User:Brainandforce/sandbox/Telescope diagonal

A telescope diagonal is an optical element used to redirect light from a telescope so that the image it forms is conveniently placed for an observer.

The flat secondary mirror of a Newtonian telescope performs a similar function to a diagonal; separate diagonals are rarely, if ever, used with Newtonians.

Types of diagonals

Different types of diagonals are used to

Star diagonals

Star diagonals, or 90° diagonals, are the most commonly used by amateur astronomers. These use either uses a flat mirror oriented at a 45° angle or a Prism (optics) to redirect the light path at a 90° angle. When installed in an orientation which points the eyepiece directly upwards, a star diagonal will reverse the top and bottom of the image. For most amateur telescopes, which produce an inverted image without a diagonal, this results in an image which is erect but mirrored left to right. However, some telescopes, such as the Gregorian telescope design, produce an erect image without a diagonal, and so the resulting image is reversed from top to bottom.

The choice of prism

Compared to prism diagonals, mirror diagonals are lighter, have only one optical surface (as opposed to three for a prism), cost less for a given size, and do not introduce any chromatic aberration. However, they are also prone to scattering light, which reduces contrast, particularly for bright objects. Scattering can worsen over time for mirrors whose coatings are prone to degradation, but modern Dielectric mirror are highly durable and scatter light minimally. Additionally, a mirror may be more prone to misalignment, especially if the diagonal is dropped.

Mirror diagonals also take up more backfocus than a prism diagonal: the change in medium shortens the light path in a prism diagonal

Correct image diagonals

More complex designs may be used to simultaneously reposition an image and reorient it so that it is right side up and unmirrored. The Amici roof prism is used for this purpose

Amici prism diagonals are commonly used on finderscopes to assist astronomers referencing star charts so that they do not have to account for the change in direction induced by a star diagonal. In this application, the diffraction spike is of little importance and may not even be visible at the low, usually fixed power of the finderscope.

45 degree correct image diagonals are commonly used and supplied with telescopes for terrestrial observation. Aside from the correct image, the angle is usually

Aberrations

Diagonals can introduce aberrations that affect the image produced by the telescope. While some of these aberrations may be caused by poor manufacturing or misalignment of the diagonal's optical elements, others are inherent to the designs of certain diagonals.

A prism diagonal refracts light entering the first surface and leaving the last surface, and the amount of refraction Dispersion (optics). This refraction can introduce spherical aberration and coma, as well as spherochromatism and .

Generally, telescopes with focal ratios faster than f/7 are usually equipped with mirror diagonals so that aberrations from a prism do not affect the image. However, some telescopes have been built specifically to accomodate a prism in the light path to produce a fully corrected image.

External links

• www.example.com