2" DIAGONALS
 

ERECTING PRISMS

 When using a refractor or cassegrain style telescope a unique problem can sometimes occur. Depending on the telescope's position, it might be very uncomfortable for the viewer to look straight through the eyepiece! For that reason, a simple piece of equipment was invented - the 90° diagonal.

A diagonal or star diagonal, as it is sometimes known, contains an angled mirror or prism that allows for comfortable viewingWhen using a refractor or cassegrain style telescope. There are two types of diiagonals:
The first design is the mirror diagonal. Many star diagonals use a high quality mirror set at a 45° angle inside the diagonal. This allows the telescope's image to appear at a 90° angle to the rear cell where it is then magnified by the eyepiece. The image in your eyepiece will be correctly oriented vertically, but reversed left-to-right horizontally. At first this change in direction can be slightly confusing when comparing the image to a star chart, but a high quality mirror diagonal provides very little light loss in the optical path.

The second design is the prism diagonal. Inside the angled body, an Amici roof prism bends the light path in the same fashion as binoculars. Because it is not reflecting, but bending, this design allows it to be "image erecting" or "image correcting" both horizontally and vertically.  It is important to note that unless a high quality prism is used, it can reduce the total amount of light transmitted to the eyepiece. Some prism diagonals may also introduce chromatic aberration when used with short focal-length telescopes. In spite of these limitations they are an excellent choice for viewing terrestrial subjects where orientation matters the most and a small amount of light lost is not a problem.

   Very often the diagonal supplied by telescope manufacturers are not the best of quality. In many cases it would be a signiificant improvement to upgrade to a higher quality diagonal. Diagonals, like eyepieces, come in three sizes: .965", 1.25" and 2".If  your diagonal is .965" in size you should definitely consider upgrading to a hybrid diagonal which will convert your scope to accept the far superior 1.25" size eyepieces. By the same token, many scoopes are capable of being upgraded to a 2 inch diagonal.