Robtics | Diagonals/Mirrors/PrismsA diagonal (star diagonal, diagonal mirror) is an accessory that makes you able to look-around-the-corner. It allows more convenient and comfortable viewing when the telescope is pointed at, or near the zenith (i.e. directly overhead). To accomplish this the diagonal uses an angled mirror or a prism. The result is both the same and one is not per se better than the other.

Diagonals are used most often in refractor and catadioptric telescopes and is placed between the telescope and the eyepiece. Diagonals are available in 1.25 inch and 2 inch diameters. The 2" diagonals allow larger longer focal length low power 2 inch barrel eyepieces for a greater field of view and mostly come with an adapter to 1.25" to accommodate for both sizes.

Standard prism diagonals offer a transmission roughly between 70 and 85 percent. Standard diagonal mirrors reflect roughly between 80 and 90 percent. High quality mirror diagonals come with dielectric coatings and can have up to 99% reflectivity!

There are different models of diagonals are available. Most diagonals turn the telescope's image 90 degrees. There are also versions that gives 45 or 60 degrees. These are most often used by observers who also use their scopes for terrestrial viewing.

An Amici prism is a type of roof prism which allows an upright image without left-right mirroring. This means that what is seen in the eyepiece is the same as what is seen when looking at the sky, or a star chart or lunar map. The disadvantage of typical 'correct image' Amici roof prism diagonals is that because the light path bounces around through a piece of glass, the total amount of light transmitted is less and the multiple reflections required can introduce optical aberrations. For this reason amici prisms cannot be used for high magnification observing.

Can't find what you're looking for or need help selecting the best diagonal for you? Contact Robtics! We are happy to help!

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