Meade's #777 Off-Axis Guider provides a means during long-exposure astrophotography for the photographer to monitor the tracking of the telescope, assuring that the telescope remains precisely positioned upon the object being photographed.
Like a T-Adapter, the #777 Off-Axis Guider couples the camera body to the telescope in a straight-through path. But unlike common t-mount adapters, it also diverts a small amount of the incoming light to be diverted at a right angle, allowing a star's position to be examined for tracking errors when compared with a known position upon an illuminated reticle eyepiece at the beginning of the exposure. Position corrections can then be effected using the telescope's hand controller or drive corrector.
#777 Off-Axis Guider