Star diagonal

Mirror diagonals produce an image in the eyepiece that is correctly oriented vertically, but is reversed left-to-right horizontally.

The major advantage to mirror diagonals is that they cost less to produce to a high degree of optical accuracy compared to a prism and that they do not introduce any color errors to the image.

The major disadvantage of mirror diagonals is that unless the reflective coating is properly applied they can scatter light rendering lower image contrast compared to a 90° prism.

The natural color dispersion properties (overcorrection) of the prism works to lessen or nullify the undercorrection of the objective lens.

However, prism diagonals may introduce chromatic aberration when used with short focal-length scopes although this is not a problem with the popular Schmidt-Cassegrain and Maksutov-Cassegrain telescopes, which have long focal lengths.

[3] Even an expensive star diagonal will deliver poor performance if it is not in alignment with the optical axis of the telescope.

Since the mirror or prism of the star diagonal is located nearly at the focal plane of the instrument, surface accuracy of greater that 1/4 wave is more in the line of advertising than any increase in optical performance.