Meade's #1220 Field De-Rotator allows long exposure times to yield sharp images while your telescope is being used in the altazimuth mode.
Typically with astrophotography in the altazimuth mode, your exposure will be limited to a maximum of 5 minutes. Beyond this arbitrary 5 minute barrier, and often before it, the rotation of the field visibly turns stars in the image into unattractive curved streaks. This occurs because when your telescope is in altazimuth mode it is not rotating around the same axis as the planet itself. Rotating upon the celestial pole(the axis the Earth is based upon) is not crucial for lunar and planetary astrophotography, because the exposures needed for these subjects are short enough to make it appear unimportant. Deep space exposures of galaxies and nebula, however, are another matter.
There are two options for preventing stars from streaking during long exposures: you can mount your scope upon an equatorial wedge and use the scope in the equatorial mode, or you can employ a field derotator.
The Meade #1220 Field De-Rotator integrates with your telescope's Autostar II computer exclusively for imaging in the altazimuth mode, and is not used with the scope mounted upon an equatorial wedge. The Autostar determines the amount of field rotation during the exposure based upon the region of the sky being imaged, and automatically rotates the field derotator(along with its attached camera) in the opposite direction by the exact amount needed to compensate for the field rotation.
Many users prefer a field derotator to employing an equatorial wedge because they have one less heavy chunk of equipment to carry into the field as well as less time aligning the scope. A field derotator also allows a broader declination range than a wedge, and less chance of interference between your mount and eyepiece-side accessories.