* Astronomy

A star diagonal is designed to place the eyepiece at comfortable viewing position on an astronomical telescope.
A star diagonal is either constructed from a prism or mirror, and reflects the path of the light at, usually, a 90-degree angle to the telescope's optical axis.
Newtonian-type reflecting telescopes already have a 'star diagonal' by design.

Standard star diagonals come in three main barrel sizes; 0.965", 1.25"(31.7mm) and 2" (50.8mm).
The 2" diagonal is likely to have an adaptor fitting, so that smaller 1.25" eyepieces can be used.
One feature to look out for is that the barrel of the diagonal is threaded to allow filters to be used.  48mm filters are the usual size fitted to 2" diagonals.
Most higher priced telescopes are adaptable to use 1.25" and 2" eyepiece diameters.

A larger 2" diagonal on a telescope will allow the use of lower power (lower than 32mm) eyepieces to give an apparent field of view unobstructed by the mounting barrel. 
With short focus refractors there is little reason to upgrade to a 2" diagonal (apart from the need of a wide field of view required by astrophotography - a rather mute point).
2" eyepieces are considerably more expensive and heavier compared to 1.25" eyepieces.

Good quality diagonals will have a barrel that is recessed to stop the diagonal from slipping out of the focuser.
Another feature to look out for, but is not essential, is a compression ring which prevents the tip of the locking screw from scratching the surface of the eyepiece barrel.

Good quality mirror star diagonals will have a surface with 99% reflectivity, and lower priced diagonals with a seemingly poor 91% reflectivity; However, the reality is that most observers cannot tell the difference between the two.

Since the mirror or prism of the star diagonal is located nearly at the focal plane of the instrument, surface accuracy of greater that ¼ wave is more in the line of advertising than any increase in optical performance.

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-degree prism. Also they deteriorate with age as the reflective surface oxidizes. The newer Dielectric mirrors have largely solved the deterioration problem, and if properly made the Dielectric mirrors scatter less light compared to conventional mirrors. With short focal length instruments a mirror diagonal is preferred over a prism.

A well-made conventional 90-degree prism star diagonal can transmit as much or more light as a mirror, and do so with higher image contrast since there is no possibility of light scattering from a reflective metallic surface as in a mirror diagonal. Also a prism will never degrade over time as a mirror will since there is no reflective metal coating to degrade from oxidation. However prism diagonals may introduce chromatic aberration when used with short focal-length scopes although this isn't a problem with the popular Schmidt-Cassegrain and Maksutov Cassegrain telescopes, which have long focal lengths. On longer focal ratio telescopes a well-made 90-degree prism diagonal is the optimum choice to deliver the highest image contrast short of using the telescope without a diagonal entirely. However prisms seem to be falling out of favour probably due to marketing forces which have been favouring short focal length instruments which tend to function better with a mirror diagonal. In some special cases however, the colour dispersion effects of a prism diagonal can be used to advantage to improve the performance of undercorrected refractor objectives (regardless of focal length) by shifting the spherical and colour correction of the objective closer to the design optimum. The natural colour dispersion properties (overcorrection) of the prism works to lessen or nullify the undercorrection of the objective lens.

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Collimate A Star Diagonal

Home made star diagonal
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