Diagonals are used to redirect the viewing position of the eye or CCD camera - for example, rather than looking straight through the scope (which may be uncomfortable, if the scope is mounted low, and aimed straight up), a 90-degree diagonal makes it possible to look down in order to see through the scope (or, in the example of the scope pointing up, a 90-degree diagonal will allow looking horizontally in a much more comfortable position). There are also 45-degree diagonals; the one in the photo above again just makes it easier to look through the scope. However, the WO 45-degree diagonal shown below is actually an Amici prism, which inverts the image from left-to-right, so that it does not move opposite from the movement of the scope. Diagonals come in both 1.25" and 2" sizes, to fit corresponding eyepieces and CCD camera nosepieces. They can have a screw to hold the eyepiece (or camera) in place, or - in better diagonals - a brass compression ring that is tightened with a screw, so that the eyepiece barrels and camera nosepieces are not marked when the diagonal screw is tightened down.

William Optics Dielectric Diagonal

TheWilliam Optics diagonals are very high quality, in terms of fit and finish. The come in several different versions, including quartz, carbon body, and dielectric coatings. The dielectric diagonals have the lowest light loss, but are less expensive than the quartz versions.

Meade Dielectric 99% Diagonal

The Meade dielectric 99% reflectivity diagonal is one of their best, having multilayer dielectric coatings which increase the reflectivity from 96% to 99%. While this is only a 3% increase, it may help with very dim deep space objects. These diagonals are generally used only for visual observing, and not astro-imaging.

William Optics 45-degree Amici Prism

The WO 45-degree Amici prism is a specialized diagonal for terrestrial viewing, where it can be very confusing to move the scope in one direction, and the image moves in the opposite direction. This diagonal allows for "correct image" viewing. Some finder scopes are called "RACI", which means "right angle correct image" - so that they present the image in the same orientaion as looking at the sky; this WO diagonal performs a similar function, but is in a 45-degree format, rather than 90 degrees.

Televue 1.25" Everbright Diagonal

The Televue Everbright series of diagonals use a dielectric multilayer coating to achieve 99% reflectivity. They are considered some of the best-quality diagonals made for amateur astronomy.