It’s the age-old question that is always heard in star viewing parties. “Which is a better diagonal? Erecting prisms or Mirror diagonals?”. “Mine performs better”, “no, mine gives sharper images”, and so on for hours. So, what’s the definitive answer? well… The difference between prism and mirror diagonals is in their design and materials. Prisms use lenses while mirror diagonals obviously use...

Source