The Alpine Astronomical Baader 2" ClickLock BBHS Star Prism Diagonal gives you a nice full aperture with a precision prism and optical coatings for the performance you demand from your diagonals.

The BAK4 prisms are treated with a combination of anti-reflection Phantom Group multi-coatings on the portions of the prism where light passes through and Broadband Hard Silver (BBHS) coatings, sealed with a dielectric coating, on the prism's reflective surfaces. This combination of complementary coatings boosts reflectivity to greater than 99% for minimal light loss throughout its long life. The broadband coatings uniformly transmit all wavelengths of light between 390 and 2000nm for accurate color rendition and enhanced clarity. As a star diagonal, the views through it will appear right side up, but vertically reversed.

Unlike conventional compression ring and set screw holders, the ClickLock system uses three long roller bearings to hold the eyepiece firmly centered in place without marking or marring it, while eliminating the wobble and off-axis tilt. A simple 20° twist locks and unlocks the clamping system for fast eyepiece changes even while wearing gloves. There is no need to fumble with small set screws or for using excessive force and torque to hold the eyepiece in a fixed position. A 2" nosepiece that is made to accept standard 2" eyepiece filters rounds out the diagonal features.