Eyepiece accessories 6101 Diagonals

A star diagonal (Figure 6.5) is a right-angle adapter, now almost universally used with refractors and catadioptrics so that observers do not have to bend their necks backward. It is called a star diagonal to distinguish it from a sun diagonal, which reflects only part of the light and was formerly used by solar observers. Some Maksutov-Cassegrains, such as the Questar and Meade ETX-90, have a diagonal mirror built in that can be flipped out of the way.

Figure 6.6 shows how diagonals work. Most star diagonals are 90° prisms and transmit about 90% of the light that reaches them. The losses occur by reflection where the light enters and exits the glass; the internal reflection is total.

The optical quality of prism diagonals varies. Total flatness is not critical because each point in the image reflects off only a small part of the diagonal surface. Local smoothness is more important and is generally good.

However, many cheap prism diagonals are miscollimated - they are not centered on the optical axis and not aligned at exactly 90°. To check for this problem, aim the telescope at a terrestrial object and make sure the same point remains centered as you turn the diagonal from side to side. Miscollimation is often easy to fix by taking the diagonal apart and putting it together again, making sure the prism fits into its mount properly.

Mirror diagonals of higher quality are available from Tele Vue and Lumicon. Their flatness meets the same rigorous standards as Newtonian secondaries.

Figure 6.5. This Meade 2-inch mirror diagonal attaches directly to the back of the telescope. An adapter for l|-inch eyepieces is provided with it.

Figure 6.5. This Meade 2-inch mirror diagonal attaches directly to the back of the telescope. An adapter for l|-inch eyepieces is provided with it.

Mirror 90° prism Roof prism

Figure 6.6. What's inside a star diagonal. Roof-prism type (right) gives an erect image.

Good mirror diagonals transmit slightly more light than prisms, typically about 95%. Also, unlike a prism, a mirror cannot introduce any chromatic aberration, even with low- f -ratio telescopes.

Despite these concerns, my own experience is that except for miscollimation, almost all the diagonals I have ever tried are good. Slight differences in light transmission are not perceptible, and optical quality, as judged by star tests, is not a problem. The limitation I most often run into is that a small diagonal may not completely illuminate the field of a low-power eyepiece.

A diagonal with a roof prism gives an erect image that is correct left to right. In place of the flat reflecting surface, it has a structure like a roof, with two surfaces at a 90° angle. As a result, the light is reflected twice instead of once and does not come out mirror-imaged. Each point in the image receives light from both surfaces, so the 90° angle is critical; even the slightest error results in blurring or doubling of the image. For that reason, roof prisms are not used for high-power work.

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