A star diagonal is an angled mirror or prism used in telescopes that allows viewing from a direction that is perpendicular to the usual eyepiece axis. It allows more convenient and comfortable viewing when the telescope is pointed at, or near the zenith . Also, the resulting image is right side up, but is reversed from left to right. Star diagonals come in all price ranges, from as low as thirty dollars up to hundreds of dollars. However, even an expensive star diagonal will deliver poor performance if it is not in alignment with the optical axis of the telescope. A telescope in perfect collimation will be thrown out of collimation by a misaligned star diagonal and often this misalignment will determine the image quality of the telescope to a larger extent than the surface accuracy of the prism or mirror.
However I can do it by hand from microscope Binoviewer Broken, It nice work with 90 degree and Focuser 2 cm operation