Introduction of the GP-B Telescope
The GP-B Telescope, strictly speaking, is a star tracker telescope.
Telescope Geometry
The GP-B telescope consists of a concave primary mirror which has a built in aspherical correction, a convex secondary and a tertiary mirror.
These mirrors focus the star light on the front end of the telescope where a window supports both the secondary mirror and the imaging optics.
At the focal point, the star image is bisected by two equivalently orthogonal roof prism mirrors.
The bisected images are refocused by field lenses to the corresponding detectors.
The difference in signal strength on the detectors of each axis gives the inccident angle of the incoming star light.
The effective focal length of the telescope is 150 inches and the aperture is 5.66 inches with a 2.85 inch obscuration.