. Adjust the telescope in Dec. (with the hand controller)

to place the star on the R.A. crosshair.

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