Orion Sirius EQ?G, Manual, Page: 21 / 28

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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.
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).
Figure 15.2.
Aligning the R.A.
motion of the star
with the crosshairs.
Figure 16.
Adjust the telescope in Dec. (with the hand controller)
to place the star on the R.A. crosshair.
.