![]() ![]() ![]() Aberration – defects in images produced by a telescope (or optical system).If you’re limited to a 1.25” format, consider a prism diagonal instead of a mirror, as they are on average cheaper for the same or better performance in most telescopes, as mentioned above. Most mirror diagonals without dielectric coatings are likely to be cheap and of low quality. Dielectric mirrors have extremely durable coatings with many protective layers to minimize scatter and make them nearly impossible to scratch or chemically damage. If you’re using a 2” star diagonal, it’s likely to be a mirror diagonal. Best Mirror Star Diagonal: Our Recommendations Twist-lock diagonal designs are usually a little more secure than brass compression rings, but are usually more expensive and slower to interchange eyepieces with. Set screws can also cause cosmetic damage from digging into your eyepiece. The reasoning for this is that a tripod-mounted telescope, especially when on an equatorial mount, can have the eyepiece at angles where it could fall out due to an improper grip, whereas Dobsonian telescopes usually have gravity to help as a backup should you screw something less-than-tightly. Most good star diagonals use either a compression ring or a rotating-lock adapter to grip your eyepieces. Also, smaller catadioptric telescopes with apertures less than 8″ may be able to physically fit a 2″ diagonal, but they may have baffle tubes that limit the usable field of view to a 1.25″ format eyepiece without causing vignetting. The added weight alone may also be too much for smaller and less-than-steady mounts. Some telescopes may be unable to both clear their fork mounts with proper balance or simply be unable to balance on their mount entirely with a heavy 2” diagonal and eyepieces. While using a 2” diagonal with any scope that can physically accept it would be nice, in reality, this isn’t always beneficial. ![]() This makes them a little more rugged and ideal for use in dusty and dirty environments, especially if you are using a more expensive instrument. Lastly, the prism diagonals’ solid prism optics essentially “seal off” a telescope tube when they are installed in your focuser. Mirror diagonals can also go out of collimation if dropped, though this is rare and user adjustment is usually possible to fix the problem. However, silver dielectric mirrors are available for this purpose as well. While a niche concern, those using night vision eyepieces may want a prism diagonal because prisms transmit the near-infrared light picked up by night vision devices significantly better than aluminum mirrors. Prism diagonals are also rarely available in 2” format, and those that are tend to be heavy and expensive. However, in refracting telescopes with fast focal ratios of f/6 to f/7 or below, prisms can add additional chromatic aberration to the view. The biggest difference in performance between the two types is that prisms scatter light less than mirrors and thus can produce a slightly sharper image on bright targets, such as planets. Prism and mirror diagonals can both achieve excellent light transmission even the cheaper units sold today usually have 93% or better transmission, and all of the units we recommend are 98% efficient or better. Amici prisms produce annoying diffraction spikes and other effects on astronomical objects, and cheap ones are often supplied with beginner telescopes. Astronomical prism star diagonals use a triangular prism and should not be confused with Amici “erecting prism” and other designs that can be used for terrestrial observing. Prism or Mirror?ĭiagonals use either a mirror or a prism to bend light at a 90-degree angle. If you’re concerned about them affecting the sharpness of your views, it’s usually possible to adapt a diagonal to the back end port, but this may cause clearance issues on a mount. A few telescopes have built-in “flip mirror” diagonals, which can often be of low quality. Most telescopes that need one come with a star diagonal supplied, but it may be of low quality or only in 1.25” format. Diagonals with poorly polished optical surfaces can induce scattering and warp the image with aberrations like astigmatism, and cheap diagonals with low reflectivity can dim the image through your telescope. Diagonals are often an overlooked part of a telescope, but they are a tremendously important optical accessory for achieving sharp views. ![]()
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