Orion 90 EQ Polar Alignment, Use of the R.A. and Dec. Slow-Motion Control Cables

to compensate for that motion, allowing you to easily “track" the movement of astronomical objects, thereby keeping them from drifting out of the telescope’s field of view while you’re observing.

This is accomplished by slowly rotating the telescope on its right ascension (R.A.) axis, using only the R.A. slow-motion cable. But first the R.A. axis of the mount must be aligned with the Earth’s rotational (polar) axis - a process called polar alignment.

Polar Alignment

For Northern Hemisphere observers, approximate polar alignment is achieved by pointing the mount’s R.A. axis at the North Star, or Polaris. It lies within 1° of the north celestial pole (NCP), which is an extension of the Earth’s rotational

	Little Dipper
	(in Ursa Minor)
Big Dipper	N.C.P.
(in Ursa Major)		Polaris
Pointer
Stars

Figure 5. To find Polaris in the night sky, look north and find the Big Dipper. Extend an imaginary line from the two "Pointer Stars" in the bowl of the Big Dipper. Go about five times the distance between those stars and you'll reach Polaris, which lies within 1° of the north celestial pole (NCP).

axis out into space. Stars in the Northern Hemisphere appear to revolve around the NCP.

To find Polaris in the sky, look north and locate the pattern of the Big Dipper (Figure 5). The two stars at the end of the “bowl" of the Big Dipper point right to Polaris.

Observers in the Southern Hemisphere aren’t so fortunate to have a bright star so near the south celestial pole (SCP). The star Sigma Octantis lies about 1° from the SCP, but it is barely visible with the naked eye (magnitude 5.5).

For general visual observation, an approximate polar align‑ ment is sufficient.

1.Level the equatorial mount by adjusting the length of the three tripod legs.

2.Loosen the latitude lock t-bolt. Turn the latitude adjust‑ ment t-bolt and tilt the mount until the pointer on the latitude scale is set at the latitude of your observing site. If you don’t know your latitude, consult a geographical atlas to find it. For example, if your latitude is 35° North, set the pointer to 35. Then retighten the latitude lock t-bolt.

The latitude setting should not have to be adjusted again

unless you move to a different viewing location some dis‑ tance away.

3.Loosen the Dec. lock knob and rotate the telescope opti‑ cal tube until it is parallel with the R.A. axis, as it is in Figure 1. The pointer on the Dec. setting circle should read 90°. Retighten the Dec. lock lever.

4.Loosen the azimuth lock knob at the base of the equa‑ torial mount and rotate the mount so the telescope tube (and R.A. axis) points roughly at Polaris. If you cannot see Polaris directly from your observing site, consult a com‑ pass and rotate the mount so the telescope points North. Retighten the azimuth lock knob.

The equatorial mount is now polar aligned for casual observ‑ ing. More precise polar alignment is recommended for astrophotography.

From this point on in your observing session, you should not make any further adjustments in the azimuth or the latitude of the mount, nor should you move the tripod. Doing so will undo the polar alignment. The telescope should be moved only about its R.A. and Dec. axes.

Use of the R.A. and Dec. Slow-Motion Control Cables

The R.A. and Dec. slow-motion control cables allow fine adjustment of the telescope’s position to center objects within the field of view. Before you can use the cables, you must manually “slew" the mount to point the telescope in the vicini‑ ty of the desired target. Do this by loosening the R.A. and Dec. lock knobs and moving the telescope about the mount’s R.A. and Dec. axes. Once the telescope is pointed somewhere close to the object to be viewed, retighten the mount’s R.A. and Dec. lock knobs.

The object should now be visible somewhere in the tele‑ scope’s finder scope. If it isn’t, use the slow-motion controls to scan the surrounding area of sky. When the object is visible in the finder scope, use the slow-motion controls to center it. Now, look in the telescope’s eyepiece. If the finder scope is properly aligned, the object should be visible somewhere in the field of view. Once the object is visible in the eyepiece, use the slow-motion controls to center it in the field of view.

The Dec. slow-motion control cable can move the telescope a maximum of 25°. This is because the Dec. slow-motion mech‑ anism has a limited range of mechanical travel. (The R.A. slow-motion mechanism has no limit to its amount of travel). If you can no longer rotate the Dec. control cable in a desired direction, you have reached the end of travel, and the slow- motion mechanism must be reset. This is done by first rotating the control cable several turns in the opposite direction from which it was originally being turned. Then, manually slew the telescope closer to the object you wish to observe (remember to first loosen the Dec. lock knob). You should now be able to use the Dec. slow-motion control cable again to fine adjust the telescope’s position.

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