If you're looking to upgrade your standard 1.25" diagonal with a 2" model, check out this 2" Mirror Diagonal from Founder Optics!

The compression ring twists to lock in 2" barreled accessories without marring and fiddling with small screws in the dark.  You will also receive a 2" to 1.25" compression ring adapter for your 1.25" barreled accessories.  The telescope side slides into 2" focusers.  In order to use this diagonal with a catadioptric telescope you would need an SCT adapter.

This Founder Optics 90-degree mirror diagonal has a dielectric coating with over 99.99% reflectivity for bright images.