| The Meade 16” LX200 telescopes are capable of photography in their standard altazimuth mode (either on the Super Giant field tripod or on the permanent altazimuth pier). However, altazimuth exposure times are limited to a maximum of 5 minutes (usually less, depending on which part of the sky is being imaged). Field rotation will start visibly turning the stars at the edge of the photographic field into curved streaks, rather than keeping them point-like, as this five-minute limit is reached. Field rotation is due to the altazimuth mount 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 and fractions of a second. However, deep space exposures of nebulas and galaxies require much longer exposure times, many of them well over the five-minute maximum. This results in streaked and unattractive images with an altazimuth mount scope. To keep stars from streaking during long exposure photos with the altazimuth-mounted scopes (this is not an issue with a 16” LX200 mounted on a permanent equatorial pier), you can correct for field rotation by installing the #1222 electronic field derotator between your camera and your telescope. The field derotator integrates with your scope’s Autostar II computer for imaging in the altazimuth mode. The Autostar calculates the amount of field rotation during an altazimuth photograph, based on the region of the sky being imaged. It then automatically rotates the field derotator and its attached camera in the opposite direction by the precise amount needed to compensate for the field rotation. The only drawback of the field derotator on the 16” LX200 might be the loss of the telescope’s PPEC (Permanent Periodic Error Correction) system when the field derotator is used. PPEC only works with a scope that is equatorially mounted on a pier. It does not work in the altazimuth mode. In general, however, the accuracy of the 16” LX200’s drive system is such that the field derotator will permit more than acceptable long exposure images. If exceptional accuracy is needed for critical imaging or scientific measurement purposes, however, you might want to consider going with an equatorially mounted 16” scope and training its PPEC system rather than using a field derotator on an altazimuth mount scope. |
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