| Altazimuth mount computerized telescopes (Celestron NexStar and CPC series; Meade LX90, LX200, and RCX400 series) are capable of astrophotography in their standard altazimuth mode. However, exposure times are limited to a maximum of 5 minutes (usually less). Field rotation will start visibly turning the stars at the edge of the 35mm or large CCD camera photographic field into curved streaks, rather than keeping them point-like, as this five-minute limit is approached. Field rotation is due to the telescope not rotating around the same axis that the Earth does (the celestial pole). Field rotation is not a problem for lunar and planetary photography, where exposure times are measured in seconds, or fractions of a second. Deep space exposures of nebulas and galaxies require much longer exposure times, however. Many deep space exposures are well over the five-minute maximum, particularly tri-color CCD images that require multiple exposures through different color filters. To keep stars from streaking during long exposure photos, a computerized altazimuth mount scope can be installed on an equatorial wedge. A wedge is a heavy die-cast mount that bolts to the top of the telescope tripod. The telescope is in turn bolted to the adjustable tilt plate of the wedge. The tilt plate pivots up and down to let you point the scope’s fork arms towards the north celestial pole so that the polar axis of the telescope (the right ascension axis) is parallel to the earth’s axis. This aligns the telescope’s axis of rotation with the Earth’s axis of rotation, allowing the telescope’s drive to track the stars without streaking. The angle of tilt will depend on your latitude when the telescope is level. The angle between the horizon level and the polar axis will equal your latitude when the telescope’s axis of rotation is parallel with that of the Earth’s. |
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