A 90° erecting prism diagonal providing a correct left/right and up/down image, designed exclusively for William Optics RedCat / SpaceCat 51 models in versions I, IX and II (not compatible with version II-U or with larger models such as RedCat 61 / 71). It enables terrestrial viewing with a correctly oriented image and allows the use of 1.25" eyepieces (note that due to the limited backfocus, not all eyepieces will reach focus).
 
The housing is CNC-machined from 6061T anodized aluminum, and the BaK4 prism features anti-reflection coatings; the interior is matte-blackened to maximize contrast.
 
Mounting requires removing the rear tilt adapter on the telescope to expose the M56x0.75 thread (a 1.5 mm hex key is required and not included). After removing the adapter, the internal M48 thread for 2" filters is no longer accessible.