Meade 7” through 14” LX200GPS telescopes are capable of photography in their standard altazimuth mode. However, 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, you can either correct for field rotation by installing an electronic field derotator between your camera and your altazimuth telescope or by mounting your scope on an equatorial wedge and using your scope in the equatorial mode.
The field derotator integrates with your scope’s Autostar II computer for imaging in the altazimuth mode. It is not used with the scope mounted on an equatorial wedge. 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 advantages of a field derotator are that you have one less heavy piece of equipment to take out into the field and assemble (an equatorial wedge) and you eliminate the extra time and effort needed to polar align the scope and equatorial wedge. In addition, you can image near the north celestial pole and the southern horizon with the field derotator. If you have an electric focuser, flip mirror system, and camera stacked on the rear of your equatorially-mounted scope, the equipment stack might be long enough to hit the drive base of your scope before the scope reaches the area around the pole. Since the field derotator works in the altazimuth mode, where the fork arms point straight up at the zenith, aiming at the celestial pole 30° or 40° above the northern horizon is not a problem. Also, with an equatorially-mounted scope, the front end of optical tube can often hit the drive base of the scope before the optical tube can be tilted low enough to aim at the southern horizon. Again, the altazimuth orientation of the scope using a field derotator eliminates this possibility.
Some of the drawbacks of the field derotator involve these very advantages, however. If you have a long enough equipment stack on the back of your scope – electric focuser, flip mirror system, camera, and now also a field derotator – you may have problems aiming the optical tube at the zenith with the field derotator in use. Whether your camera hits your scope’s drive base depends on how much swing-through clearance your scope has and how much equipment you feel you need to stack on your scope’s rear cell.
A more significant drawback might be the loss of your scope’s PPEC (Permanent Periodic Error Correction) when the field derotator is used. PPEC only works when the scope is equatorially-mounted on a wedge. It does not work in the altazimuth mode. If your scope has a drive system with low periodic error in both drive chains, the field derotator will permit perfectly acceptable long exposure images. If, on the other hand, your drive has enough periodic error to show up as slightly streaked stars in images taken in the equatorial mode (even if the errors can be eliminated by training the drive using the PPEC system), those errors will also show up using the field derotator. If you have reservations concerning the accuracy of your scope’s drive system, consider using the appropriate equatorial wedge and training your scope’s PPEC system rather than investing in a field derotator.
In the end, neither field derotator nor equatorial wedge is the perfect solution. You have to weigh the advantages and disadvantages to you of a field derotator over an equatorial wedge – weight, setup time, equipment stack and PPEC issues, etc. – in order to decide which route to take to reach your astrophotography goals.