a.b.
6.Setting Up and Using the Equatorial Mount
When you look at the night sky, you no doubt have noticed that the stars appear to move slowly from east to west over time. That apparent motion is caused by the Earth’s rotation (from west to east). An equatorial mount (Figure 5) is designed to compensate for that motion, allowing you to easily “track” the movement of astronomical objects, thereby keeping them from drifting out of your 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 the built in motor drive. But first the R.A. axis of the mount must be aligned with the Earth’s rotational (polar)
c.d.
Figure 4a-d. Proper operation of the equatorial mount requires that the telescope tube be balanced on the R.A. and Dec. axes. (a) With the R.A. lock lever released, slide the counterweight down the counterweight shaft until it just counterbalances the telescope tube.
(b)When you let go with both hands, the tube should not drift up or down. (c) With the Dec. lock lever released, loosen the tube ring lock clamps a few turns and slide the telescope forward or back in the tube rings. (d) When the tube is balanced about the Dec. axis, it will not move when you let go.
have either too much or too little counterweight. Remove counterweight, or add optional counterweights if needed.
Dec lock lever
Front opening
| (Dec) | axis |
Declination |
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| Right |
Ascension | |
(R |
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. | . |
| ) |
| axis |
R.A. lock lever
Polar axis finder scope
Latitude scale
Latitude adjustment
3.Retighten the counterweight lock knob. The telescope is now balanced on the right ascension axis.
4.To balance the telescope on the declination axis, first tight- en the R.A. lock lever, with the counterweight shaft still in the horizontal position.
5.With one hand on the telescope optical tube, loosen the Dec. lock lever. The telescope should now be able to rotate freely about the declination axis.
6.Loosen the knurled ring clamps on the tube rings a few turns, until you can slide the telescope tube forward and back inside the rings (this can be aided by using a slight twisting motion on the optical tube while you push or pull on it) (Figure 4c). If the mounting plate is connected directly to your telescope’s tube (i.e. tube rings are not utilized), you can balance the telescope in Dec. by sliding the mounting plate forward or back in the slot on top of the equatorial mount.
7.Position the telescope in the tube rings so it remains hori- zontal when you carefully let go with both hands. This is the balance point for the optical tube with respect to the Dec. axis (Figure 4d).
8.Retighten the knurled ring clamps.
The telescope is now balanced on both axes. When you loos- en the lock lever on one or both axes and manually point the telescope, it should move without resistance and should not drift from where you point it.
Figure 5. The Sirius EQ-G mount.
Polar Alignment
For Northern Hemisphere observers, approximate polar align- ment is achieved by pointing the mount’s right ascension axis at the North Star, or Polaris. It lies within 1° of the north celes- tial pole (NCP), which is an extension of the Earth’s rotational axis out into space. Stars in the Northern Hemisphere appear to revolve around the NCP.
| Little Dipper | |||
| (in Ursa Minor) | |||
Big Dipper | N.C.P. | |||
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(in Ursa Major) |
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| Polaris |
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Pointer |
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| Cassiopeia |
Stars |
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Figure 6. 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).
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