Setting circles

Many computerized telescopes have setting circles (Figures 4.12,4.13) just like those on noncomputerized telescopes. The declination circle is permanently fixed in position; the right ascension circle can be rotated to align with the celestial sphere.

The declination circle reads 90° when the telescope is pointed directly away from its base, ready for polar alignment. (This is important; use a bubble level to check it and make adjustments as necessary.) Apart from that, the main use of setting circles is as a "sanity check" on the computer's performance, since they read the absolute position of the tube, not just its relative motion.

To use setting circles, first polar-align as accurately as possible; they work only in equatorial mode. Then go to any convenient star. The declination circle

Figure 4.12. Meade LX200 setting circles: declination (top), right ascension (bottom).

should show that star's declination; if it does not, either the declination circle or the polar alignment is inaccurate.

The right ascension circle rotates freely. Set it so that the pointer indicates the star's right ascension. Then ascertain whether the right ascension circle is driven by the telescope's tracking motor. If so (as on the Meade LX200), it will remain accurate throughout the evening; if not (as on the ETX-90), it is accurate only when you have just set it.

The right ascension circle may have two sets of numbers on it, one reading forward (1...2...3...) and one backward (3... 2... 1...). Assuming you are

Figure 4.13. Meade ETX-90 setting circles: declination (top), right ascension (bottom). LX90 is similar.

north of the Earth's equator, if the circle is attached to the base, as on the LX200, use the scale that reads forward. If the circle moves with the telescope, as on the ETX-90, use the scale that reads backward. When in doubt, set the circle for one star, then go to another star and see if it is correct.

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