Orion 120 EQ instruction manual The Polar Axis Finder Scope, Aligning the Polar Axis Finder Scope

Little Dipper

(in Ursa Minor)

Big Dipper	N.C.P.
(in Ursa Major)

Polaris

				r	Cassiopeia
			te
	in
o
P				rs
		ta
S

Figure 8. 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 Polaris.

To find Polaris in the sky, look north and locate the pattern of the Big Dipper (Figure 8). 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 bare- ly 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 one of the latitude adjusting T-bolts and tighten the other to 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. The latitude setting should not have to be adjusted again unless you move to a different viewing location some distance away.

3.Loosen the Dec. lock lever and rotate the telescope opti- cal tube until it is parallel with the R.A. axis. The pointer on the Dec. setting circle should read 90°. Retighten the Dec. lock lever.

4.Move the tripod so the telescope tube (and R.A. axis) points roughly at Polaris. If you cannot see Polaris direct- ly from your observing site, consult a compass and rotate the tripod so the telescope points North. There is a label bearing a large “N” at the base of the equatorial mount (Figure 9). It should be facing North.

The equatorial mount is now approximately polar-aligned for casual observing. More precise polar alignment is required for astrophotography and for use of the manual setting circles.

Note: From this point on in your observing session, you should not make any further adjustments to 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.

The Polar Axis Finder Scope

A nice feature of the AstroView 120 is the polar axis finder scope housed inside the R.A. axis of the equatorial mount (see Figure 3). When properly aligned and used, it makes accurate polar align- ment quick and easy to do. Alignment need only be done once, unless the polar finder gets bumped or otherwise shifts its position.

Remove the round cover cap from the front opening in the R.A. axis of the mount (see Figure 7). Look through the polar finder at a distant object during the day. Focus the polar finder so that the images and reticle are sharp by rotating the eyepiece end of the finder. Notice that the reticle pattern consists of a crosshair with a circle around the middle. On the circumference of this circle is a tiny circle; this is where Polaris will be placed for accurate polar alignment once the finder is properly aligned. Alignment is best done during the day, before going out into the field at night.

Aligning the Polar Axis Finder Scope

Aligning the polar axis finder scope so that it will accurately point at the true north pole is a two-step procedure. First, the polar finder must be rotated in its housing so that the small circle in which Polaris will be placed in is in the proper initial position. Next, the polar axis finder must be adjusted so that it points directly along the mount’s R.A. axis.

We will start by aligning the polar finder rotationally. Refer to Figure 3.

1.Loosen the large setscrew just above the R.A. setting circle. Rotate the R.A. setting circle until the line above the “0” on the setting circle lines up with the pointed indicator that is cast into the mount (located directly below the large setscrew; see Figure 3). Retighten the set screw.

Figure 9. For polar alignment, position the telescope so that the “N” label at the base of the mount faces north. The two azimuth adjustment knobs above it (arrows) are used to make fine adjustments to the mount’s azimuth position.

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