It was mentioned that 2-inch barrel-format eyepieces will, naturally, require a 2-inch star diagonal in the telescopes. There are numerous 2-inch diagonals on sale from numerous companies, but before deciding on which brand to buy, a prospective user of 2-inchers must also decide which style diagonal to purchase; 2-inchers come in two distinct flavors: SCT and refractor.
SCT diagonals are made expressly for our CATs, and as shown in Plate 41, incorporate an integral threaded ring that allows them to screw directly onto the scope's rear port (or a reducer/corrector). The other type, the refractor style, looks just like an oversize 1.25-inch diagonal. It has a plain barrel that is designed to be inserted into a refractor's focuser. Since there is no threaded ring and a 2-inch barrel will not fit into a standard visual back, another item is needed before a refractor diagonal can be used with most SCTs (Meade Microfocuser-equipped SCTs accept 2-inch refractor diagonals directly), an inexpensive accessory called a 2-inch adapter or, interchangeably, a 2-inch visual back. Whatever it is called, this item is nothing more than a threaded tube that can be screwed onto the rear port and into which 2-inch devices can be inserted. One also features a setscrew or compression ring to hold the diagonal or other item in place. If possible, choose a model with a compression ring to best hold the 2-inch diagonal and eyepiece combination securely.
Which style diagonal is best? Both work fine. In the past, users were often advised to choose refractor diagonals since until recently there was a larger selection of quality models available in that style. Today, very high-quality SCT 2-inch diagonals have become available from companies like William Optics and TeleVue, and an SCT-style diagonal is often more convenient since there is no 2-inch visual back to install and keep up with.
In addition to choosing the style of diagonal, you must decide on the coating type. A standard aluminized diagonal's mirror will reflect about 88% of the light that strikes it. Premium dielectric-coated diagonals like William Optics Dielectric Carbon Fiber diagonal ($168) can reflect as much as 99% of incoming light. Dielectric coatings achieve this high reflectivity thanks to their multiple layers of different and sometimes-exotic materials. The choice of material for these layers allows manufacturers to tune diagonal mirrors for maximum reflectivity in visible light. Is a dielectric diagonal worth the extra money (about $100 more than standard aluminum)? That depends. There is not much difference visually between an 88% diagonal and a 99% diagonal, but there is some.
Is a 2-inch diagonal something a new SCT user needs? Only if 2-inch eyepieces will be used. A 2-inch diagonal will offer no improvement over a 1.25-inch model of similar optical quality. Think long and hard before buying a 2-incher if only
Plate 41. (2-inch Star Diagonal) William Optics 2-inch SCT-style dielectric star diagonal. Credit: Author.
1.25-inch eyepieces will be used. Sure, 2-inch diagonals come with 1.25-inch eyepiece adapters, but 1.25-inch eyepieces do not always work well in 2-inch diagonals. These adapters may place some 1.25-inch oculars far enough back that they will not reach focus. This problem is exacerbated with a reducer/corrector in place.
Forgoing a 2-inch star diagonal does not mean sticking with the cheap diagonal that came with the scope. The 1.25-inch models are every bit as good as the top 2-inchers that are available from William Optics, TeleVue, Astro-Physics, and other manufacturers. Dielectric 1.25-inch diagonals are also relatively inexpensive, costing as little as $80. Which is best, a prism or mirror diagonal?. Although 2-inch diagonals almost always use mirrors, 1.25-inchers sometimes use prisms to divert the incoming light 90°. Mirror diagonals are easier to make well than prism diagonals and are usually better optically for that reason.
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