A star diagonal is a mirror or prism in a housing that bends the light path through ninety degrees, so you look down into the eyepiece rather than along the tube. On refractors and catadioptrics it is not optional in practice.
Why it exists
Without one, a refractor pointed high in the sky puts the eyepiece close to the ground and you observe lying down. The diagonal turns the light through a right angle and puts the eyepiece somewhere a person can reach. Newtonian reflectors do not use one, because their eyepiece is already at the top of the tube pointing sideways.
Mirror or prism
A dielectric mirror diagonal reflects almost all of the light that reaches it and is the current standard. A prism uses total internal reflection, can be marginally better in slow instruments and worse in fast ones, and is heavier. For most owners the mirror is the sensible default and the difference is small.
What it does to the image
A diagonal gives a mirror reversed image: the right way up but flipped left to right. That is harmless for astronomy and confusing when matching a star chart, which is why observers learn to read charts mirrored or use software that can flip. An erecting prism corrects it for terrestrial use at some cost in sharpness.
Sizes and quality
Two barrel sizes, 1.25 inch and 2 inch, matching the eyepieces. A cheap diagonal can degrade an otherwise excellent instrument, and a poorly figured mirror shows as softness that people blame on the telescope. It is a component worth buying once and keeping across instruments.
Questions people ask about telescope
What is a star diagonal?
A mirror or prism that bends the light path ninety degrees so the eyepiece sits at a comfortable angle on a refractor or catadioptric.
Why is my telescope image reversed?
A diagonal flips the image left to right. It is normal, it is harmless for astronomy, and it only matters when matching a printed chart.
Mirror or prism diagonal?
A dielectric mirror for most instruments and most owners. Prisms have a narrow advantage in slow telescopes and are heavier.