Image derotation device for relaying optical paths off or across gimbals

DETAILED DESCRIPTION The present invention achieves these objects by providing means to rotatably mount the gimballed portion of the system for rotation about a gimbal axis which is centrally located in the housing of the system and is parallel to and displaced from the optic axis of the objective lens means and associated optics in the gimballed portion of the system.
The optic axis of the gimballed portion thus describes an arc of constant radius about the gimbal axis during the typically 180 degrees of permitted rotation in azimuth about the gimbal axis.
The optic axis from the gimballed portion provides an optical path which leads from it to a V reflector type optical path folding means which is mounted for rotation not about the optical axis but about a projection of the gimbal axis itself.
The path folding means is rotated independently of, but simultaneously with, the gimballed portion and rotates at an angular rate equal to half of the angular rate of rotation of the gimballed portion.
At any point in its rotation the optical path folding means is positioned to lie on the optical axis from the objective lens.
The optical path folding means consists of a reflector assembly comprising two plane reflecting surfaces intersecting each other at a 90.
degree.
angle along a line forming an apex of the V cross section assembly which apex line intersects the gimbal axis at a 90.
degree.
angle at the point of rotation of the path folding means.
The optic path from the objective intersects one of the plane reflecting surfaces at an angle which is preferably 45.
degree.
.
That surface is positioned so that the optical path from the objective lens is folded by the first surface and directed to the other reflecting surface which again folds it so that it emerges in a direction parallel and opposed to the direction of the optical path along the optic axis of the gimballed portion of the system and is displaced therefrom.
It can be shown that the optical path lengths through the V cross-section reflector assembly comprising the optical path folding means is the same in any given position of the path folding means for any point on the image